Synastry: Birth Chart Compatibility Between Two People

An online tool for comparing two birth charts: a bi-wheel, a grid of cross-aspects, each person’s planets in the other’s houses, and a reading across six themes. Instead of an invented compatibility percentage it shows how much of the reading actually rests on the data you entered.

How to compare charts

1
Enter both people's details

Date, time and place of birth for each. If a time is unknown, say so rather than entering noon — the tool will handle the uncertainty honestly.

2
Set how precisely each time is known

Chosen separately for each person. It decides which conclusions come out marked reliable and which come out marked shaky.

3
The themes, the aspects and the house overlay

First the share of supported conclusions, then the themes, then the aspect list with its markers, then the house overlays.

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Compare two charts and see which conclusions survive an imprecise birth time

First person
Second person
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Features

A bi-wheel and an aspect grid, not just prose
The first person’s chart inside, the second’s planets outside, aspect lines between them — red for the hard ones, blue for the easy. Beside it a planet-against-planet grid, where an empty column shows at a glance a planet that is not engaged in this pairing at all; a list of sixty rows cannot say that.
A data-support share instead of a compatibility percentage
The same couple scores 62% on one site and 87% on another, and none of them publishes a formula. We do not print such a number: we compute what is genuinely computable from your input.
It accounts for birth-time uncertainty — in both charts at once
The Moon covers about 13° a day: with an unknown time it could be anywhere in a band wider than any orb we use. In synastry that uncertainty compounds across two charts, and every aspect is marked with whether it survives both.
Without a birth time, houses are not invented
With an unknown time the Ascendant and houses are not "less accurate" — they are undefined. Such a chart is built without them, and everything depending on houses simply stays out of the comparison.
Aspects are directional, not shared
"Her Venus to his Mars" and "his Venus to her Mars" are different statements about different people. Both are listed separately and each names its owner.
A conjunction is not counted as a plus
Whether it helps or grinds depends entirely on which planets. Scoring conjunctions as automatically positive is how calculators manufacture flattering numbers; here they weigh zero.
Two numbers per theme instead of one
How easy it is, and how much is going on there at all. "Almost nothing happens between you here" and "a lot happens and it is hard work" are opposite situations that a single figure renders identical.

What you can do

  • See the bi-wheel: your chart inside, your partner’s planets outside
  • Read the planet-against-planet aspect grid at a glance
  • Compare two birth charts and get the list of cross-aspects
  • See which conclusions survive both people's birth-time uncertainty
  • Break the comparison down by theme: attraction, warmth, talk, action, commitment, change
  • See which of your partner's houses your planets fall into
  • Download the analysis as a PDF or share a link

Why we do not show a compatibility percentage

Because it cannot be computed honestly. Producing one number means assigning weights to aspects, planets and themes — and no agreed set of such weights exists in astrology, so every site invents its own and publishes none. That is why the same couple gets different percentages on different sites, and why no site stands behind its figure. We show what is genuinely computable: how many aspects there are, how exact they are, and what share of them rests on your data rather than on a substituted noon.

Frequently asked questions

What is synastry?

A comparison of two birth charts: the angles one person's planets make to the other's, and which of the partner's houses they fall into. It is a traditional astrological technique, not a measurement of anything physical.

Why is there no compatibility percentage?

Because it cannot be derived from anything checkable. Any such number comes from multiplying invented weights, and calculators do not publish theirs. We show the structure instead: where the pull is, where the friction is, how exact the aspects are, and how much of it rests on real data.

What if a birth time is unknown?

Say so. The chart is then built without houses or an Ascendant, and Moon aspects are marked unsupported: the Moon covers about 13° a day, so with an unknown time its position is uncertain across a band wider than any orb. Substituting noon and staying quiet would be easier, and the result would be fiction.

What does the percentage beside the names mean?

The share of aspects that survive the time precision you stated. It is not "how well you suit each other" but "how much this reading can be leaned on at all". It used to sit large and green at the head of the report — exactly where other calculators put a compatibility score — and was read as one. It is now a small chip beside the names: it describes the quality of the input, not the couple. If the share is low, find an accurate birth time before reading on.

Why is a conjunction not counted as a good aspect?

Because it only means "these two planets sit together". Venus with Jupiter and Mars with Saturn are conjunctions of identical geometry and entirely different meaning. An automatic bonus for conjunctions is the simplest way to inflate a final score, and we do not use it.

Do studies find any link between charts and relationships?

No. Astrology is not a scientific method, and controlled studies do not find correspondences between charts and character or relationships. The tool computes the geometry honestly and reports the uncertainty honestly, but the interpretation remains a tradition rather than knowledge about the world.

What travels inside the share link?

The calculation runs in your browser. The share link carries only the birth details you entered — the interpretation is not in it and is recomputed on the device of whoever opens it.

💡 Want us to improve this tool just for you?

We can! Just send us a quick message with your idea. If you'd like to discuss it in detail, leave your email and we'll get back to you. You can stay anonymous.

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