Reef Dosing & Chemistry Calculator

A reef dosing calculator tells you exactly how much of a supplement to add to move a water parameter to where you want it, so you stop guessing and start dosing with numbers. Enter your tank's net water volume, your latest test reading, and the target you are aiming for, and it returns the amount to add for calcium, alkalinity, or magnesium. It also handles the small conversions that trip people up, such as dKH to meq/L, and it works out a steady daily dose that matches what your corals consume. Everything runs in your browser, so the numbers you type stay on your device and the answer comes back the instant you enter them.

Keeping a reef tank stable is mostly a chemistry problem. Corals pull calcium and carbonate out of the water day and night to build their skeletons, so the levels drift down and you have to put them back. Do it by eye and you either underdose and stall growth, or overdose and swing the parameters, which stresses the animals you are trying to keep. A calculator turns the job into arithmetic you can trust. This page covers the big three parameters, the ranges worth aiming for, the formula behind every dose, how to use the tool step by step, and how to change things slowly enough that your tank stays steady.

What a reef dosing calculator does, and who needs it

A reef dosing calculator takes three inputs, your water volume, your current level, and your target level, and returns a single output: how much supplement to add. Behind that simple exchange is chemistry that would otherwise mean pulling out molar masses and unit conversions every time you test. The tool holds that for you and gives back a dose in grams or milliliters, plus the conversions between the units different test kits and products use.

The people who need one keep saltwater reef aquariums, from a first nano tank with a few soft corals to a large system packed with stony corals. Anyone dosing a two part calcium and alkalinity system, adding baking soda or soda ash for alkalinity, correcting a low magnesium reading, or setting up a daily maintenance dose will reach for a calculator to get the amount right. New reefers use it to avoid the classic beginner mistake of dumping in too much. Experienced keepers use it to dial in a daily dose and to plan a slow correction when a test comes back off. If you keep corals and you test your water, this is a tool you will open often.

What sets this one apart from a single-parameter box is that all three parameters sit on screen at once. You can see that magnesium is the reading holding the other two back, that alkalinity needs a small nudge, and that calcium is a longer job, without running three separate calculations and holding the results in your head.

The big three: calcium, alkalinity, and magnesium

Reef chemistry has many parameters, but three drive coral growth and get dosed the most. They are usually called the big three.

Calcium is the mineral corals use, along with carbonate, to build their calcium carbonate skeletons. It is measured in parts per million, or ppm. A reef running plenty of stony corals goes through calcium steadily, and if it falls too low, growth slows and then stops.

Alkalinity is the water's supply of carbonate and bicarbonate, the other half of the skeleton building reaction, and it also buffers pH against sudden change. It is measured in degrees of carbonate hardness, dKH, or in milliequivalents per liter, meq/L. Alkalinity is usually the parameter that moves fastest, because corals draw it down quickly, which is why so many reefers test it most often.

Magnesium is the quiet third member. Corals use only a little of it, but it plays a critical supporting role: magnesium keeps calcium and carbonate dissolved in the water so they stay available instead of precipitating out. When magnesium is too low, calcium and alkalinity become hard to hold no matter how much you dose. Magnesium is measured in ppm, like calcium.

The three work as a set. Calcium and alkalinity are consumed together and dosed together, and magnesium is the enabler that lets the other two behave. Get all three into range and keep them steady, and most coral trouble tied to chemistry goes away. That is why the tool flags a low magnesium reading before anything else, and why it will tell you to fix that first.

The target ranges to aim for

There is a well established band for each parameter that most reef keepers work within. These come from the chemistry of natural seawater and from years of hobby experience, so treat them as sensible zones rather than exact points to chase. Each card in the calculator shades that band on its bar, so you can see at a glance whether your reading sits inside it.

• Calcium: about 400 to 450 ppm. Natural seawater sits near 420 ppm, and anywhere in this band supports healthy skeleton growth.
• Alkalinity: about 8 to 11 dKH, with many keepers settling around 8 to 9. Natural seawater is closer to 7 dKH. A lower, stable alkalinity is often gentler for stony corals, while a slightly higher figure can push faster growth if the rest of your chemistry supports it.
• Magnesium: about 1250 to 1350 ppm. Natural seawater is near 1285 ppm. Keeping magnesium in this band is what lets you hold calcium and alkalinity where you want them.

