Synastry Chart Calculator
Synastry is the astrology of two people. It lays one birth chart over another and looks at the angles between your planets and theirs, and at which of their houses your planets fall into. It describes where two people click without trying and where they rub. Enter both people’s birth dates, times and cities, leaving a time blank if you don’t know it. The calculator draws a bi-wheel, lists every aspect between the two charts with the tightest first, and gives the pair a synastry score out of 30.
What synastry is, and what this calculator does
Sun-sign compatibility asks whether a Leo and a Scorpio get along. Synastry asks something far more specific: how does this person’s whole chart meet that person’s whole chart? It compares every planet, from the Sun and Moon out to Pluto, so two Leos can have very different synastry with the same partner, because their Moons, Venus signs and Ascendants differ.
The calculator computes both birth charts from the same astronomical ephemeris and draws them as one bi-wheel, the first person’s chart inside and the second person’s planets around the outside. Below it is the list of cross-aspects, meaning the aspects between one person’s planets and the other’s, each with its orb: how many degrees it falls short of exact. The tightest come first. Where birth times are known, you also get house overlays. Finally there’s a synastry score out of 30 with a letter grade, which has its own section further down.
The four core contacts
A synastry list can run to dozens of aspects, and you don’t need to read them all. Most astrologers start with four kinds of contact, because these are the ones two people tend to feel in ordinary daily life.
Moon to Moon. The Moon is your emotional rhythm and what makes you feel safe. When the two Moons are in a harmonious aspect, you often know what the other person needs without asking. When they square or oppose each other, the comfort one of you offers may be exactly what the other doesn’t want: one reaches for a hug, the other thinks giving space is the kind thing. Over the years, this pairing says a lot about whether being together feels restful or tiring.
Sun to Moon. One person’s Sun is who they’re becoming; the other person’s Moon is who they are when they relax. A strong link between them is one of the classic signatures of a lasting bond, where the Sun person feels supported and the Moon person feels seen. A square is common too, and means one person’s goals and the other’s emotional needs will sometimes collide.
Venus to Mars. Venus is how you love and how you want to be treated; Mars is how you pursue and act. Contacts between one person’s Venus and the other’s Mars are the most direct source of attraction and chemistry. Conjunctions and trines bring an easy spark. Squares and oppositions bring a hotter one, along with more arguments about pace, where one of you wants to slow down and the other is already impatient. Direction matters here: your Venus to their Mars and your Mars to their Venus tell different stories.
The Ascendant to personal planets. When someone’s Sun, Moon, Venus or Mars sits near your Ascendant, their presence feels immediate, as if they switch something on in you just by walking into the room. This contact needs a birth time, since there’s no Ascendant without one, which is one more reason to track down exact times if you can.
Harmonious, challenging and minor aspects
Trines and sextiles are the harmonious aspects. In those areas the two of you fit together without much effort, often so smoothly that neither of you notices. The flip side is that ease can be taken for granted, and it rarely pushes anyone to change.
Squares and oppositions are the challenging aspects. The two of you want different things in that area, which creates friction and also most of the spark. Say one person’s Mars squares the other’s Moon: one speaks bluntly and moves fast, the other needs gentleness first, and in an argument one pushes while the other retreats. Seeing the pattern laid out makes it easier to pause next time and try another approach. Conjunctions depend on the planets involved; two energies stacked in one place can feel sweet or crowded.
Minor aspects include the semi-sextile (30 degrees), the semi-square (45), the sesquiquadrate (135) and the quincunx (150). Their orbs are small and their weight is light, so they’re best read after the major aspects, to explain subtler things the big ones don’t cover, like a vague awkwardness or a quiet, unspoken understanding.
Two rules of thumb help. Tighter orbs usually feel stronger, so the first few lines on the list are often the themes that come up most. And repetition matters: if several tight aspects involve the same planet, say one person’s Moon, that planet is probably the center of gravity in the relationship.
Going further: Saturn, the Nodes, Pluto and the 7th house
Saturn contacts to the other person’s personal planets are about commitment, responsibility and staying power. The Saturn person brings structure and weight; the other may feel steadied, or watched and judged. Many long relationships have strong Saturn ties. They make things serious, and the price is that they can sometimes feel heavy.
The lunar Nodes are the two points where the Moon’s orbit crosses the ecliptic. Someone’s planet on your South Node can feel strangely familiar, as if you’ve met before. On your North Node, they may pull you toward something unfamiliar that’s worth growing into. Familiarity is seductive, but it isn’t proof that a relationship was written in advance; how you get along day to day still decides it.
Pluto contacts to personal planets run at high intensity: fascination, the sense of being seen right through, the urge to hold on tight, sometimes deep change. These are hard relationships to forget. Keep an eye on the balance of power so that caring doesn’t turn into control.
The 7th house is the house of partnership. When someone’s planets fall in your 7th, they often feel like a person you could stand beside and commit to, and the feeling is stronger if a planet sits right on your Descendant.
House overlays: where your planets land in their chart
House overlays place one person’s planets in the other’s twelve houses to see which area of life they touch. Your Venus in their 5th house can make time together feel playful and romantic to them. In their 4th, being with you may feel like home. In their 10th, you may influence their goals and direction.
Overlays need the birth time of the person whose houses you’re using. To see which of their houses your planets fall in, you need their time, and the reverse needs yours. Overlays aren’t symmetrical either. Your Sun in their 7th says nothing about where their Sun falls in your chart, so read the two directions separately.
Synastry, composite and Davison charts
Relationship astrology has three main techniques. Synastry, what this page does, keeps both birth charts intact and reads the aspects and overlays between them. It describes how each person affects the other.
