This free synastry calculator compares two people's birth charts and shows, in one instant read, how their planets connect: the aspects between them, the houses they activate in each other, and an optional composite chart for the relationship itself. You enter a birth date, time, and place for each person, and the tool builds both natal charts, overlays them, and lays out the compatibility picture with the meaning of each contact spelled out. There is no sign-up, no email to hand over, and no paid report behind a wall. Both people's birth details stay in your browser, so two sets of private information never leave the device.
A single birth chart is interesting on its own, but a relationship lives between two people, and that is exactly what synastry looks at. By placing one chart against another and reading the angles where the planets meet, a synastry calculator turns two separate sky maps into a portrait of a connection. You do not have to believe astrology is literal to find this compelling. The astronomy is exact, the same two births always produce the same grid, and the result is a rich, shareable way to reflect on how two people fit.
What synastry is
Synastry is the branch of astrology that studies relationships by comparing two natal charts. The word comes from Greek roots meaning a bringing together of stars. Rather than reading one person in isolation, synastry asks what happens when two charts are laid over each other: where one person's Sun falls in the other's chart, whether their Moons are in easy or tense angle, how their Venus and Mars line up, and which areas of life each person tends to stir in the other.
The tradition treats each planet as a different function of a person. The Sun is core identity and vitality, the Moon is emotion and inner needs, Mercury is thinking and talking, Venus is love and values, Mars is drive and desire, and the slower planets carry broader themes of commitment, change, and depth. When you compare two charts, you are really comparing these functions across two people, asking how one person's way of loving meets the other's way of acting, and so on through every pair.
It helps to be clear about what this is and is not. Astrology is a symbolic system, not a science, and no study has shown that the positions of the planets determine whether two people are compatible. We make no such claim. What a synastry calculator offers is a structured mirror for a relationship, a set of prompts framed as planetary contacts, that many people find genuinely useful for reflection, for fun, and for putting words to a dynamic they already sense. Read in that spirit, it is a warm and thought-provoking thing to share with another person.
What a synastry calculator compares
A synastry calculator produces three layers, and each answers a different question about the pair.
The first layer is the inter-aspect grid. This is the heart of synastry: the set of angles between person A's planets and person B's planets. An aspect is a specific angular relationship, like zero degrees or ninety degrees, and each carries a flavor, some easy and flowing, some tense and energizing. The grid lists every significant contact, so you can see at a glance where the two charts harmonize and where they rub.
The second layer is the house overlays. Each birth chart is divided into twelve houses, each covering an area of life such as home, work, romance, or shared resources. An overlay shows which of person A's houses each of person B's planets falls into, and the reverse. This tells you not just that two planets connect, but where in life that connection tends to show up.
The third layer is the optional composite chart, a single derived chart that represents the relationship as its own entity. Where the aspect grid and overlays describe how two people affect each other, the composite describes the character of the bond itself. Together the three layers give a rounded reading, and a good synastry calculator lays them out side by side.
How a synastry calculator works
Underneath the interpretation, the process is precise and repeatable. Here is the method in plain language, without the mathematics of Section 4.
Two natal charts first
Before anything can be compared, the tool builds a complete natal chart for each person. A natal chart is a snapshot of the sky at the moment and place of a birth: the positions of the Sun, Moon, and planets along the zodiac, plus the ascendant, the point rising on the eastern horizon, and the twelve houses that hang off it. To place the planets, the tool uses an ephemeris, a precise table of where every body sits at any given instant, and the same tropical zodiac used by Western astrology. This is the identical engine that powers a single natal chart reading, run twice, once per person.
The birth date and place set the planetary positions, and the birth time is what pins down the fast-moving Moon and fixes the ascendant and the house cusps. That is why a birth time matters so much for a full reading and why a comparison without one is only partial.