The single most important idea here is that stability beats perfection. A tank held steady at 8 dKH does better than one that swings between 8 and 11 from week to week, even though both readings fall inside the range. Pick a target you can hold, then dose to keep it there rather than nudging it around. The calculator helps with both halves of that: reaching a target, and then maintaining it.

The dosing formula: how much to add

Under every reef dosing calculator is one relationship. To raise a parameter, the amount of supplement you add depends on how far you want to move the level, how much water you are treating, and how strong the supplement is. In plain terms:

dose = (target level minus current level) times net water volume, divided by the supplement's strength.

The first part, target minus current, is your gap, how many ppm of calcium or magnesium, or how many dKH of alkalinity, you need to add. The second part, water volume, is the actual volume of water in the system. The third part, strength, is how much one unit of the supplement raises the parameter in a given volume, a figure you read from the product label or from a known value for a raw chemical.

That is the whole idea. Every branded calculator is this formula with the strength values already filled in for specific products. This one fills in the raw chemicals from their molar masses, which means you can check the figures rather than take them on trust. Baking soda works out at about 8.8 dKH per gram per gallon, soda ash at about 14.0, calcium chloride dihydrate at about 72 ppm of calcium per gram per gallon, and magnesium chloride hexahydrate at about 31.6 ppm of magnesium. Flip those around and you get the numbers the tool actually uses: roughly 0.113 grams of baking soda per gallon for each 1 dKH, and about 0.014 grams of calcium chloride dihydrate per gallon for each 1 ppm.

Understanding the formula means you are never at the mercy of a chart you cannot check, because you can see why the answer is what it is.

Units and conversions: dKH, meq/L, and ppm

Alkalinity causes the most confusion because it comes in more than one unit, and test kits and products do not all agree on which to use. Here are the conversions the calculator handles for you.

1 dKH equals 0.357 meq/L, and it equals about 17.86 ppm expressed as calcium carbonate. Turning that around, 1 meq/L equals 2.8 dKH, and it equals about 50 ppm as calcium carbonate. So if your kit reads in meq/L and your target is written in dKH, you multiply meq/L by 2.8 to compare them. The tool has a switch for this: pick dKH or meq/L and every alkalinity figure on screen, including the safe daily cap and the product strength, is restated in the unit you chose. A live line under the cards shows your current reading in all three units at once.

Calcium and magnesium are simpler, since both are almost always given in ppm, which is the same as milligrams per liter. No conversion is needed there. A target of 430 ppm calcium means 430 ppm on any kit that reports ppm.

Volume has its own switch, US gallons or liters, and swapping it converts your entered volume rather than reinterpreting it, so a 34 gallon system becomes 128.7 liters and the doses stay the same. That matters when a product chart is written for liters and your tank is described in gallons.

The practical advice is to choose one alkalinity unit and stay with it across your kit, your notes, and the calculator. Mixing dKH and meq/L in your head is where dosing errors creep in.

How the big three relate: the consumption ratio

One relationship is worth knowing because it explains why two part dosing works. When a coral lays down skeleton, it uses calcium and carbonate in a fixed proportion. In tank units, roughly 20 ppm of calcium is consumed for every 2.8 dKH of alkalinity, which works out to about 7 ppm of calcium for each 1 dKH.

That is why calcium and alkalinity fall together and why a balanced two part system doses them in matched amounts. If your alkalinity drops by 1.4 dKH over a day, your calcium has dropped by roughly 10 ppm over the same day, and a balanced dose replaces both. The calculator runs this check for you. Enter the daily drop you measured for both parameters and the ionic balance note tells you whether the pair sits on the 7 ppm per dKH line, or whether one is running ahead of the other.

It also explains a common observation: if you test and find alkalinity has fallen but calcium looks steady, the calcium change may simply be too small to show clearly on a hobby kit, because the calcium swing is about seven times smaller in dKH terms. Magnesium is consumed far more slowly and is often topped up by regular water changes rather than daily dosing, which is why its daily field is usually left at zero.

How to use the Toolfiddle reef dosing calculator

The flow is short and built to be used beside the tank, with the doses updating as you type. There is no submit button and no cap on how many tanks or scenarios you run.