A composite chart takes the midpoint of each matching pair of planets, your Sun and their Sun, your Moon and their Moon and so on, and builds a new chart from those midpoints. It represents the relationship itself, almost like a third character with its own temperament. A Davison chart also describes the relationship, but it’s cast for a real moment and place: the midpoint between the two birth times and the midpoint between the two birthplaces.
They answer different questions. Synastry asks how two people affect each other; composite and Davison charts ask what the relationship is like as a unit. This calculator does synastry only, and both the score and the AI analysis are built on the cross-aspects between the two birth charts.
How to read the synastry score
The calculator gives each pair a score out of 30 and one of six grades: D, C, B, B+, A or A+. The method follows the approach of the Italian astrologer Ciro Discepolo, scoring the cross-aspects between key planets in the two charts: which planets touch, by what aspect and how closely.
Read the score as a measure of how easily two charts work together. A high score means many smooth contacts between the important planets, so plenty fits without much effort. A low score means more places where the two of you will need to adjust to each other and talk things through. It doesn’t say whether you can be together, and it isn’t a verdict on whether the relationship is worth having.
A low score doesn’t mean you’re wrong for each other. Relationships with many challenging aspects have more friction and often more charge, and plenty of long-term couples learned to understand each other precisely by working through it. A high score still takes effort; two people who get along easily can still drift apart over real-life problems. So after the score, spend your time on the aspect list underneath: which lines lift the number, and which pull it down.
Signed in, you can ask the AI for a closer analysis. It rates four areas, attraction, harmony, shared hopes and passion, separately from each person’s point of view. The two ratings for the same area can differ: one of you may feel a strong pull while the other mostly feels at ease, and that asymmetry is worth talking about in itself. The AI also writes up the relationship’s key themes, the issues you’ll most need to face together and the aspects they come from. These ratings are a reading of the charts, not a measurement of how either of you actually feels.
Keeping synastry in perspective
Synastry describes tendencies, not outcomes. It puts the easy places and the sticking points on the table so that two people can see each other’s needs a little sooner. It can’t tell you whether a relationship will start, how long it will last, or whether you’ll marry or split. Those depend on what the two of you do every day.
Don’t forget each person’s own chart, either. If one person finds closeness frightening, that theme will come up with almost anyone, and blaming it all on “our bad aspects” misses the part you can change yourself. And remember that someone else’s birth details are personal information, best used with their knowledge and consent.
How to ask the AI good questions
You can also ask the AI anything about the two charts: the first question needs no sign-in, follow-ups do. It answers from both birth charts and the cross-aspects between them, so specific questions get more useful answers.
Instead of “Are we compatible?”, try: “When we argue, I want to talk it through right away and they need to cool off first. Where does that show up in our charts?” Or pick the tightest aspect on the list, or the one pulling your score down, and ask how it might play out in everyday life.
FAQ
How is synastry different from Sun-sign compatibility?
Sun-sign compatibility compares only two Sun signs, which is roughly the same as comparing birth months, so millions of pairs get the same answer. Synastry uses both full charts: the Moon, Venus, Mars, the Ascendant, and the exact angles and houses between them. Two people who share a Sun sign can have very different synastry with the same partner.
Do we need exact birth times for synastry?
It helps, but it isn’t required. The Sun, Mercury, Venus, Mars and the slower planets move little in a day, so a birth date alone places them closely enough to read their aspects. The Moon is the exception: it travels 12 to 15 degrees a day, so aspects involving it should be taken more lightly. Without a time there’s also no Ascendant and no houses for that person. If one of you knows your time, overlays in that direction still work.
We have lots of squares and oppositions. Are we a bad match?
No. Challenging aspects show where two people want different things, which can cause friction and can also push both of you to grow. Many long relationships have plenty of them. The useful step is to see where the friction comes from, say different emotional rhythms or different ways of making decisions, and then work out a way of handling it together.
What kinds of relationships can synastry be used for?
Any two people: partners, friends, parents and children, siblings, colleagues, business partners. The focus changes with the relationship. Romance leans on the Moon, Venus, Mars and the 7th house; family on the Moon and Saturn; working relationships on Mercury and Saturn. Attraction and passion in the AI analysis are mostly romantic ideas, so for other relationships, take those two lightly.
Our synastry looks great. Why didn’t it work out?
Synastry describes how easily two people get along. It doesn’t cover timing, circumstances, plans for the future, or whether both people were ready for a relationship. Distance, family or different values can all get in the way of an easy match, and very smooth charts sometimes lack the tension that pulls two people closer. Good synastry means plenty of natural rapport; whether you end up together is still a matter of choices.
Is there such a thing as a soulmate aspect?
The aspects people tend to call that include one person’s Sun conjunct the other’s Moon, Venus conjunct Mars, and planets on each other’s Nodes. They often bring a strong sense of familiarity or pull, but no aspect proves that someone is the one. Relationships with intense familiarity still have to work through everyday life, and relationships without these aspects can be just as deep. Rather than hunting for a soulmate aspect, look at how your tightest aspects play out in real life.
How should I read the synastry score?
The score out of 30, graded from D to A+, sums up how smoothly the key planets in two charts connect. It’s useful as a quick sense of a pair’s overall texture, not as a basis for decisions: a low score doesn’t mean you’re unsuited, and a high score still needs work. Look at which aspects are behind the number. Understanding those is worth more than remembering a grade.
How can I learn synastry on my own?
Start with your own birth chart: what planets, signs, houses and aspects mean, especially the Sun, Moon, Venus, Mars and the Ascendant. Then practice on relationships you know well, such as your parents, or you and a close friend. Begin with the four core contacts and check them against how those people actually get along; that teaches more than memorizing rules. Add Saturn, the Nodes and house overlays later. You can also pick one aspect, ask the AI how it tends to show up day to day, and compare the answer with your own experience.