The inter-aspect grid and orbs
With both charts built, the tool measures the angle between every planet in chart A and every planet in chart B. If two planets sit at nearly the same spot, that is a conjunction. At roughly sixty degrees apart it is a sextile, at ninety a square, at a hundred and twenty a trine, and at a hundred and eighty an opposition. Conjunctions, sextiles, and trines tend to feel supportive and easy; squares and oppositions tend to feel tense and charged, though tension in astrology is often what creates energy and attraction rather than plain trouble.
Real births rarely produce a perfect ninety or a perfect one hundred twenty, so astrology allows a margin called an orb. If two planets are within a few degrees of an exact aspect, the aspect still counts, and the tighter it is, the stronger it is read. Luminaries, the Sun and Moon, are given wider orbs than the other bodies because they carry more weight. A synastry calculator applies a consistent set of orbs to decide which contacts make the grid, and the tighter contacts are the ones to trust most.
House overlays
The next step maps each person's planets into the other person's houses. The tool takes person B's house cusps, worked out from B's birth time and place, and asks which house each of person A's planets falls into by its position along the zodiac. Then it does the reverse, dropping B's planets into A's houses.
This is where a contact gains its setting. A partner's Venus in your seventh house of committed partnership reads very differently from the same Venus in your tenth house of career and public life. The overlay names the arena. Because it depends on the house cusps, and the cusps depend on the birth time, the overlays are the part of the reading that most needs accurate times for both people.
The optional composite chart
Finally, the tool can build a composite chart. The most common method takes the midpoint between each matching pair of planets: the composite Sun sits halfway between person A's Sun and person B's Sun, the composite Moon halfway between the two Moons, and so on around the chart. The result is a single new chart that belongs to neither person but to the relationship, read as a picture of the bond's own nature and purpose. It is offered as an extra layer on top of the direct comparison, for those who want to see the partnership as its own chart.
The contacts that matter most
A full synastry grid can hold dozens of aspects, so it helps to know which ones carry the most weight. These are the classic pillars of a compatibility reading.
Sun and Moon contacts come first. A link between one person's Sun and the other's Moon is one of the most valued contacts in all of synastry, read as a natural fit between identity and emotional nature, the kind of pairing traditionally associated with lasting bonds. Two Moons in good angle suggest an easy emotional rhythm, a sense of feeling at home with each other.
Venus and Mars contacts describe attraction. Venus is how a person loves and what they find beautiful; Mars is drive and desire. When one person's Venus meets the other's Mars, the tradition reads chemistry and physical spark. Venus to Venus shows shared tastes and values, while Mars to Mars shows how two people's energy and tempers mesh or clash.
Ascendant contacts matter because the ascendant is the face you meet the world with. A planet from one chart landing on the other's ascendant creates that instant sense of recognition, the feeling of being seen. Sun or Moon on the ascendant is especially strong.
Saturn contacts speak to time. Saturn is commitment, structure, and endurance, so its contacts are read as the glue that keeps a bond together through years, though the same Saturn can also feel like weight or restriction. The outer planets add their own colors: Jupiter contacts feel generous and expansive, Uranus contacts electric and unpredictable, Neptune contacts dreamy and idealizing, Pluto contacts intense and transformative. A strong synastry reading weighs all of these together rather than fixing on any single one.
Two further contacts reward a look. Mercury links show how two people think and talk together, the difference between conversations that flow easily and ones that keep missing each other, which is why they count as much for friendships and working pairs as for romance. The lunar nodes add a sense of direction: when one person's planets touch the other's nodes, the tradition reads a pulled-together quality, a feeling that the meeting carries some weight. Neither of these replaces the Sun, Moon, Venus, and Mars pillars, but both add texture that the headline contacts on their own can miss.
Synastry versus a composite chart
Because the two are easy to confuse, it is worth drawing the line clearly. Synastry keeps both people's charts intact and reads the traffic between them, the aspects and overlays that show how two individuals affect each other. It is a study of interaction, of what sparks when these two particular people meet.