• Enter your net water volume. Use the actual amount of saltwater in the system, not the tank's rated size. Rock and sand displace water, so real volume is usually 80 to 90 percent of the nominal number. If your display is 40 gallons with a sump, estimate the true total and lean low if unsure. Switch to liters if that is how your equipment is described.
• Set your alkalinity unit. Pick dKH or meq/L to match your test kit. Every alkalinity figure on the page follows that choice.
• Fill in each parameter. For calcium, alkalinity, and magnesium, type the reading your test gave you and the level you want to reach. The card works out the gap and draws it on the bar, with your usual range shaded and a marker at natural seawater.
• Add your daily consumption if you know it. The used per day field takes the drop you measured over 24 hours. Leave it blank until you have measured it; the maintenance dose simply waits.
• Pick your supplement. The raw chemicals are already in the list with strengths worked out from their molar masses. If you use a bottled product, choose the custom option and copy the three figures off its dosing chart: the dose, the volume it applies to, and how much it raises the parameter.
• Read the two doses on each card. The first is the amount to close the gap. The second is the daily amount that replaces what your corals use. If the gap is bigger than a safe single-day change, the card splits it for you and tells you how many days to spread it over.
• Check the notes underneath. A low magnesium reading raises a priority note, a large correction raises a slow down note, and the ionic balance note checks your two daily figures against the 7 ppm per dKH ratio.
• Copy the plan, then dose and retest. The copy button puts the whole thing on your clipboard as plain text for a note or a message. Add supplements to a high flow area, dissolve powders in RODI water first, and test again once it has mixed, usually the next day.

A worked example: raising alkalinity with baking soda

Say you have a 40 gallon tank with a sump, and after subtracting rock and sand you estimate about 34 gallons of actual water. Your alkalinity test reads 7.2 dKH, and you want to bring it to 8.2 dKH, a gap of 1.0 dKH. That gap sits under the safe daily change, so you can do it in one dose.

Baking soda, which is sodium bicarbonate, raises alkalinity by about 8.8 dKH per gram per gallon, which is the same as needing about 0.113 grams per gallon for each 1 dKH. Running the formula, 34 gallons times 1.0 dKH times 0.113 grams gives about 3.9 grams of baking soda. You dissolve that in a cup of RODI water, then add it slowly to a high flow spot such as near a powerhead, so it disperses without a local spike. The next day you test again and expect to read close to 8.2 dKH.

You will see the familiar hobby rule of thumb written as a tenth of a gram per gallon per dKH, which would give 3.4 grams here. That shortcut lands a few percent under the chemistry, and under is the safe side of a dosing error, so both answers are fine. The calculator uses the exact figure and lets you retest rather than rounding on your behalf.

Now suppose instead your test had read 6.5 dKH and you wanted 8.5 dKH, a gap of 2.0 dKH. That is more than the roughly 1.4 dKH you should change in a single day, so the card splits it: two days at about 3.9 grams each, testing between doses. Slow and checked beats fast and sorry.

A worked example: raising calcium

Same 34 gallons of water. Your calcium reads 390 ppm and you want 430 ppm, a gap of 40 ppm. Calcium moves less dramatically than alkalinity, but you still raise it in steps rather than all at once.

Calcium chloride dihydrate, the flake or prill sold for reef use and the calcium half of most two part systems, raises calcium by about 72 ppm per gram per gallon, so it takes about 0.0139 grams per gallon for each 1 ppm. Forty ppm across 34 gallons comes to roughly 18.9 grams. That is a single day's correction, since 40 ppm is inside the 50 ppm a day the tool treats as a working ceiling.

If you use the anhydrous form instead, it is stronger per gram, about 95 ppm per gram per gallon, so the same job takes around 14 grams. Check the bag before you assume which one you have, because ice melt grades vary and some carry additives you do not want in a reef.

Because calcium and alkalinity are consumed together, once you are on target you will usually maintain calcium with the same balanced two part dose that maintains alkalinity, rather than dosing it separately. The daily field on the calcium card exists for exactly that reason: enter the drop you measured and the tool shows the daily amount, then the balance note checks it against your alkalinity figure.

A worked example: raising magnesium

Now a magnesium problem, which is often the real problem. In the same 34 gallon system, magnesium reads 1180 ppm, which is under the 1250 ppm floor, and you want 1300 ppm, a gap of 120 ppm. Low magnesium is worth fixing before you chase calcium and alkalinity, because until it is back in range those two will keep slipping no matter how much you dose. The tool raises a priority note the moment your reading drops below 1250.

