Synastry Chart Calculator

What synastry is, and what this calculator does

Sun-sign compatibility asks whether a Leo and a Scorpio get along. Synastry asks a far more specific question: 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, then draws them as one bi-wheel: the first person’s chart on the inner wheel, the second person’s planets around the outside. Below it, you get a list of the cross-aspects, meaning the aspects between one person’s planets and the other’s. It covers the five major aspects, conjunction, sextile, square, trine and opposition, each with its orb, the number of degrees it falls short of exact. The tightest come first. Where birth times are known, it also shows the house overlays: which of one person’s houses the other person’s planets land in.

Synastry isn’t only for couples. Friends, siblings, a parent and child, co-founders, an editor and a writer: any two people can be compared this way. What you pay attention to changes with the relationship. Romantic questions lean on the Moon, Venus and Mars; working relationships lean on Mercury and Saturn.

How to read the cross-aspects

A full synastry list can run to dozens of aspects, and you don’t need to read them all. Start with the five most personal points: the Sun, Moon, Venus, Mars and the Ascendant. Contacts between these are usually the ones both people can feel. Whether one person’s planet sits right on the other’s Ascendant is easy to spot on the bi-wheel. Uranus, Neptune and Pluto move so slowly that people of a similar age often have them in the same signs, so aspects among the outer planets are shared by a whole generation and deserve a lighter touch.

A few pairings astrologers look at first. One person’s Sun to the other’s Moon: does who you’re becoming land well with how they feel? Moon to Moon: do your emotional rhythms match, and do you comfort each other in ways that actually help? Venus to Mars: attraction, and the way each of you pursues and responds. Mercury to Mercury: whether you talk easily or keep talking past each other. Saturn to the other person’s personal planets: commitment and responsibility, and sometimes a feeling of weight or pressure.

Orb matters. The smaller the orb, the stronger an aspect usually feels, which is why the list puts the tightest first; the first few lines are often the themes that come up most in daily life together. Watch for repetition, too. If several of the tightest aspects involve one person’s Moon, emotional needs are probably the center of gravity in this relationship. And notice direction: your Venus to their Mars isn’t the same story as your Mars to their Venus. For each line, ask whose planet is touching whose.

Trines and sextiles are the harmonious aspects. Things flow easily there, often so easily that neither person notices. Squares and oppositions are the tense ones: the two of you want different things in that area, which can create friction and also spark. A conjunction depends on the planets involved, and can feel either sweet or crowded. Harmonious doesn’t mean good, and tense doesn’t mean bad. Say one person’s Mars squares the other’s Moon. One of you speaks bluntly and moves fast; the other needs gentleness before anything else can be heard. In an argument, one pushes and the other retreats. Seeing that pattern on the page can make it easier to pause and try a different approach next time.

House overlays: where your planets land in their chart

Besides aspects, synastry looks at house overlays: placing one person’s planets in the other person’s twelve houses to see which area of that person’s life they touch. If your Venus falls in their 5th house, time with you may feel playful and romantic to them, like an ongoing date. In their 4th house, being with you may feel like home.

Some overlays people notice most. Your planet in their 1st house tends to feel personal and immediate to them, as if it acts on who they are. The 4th house is home and belonging. The 5th is romance, fun and creativity. The 7th is partnership and commitment, and often brings a sense that this is someone to build something with. The 10th is career and public life, where you may influence their goals or direction. The 12th is the private, less spoken part of life, and contacts there can feel quietly significant.

House overlays need a birth time, specifically 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; to see where their planets fall in your chart, you need yours. If only one of you knows a birth time, you’ll see overlays in one direction only. Overlays aren’t symmetrical either. Your Sun in their 7th house says nothing about where their Sun falls in yours, so read the two directions separately.

Why there’s no compatibility score here

Many synastry tools boil everything down to a compatibility percentage. This one deliberately doesn’t. A score has to squeeze dozens of aspects pointing in different directions into a single number, and someone has to decide which aspects add points, which subtract them and how much each one weighs. Those rules are invented, they differ from site to site, and the same two charts can score very differently depending on whose formula you use.

More importantly, the same aspect can mean different things in different relationships. Saturn square Venus might feel like pressure in one couple and like the seriousness that keeps another couple committed. Tense aspects aren’t penalties, and harmonious ones aren’t insurance. Turning them into a number throws away the most useful part of the reading, which is the detail.

Synastry also can’t tell you whether a relationship will last, whether you’ll marry or whether you’ll break up. What it can do is put the easy places and the sticking points on the table, so both people can see each other’s needs a little sooner. Where the relationship goes is decided by how the two of you treat each other day to day.

If one of you doesn’t know your birth time

Leave the time blank; the chart still works. The Sun, Mercury, Venus, Mars and the slower planets move only a little in a day, so a birth date alone places them closely enough that their aspects can be read with confidence.

The Moon is the exception. It moves 12 to 15 degrees in a day, so without a birth time its position is only approximate, and any aspect involving it should be taken more lightly. If the Moon changed signs on that birthday, even the Moon sign may be one of two. With no birth time there’s also no Ascendant and no houses for that person, which means you won’t see the other person’s planets placed in their houses, or any contact to their Ascendant.

If you can, ask a relative or check the birth certificate. Even knowing “morning” or “evening” helps narrow down the Moon. And as long as the other person’s time is known, you can still see where the first person’s planets fall in the other person’s houses.

How to ask the AI good questions

Once you’re signed in, you can ask the AI about the relationship. It answers from both birth charts and the cross-aspects between them, so the more specific your question, the more useful the answer.

Instead of “Are we compatible?”, try something like: “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: “What kind of comfort do I need, and how is that different from what my partner naturally offers?” Or: “We’re starting a business together. What should we watch for in how we communicate and split the work?” You can also pick the tightest aspect on the list and ask how it might play out in everyday life.

Add real context: what the relationship is, what you’ve been stuck on lately. The AI can then connect the charts to your actual situation. It won’t predict whether you’ll end up together or when you’ll marry; it talks about tendencies and ways of relating. And remember that someone else’s birth details are personal information, so it’s best to use them with that person’s knowledge and consent.