A composite chart does something different. It fuses the two charts into one, most often by taking the midpoints of the planets, to produce a chart of the relationship as a third thing with a life of its own. It answers a different question: not how do these two people act on each other, but what is the character and direction of the bond they form together. Some astrologers also use a related method, the time-space midpoint chart, which casts a real chart for the middle moment and middle location between the two births. Both approaches describe the union rather than the individuals.
In practice the two work as a pair. Synastry shows the day-to-day chemistry and friction; the composite shows the deeper theme of the relationship. This tool builds the synastry first, since that is where the richest detail lives, and offers the composite as an extra layer for those who want it.
When a birth time is missing
It is common to know one person's birth time but not the other's, so it helps to know what you still get. The Sun, Mercury, Venus, Mars, and the slower planets move at a pace where even a whole day of uncertainty barely shifts them, so the aspects between those bodies stay reliable without a time. What a missing time removes is the ascendant, the midheaven, the house cusps, and therefore the house overlays for that person, along with a precise Moon, since the Moon can travel twelve to fifteen degrees in a single day. A synastry calculator handles this by comparing what it safely can and clearly marking the rest as unavailable rather than guessing at it. A timeless comparison is a real reading, then, just a partial one, and it is often enough to see the main planetary chemistry between two people while you track down the exact times.
How to use the tool
Using the synastry calculator takes a minute. Enter the first person's birth date, birth time, and birth place, then do the same for the second person. Press compare, and the tool builds both natal charts, overlays them, and displays the aspect grid, the house overlays, and the composite, each with a short reading of what it means.
A few habits sharpen the result. Enter birth times as accurately as you can, since they drive the ascendant, the houses, and the precise Moon; a birth certificate is the most reliable source. If one time is genuinely unknown, run the comparison anyway and read the planet-to-planet aspects, simply treating the ascendant and house overlays as unavailable. Getting the birth place right matters too, because it sets the houses along with the time. The tool never limits how many pairings you compare, so you can try several and see how they differ.
How to read your results
Start with the headline contacts rather than the full list. Look first at the Sun and Moon aspects between the two charts, then the Venus and Mars contacts, then anything touching either ascendant. Those few pillars carry most of the story, and reading them first keeps you from getting lost in a long grid.
Then notice the balance of easy and tense aspects. A connection made only of trines and sextiles can feel comfortable but flat, while one with some squares and oppositions often has more heat and growth in it. Tension is not a verdict against a pairing. Many of the strongest bonds carry plenty of it. Finally, bring in the house overlays to see where each contact lands in daily life, and glance at the composite for the overall flavor of the bond.
Hold all of it lightly. The grid is a set of prompts, not a scorecard, and no chart can decide whether two people belong together. If a reading rings true, sit with why. If it does not fit, that mismatch is worth noticing too. Synastry is at its best as a starting point for an honest conversation between two people about how they actually relate.
Why comparing two charts is so shareable
A single chart is a solo reading, but the moment two people compare charts, something social happens. Each person recognizes their own planets in the grid, sees how they touch the other's, and has an immediate reason to talk about it. That is why synastry travels so well between friends, couples, and family members. It gives two people a shared object to look at and react to together.
The comparison also invites repetition. Once you have run your own pairing, it is natural to try another: a past relationship, a sibling, a close friend, a new crush. Each pairing produces a fresh grid and a fresh conversation, and every one of them is a reason to send the tool to someone else so you can compare. A synastry calculator is, by its nature, a two-person tool, and two-person tools spread in a way that solo ones never do. That is the quiet reason relationship astrology has always been more viral than the single-chart kind.
Use cases
People reach for a synastry calculator for many reasons, most of them light and social. Couples run their charts to reflect on their dynamic and to talk through the easy and hard contacts with curiosity rather than blame. New daters check a pairing early on as a playful way to think about fit. Friends compare charts to understand why they click, and family members look at parent-and-child or sibling overlays to see long-standing patterns in a new light.