The standard hobby blend is five parts magnesium chloride hexahydrate to one part magnesium sulfate, which keeps chloride and sulfate near their seawater ratio. That mix raises magnesium by about 30.7 ppm per gram per gallon, so 120 ppm across 34 gallons needs roughly 133 grams. Magnesium is worth raising gently, on the order of 100 ppm a day, so the card splits it into two days at about 66.5 grams each rather than handing you one large number.

Add it to good flow, retest, and once magnesium holds around 1300 ppm you will find calcium and alkalinity become much easier to keep steady. This is the single most common reason a tank refuses to hold alkalinity despite careful dosing.

Daily dosing to match consumption

Reaching your targets is only half the job. The other half is holding them there, because your corals keep consuming calcium and alkalinity every day. This is where daily dosing comes in, and it is the point at which a reef tank starts to feel stable instead of like a constant rescue.

The method is straightforward. Get your parameters on target, then test alkalinity at the same time each day for several days without dosing, and note how much it falls in 24 hours. That drop is your daily consumption. Say it comes out to 0.5 dKH per day. Type that into the used per day field and the card shows the daily amount, which for 34 gallons of water and baking soda is about 1.9 grams. Dose that every day and the level holds flat. Because calcium is consumed in step with alkalinity, a balanced two part dose covers both at once, and you rarely need to touch calcium on its own.

Many reefers automate this with a dosing pump that adds small amounts through the day, which keeps the level even steadier than one daily slug. Whether you dose by hand or by pump, the calculator gives you the daily amount from your measured consumption. Magnesium usually needs only occasional topping up, often covered by water changes, so it rarely joins the daily routine unless you keep a heavy stony coral load.

One habit worth building: recheck your consumption every couple of months. As corals grow, they use more, and a daily dose that held the line in spring will fall behind by late summer. A drifting alkalinity reading in a tank that has not changed otherwise is almost always growth outrunning the dose.

Safe rates of change and responsible dosing

This is the part to take seriously, because the fastest way to harm a reef is to change its chemistry too quickly. Corals tolerate a wide range of stable conditions but react badly to sudden swings, so the goal is always slow, measured, and tested.

Cap your alkalinity changes. A widely used ceiling is about 1.4 dKH in a single day, and many keepers stay well under that, correcting by only 0.5 to 1 dKH per day. The calculator holds you to it. Ask for a bigger lift than one day's worth and the card switches to a split plan, tells you how many days it will take, and gives the amount for each day. Magnesium follows the same treatment at roughly 100 ppm a day, and calcium at roughly 50 ppm.

A few more rules that no calculator can enforce for you. Never premix the two parts of a calcium and alkalinity system, and do not add them at the same time and the same place, because concentrated calcium meeting concentrated carbonate precipitates on contact. Dose them at different times into good flow. Fix a low magnesium reading before chasing the other two. And always dose to your net water volume, since overstating the volume leads straight to overdosing.

Two habits protect you further. First, trust a retest over a single reading. Hobby test kits vary, and if a result looks surprising, confirm it before you act on it, especially before a large correction. Second, use a reliable kit and stick with the same one, so your readings stay consistent over time. The calculator is only as good as the numbers you feed it, so good testing is what makes safe dosing possible. Treat this tool as a planning aid, not a substitute for watching your tank and your test results.

Two part, soda ash, and commercial products

There is more than one way to supply calcium and alkalinity, and the calculator works with all of them because they all plug into the same formula with different strength values.

A two part system doses calcium and alkalinity from separate bottles. Part one supplies calcium, usually as calcium chloride. Part two supplies alkalinity, usually as sodium bicarbonate or sodium carbonate in solution. They are kept apart because mixing them at full strength makes them precipitate, but once diluted in the tank they stay in solution and feed the corals. Balanced two part dosing means adding matched amounts so the pair stays in the natural consumption ratio.

Baking soda and soda ash are the raw alkalinity options. Baking soda is sodium bicarbonate straight from the box. Soda ash is sodium carbonate, which you make by baking baking soda in the oven to drive off water and carbon dioxide. Both raise alkalinity, but soda ash raises it about 1.6 times more per gram and pushes pH up more strongly, while baking soda is gentler on pH. Reefers choose between them partly on whether their tank needs a pH lift. Both are in the supplement list with their own strengths.

Commercial products from the major reef brands are pre made versions of these, sometimes powders and sometimes liquids, each with its own strength printed on a dosing chart. To use any of them, pick the custom option and enter three figures from the label: the dose, the volume it treats, and how much it raises the parameter. A chart that reads five milliliters per 25 gallons raises alkalinity by 1 dKH becomes five, 25, and one, and the tool scales it to your tank. One note on powders: the conversion between weight and volume is approximate because granule size varies, so when a chart gives a teaspoon measure, dose a little under and retest rather than trusting the spoon exactly.

Your numbers stay on your device

Tank chemistry is not sensitive the way a personal photo or a tax form is, so privacy is a minor point here, but the way this calculator is built is still worth a mention. It runs entirely in your browser using your device's own processor. The tank volume, the test readings, and the targets you type are handled locally and never sent anywhere. There is no account to make, no email to hand over, and nothing about your tank is stored on a server or logged for later.

That local design has a plain practical upside beyond privacy: the tool keeps working when the network does not. Once the page has loaded, you can carry your phone or tablet to the tank, lose the wifi signal in the fish room or the garage, and still run every calculation. Nothing waits on a round trip to a distant computer, so there is no lag between typing your numbers and seeing the dose. Your reef data belongs to you, and with this calculator it stays with you.

Genuinely free, with nothing held back

This reef dosing calculator is free in the ordinary meaning of the word. Some reef tools and apps in this space put the useful parts behind a login, a subscription, or an upsell to a paid membership, and a few bury the calculator inside an app you have to install and register for before it will do the arithmetic you came for.

None of that applies here. There is no sign-up, no account, and no email required. Every parameter, every unit, and every conversion is available with nothing locked off and no premium tier waiting to charge you for the feature you actually need. You can run one calculation before a water change or a hundred while you dial in a new tank, today and again next month, without meeting a paywall or a daily cap. It was built to be a straightforward utility for reef keepers, not a teaser for a paid product, so what you see is the whole thing.

Instant and unlimited by design

Because the calculation runs on your own device, there is no waiting. You type your volume, your reading, and your target, and the dose appears at once, with all three cards updating as you adjust any figure. There is no progress bar, no queue at a busy moment, and no dependence on a fast connection.

There is also no artificial limit on how much you can use it. Server based tools sometimes ration free calculations or slow you down to manage their costs, but there is nothing to ration when the math happens in your browser. Run it as often as you like, for as many tanks as you keep, at whatever hour you happen to be testing. The page stays light because it is not weighed down with heavy advertising and tracking scripts, so it opens fast and responds instantly even on an older phone propped against the tank.

Works on any device, even offline

The calculator runs on a phone, a tablet, or a computer, in any current browser, and the layout adapts to the screen. On a phone the three parameter cards stack above the input fields, so you see the doses first and scroll down to adjust. That matters for a job you do standing next to the tank with a test kit in one hand.

Since the math happens locally, once the page has loaded no live connection is needed, so a weak signal where your tank happens to live will not stop you from working out a dose. Nothing depends on being logged in or on using a particular brand of browser. Test your water, enter the numbers on whatever device is nearby, and get your answer on the spot, whether you are at the display, at the sump, or mixing salt in the garage.

Related tools on Toolfiddle

Reef keeping involves a few side calculations, and some of Toolfiddle's other tools help with them. When a product chart lists volumes in liters and your tank is in gallons, the unit converter handles the switch cleanly. For working out the size of a water change, say what 15 percent of your system volume comes to, the percentage calculator does it in a second. To time a slow drip acclimation for new corals or fish, or to space doses out through the day, the countdown timer and the timer keep you on track. To count the days since your last test or water change, the days between dates calculator makes it easy. And if you are still working out what the tank can hold, the aquarium stocking calculator is a good companion. Each of these runs in your browser the same way, with no account and no upload.

The short version

A reef dosing calculator works out how much calcium, alkalinity, or magnesium supplement to add to reach a target level in your tank. The math is one formula: the gap between your current and target readings, times your net water volume, divided by the supplement's strength. Aim for calcium around 400 to 450 ppm, alkalinity around 8 to 11 dKH, and magnesium around 1250 to 1350 ppm, and value stability over any exact number. Change alkalinity by no more than about 1.4 dKH a day, magnesium by no more than about 100 ppm, fix low magnesium first, dose calcium and alkalinity in balance but never at the same time and place, and always retest. Once you are on target, measure your daily consumption and dose that amount each day to hold it. Do all the arithmetic here, on any device, even offline, with no account and no cost.