Others use it around moments. The start of a relationship, an anniversary, or a rough patch can prompt a look at the chart as a way to reflect together. Some people enjoy running past relationships to see, in hindsight, what the grid seemed to show. None of these uses treats the astrology as literal instruction. They treat it as a structured, playful mirror for a connection, which is what it does best.
Private by design
The first thing that sets this synastry calculator apart is that it is genuinely private. The whole comparison runs inside your browser on your own device. Two people's birth dates, times, and places are not sent to a server, not saved anywhere, and not attached to any profile, and you are never asked for an email to see the result. That matters more here than almost anywhere, because a synastry reading involves not one person's private data but two. On many astrology sites those details become records: collected, stored, and sometimes sold or used to target ads later. Here there is nothing to collect, because the chart math happens on your machine and the comparison is shown only to you.
The full read, free, with no wall
The second difference is that the full compatibility read is free, with nothing held back. When you compare two charts, the entire aspect grid, the house overlays, and the composite appear at once, each with its meaning explained. There is no teaser that shows a single flattering line and then asks for twenty or forty dollars to reveal the rest, and no email gate standing between you and the reading. That pattern, a free chart that becomes a paid report at the results screen, is common enough in astrology that people expect it. This synastry calculator does not do it. What the two births produce, you get to read in full, for free, every time.
Instant, unlimited, and on any device
Because the chart math runs on your device, the comparison appears the instant you press compare, with no waiting on a server and no queue. You can run as many pairings as you like, with no account and no cap, which is ideal for comparing a partner, a friend, and a crush in a single sitting. It works the same on a phone, a tablet, or a computer, and once the page has loaded it keeps working even if your connection drops, since nothing needs to be sent anywhere. The page stays light and quick because it is not carrying the heavy tracking scripts that clog so many free astrology sites.
Works on any device, even offline
There is no app to install and no account to create. Open the page in any modern browser and the synastry calculator is ready. The tool loads once and then holds its own logic and ephemeris, so if you lose signal partway through a session you can keep comparing charts without interruption. A phone on a train, a tablet on the sofa, a laptop with patchy café internet: all of them run the tool the same way, because the work happens in front of you rather than somewhere far off.
A note on the engine behind it
The accuracy of any comparison rests on the astronomy underneath, so it is worth saying where the positions come from. This tool uses a precise ephemeris and the tropical zodiac of Western astrology to place every planet, the same approach and the same engine as the single natal chart tool. Building a synastry chart is really running that natal engine twice, once for each person, and then measuring the geometry between the two results. That shared foundation is why a chart you build here for one person matches the one used inside the comparison, and why the numbers are consistent from tool to tool.
Related tools on ToolFiddle
If you enjoy this synastry calculator, several other ToolFiddle tools explore the same territory from different angles. The Natal Chart Calculator builds the single birth chart that this tool compares two of, and it is the natural first stop before a comparison. The Numerology Calculator reads character and compatibility through numbers drawn from a birth date and name. For a Vedic view, the Kundli tool builds an Indian birth chart and is often used for matching, and the Chinese Zodiac tool finds your animal sign and element for another take on compatibility. Every one of them runs in your browser and keeps your details on your device, exactly like this one.
A short recap
A synastry calculator compares two people's birth charts to show how they connect. It builds a full natal chart for each person from a birth date, time, and place, then reads three layers: the inter-aspect grid of angles between one person's planets and the other's, the house overlays that show which area of life each contact lands in, and an optional composite chart for the relationship itself. The contacts to watch are the Sun and Moon links, the Venus and Mars aspects, and anything touching an ascendant, with easy and tense aspects both playing a part. Take it here and both people's birth details stay on your device, the full read is free with no email and no paywall, and you can compare as many pairings as you like. It is a symbolic tradition offered for reflection, fun, and connection, not a science, and it is best enjoyed that way.