Synastry chart calculator

Give two births — date, time and city for each — and you get the two charts read against one another: the bi-wheel, the strongest contacts first, then every contact with its orb. The same form also folds the pair into a composite or a Davison chart, which are two different ways of asking the same question. Free, no account, nothing stored.

What it does, and why it is here

Comparing two charts by hand is not hard, it is just long: two full charts to cast first, then every planet of one measured against every planet and angle of the other, which is a couple of hundred angles to check before anything can be ranked.

And there is a trap in the inputs. Each birth time has to be read in its own place’s time zone, with the daylight saving in force there on that date. Two people born in different countries are the case where a hand calculation goes wrong silently — and a one-hour error moves an ascendant by fifteen degrees, which rewrites the house overlays entirely.

So the tool casts both charts, measures every pair, and ranks the result by how close each contact is rather than by whether it is traditionally called good. A tight square says more about two people than a loose trine, and a list sorted any other way buries the thing worth reading.

What you fill in

• Two births — date and time, and a city for each. The city carries the coordinates and the historical zone, separately for each person.
• Names, optional — so the tables read “Alex’s Venus, Sam’s Mars” instead of A and B.
• Reading — the choice that changes what you get back. Synastry lays one chart over the other and lists the contacts between them. Composite folds the two into a single chart of midpoints, the relationship as its own entity rather than two people meeting. Davison casts a real chart for the instant and the place exactly halfway between the two births — unlike a composite, an actual moment in an actual location, which is why some readers prefer it.
• House system — Placidus, Koch, equal or whole sign. It decides the house overlays, which is much of what synastry is read for.

What you get back

• The bi-wheel — one chart drawn inside the other, so the overlays are visible rather than tabulated.
• The strongest contacts — the short list, tightest first. This is the part to read if you read nothing else.
• Every contact, with both bodies, the aspect and the orb, so you can judge for yourself where the list stops being meaningful.
• Overall — the balance across the whole comparison, rather than a verdict.
• The composite or Davison chart, when you chose one — with its own placements, its own house cusps and its own internal aspects, read like any other chart.

What it does not do

There is no compatibility score. That is deliberate, and it is the main thing separating this from most free synastry sites. A percentage would have to weigh a Venus trine against a Saturn square on a scale nobody can defend, and the number that comes out is invented — while feeling authoritative, which is worse than useless when the subject is somebody’s relationship.

It does not interpret, either. No paragraph explaining what your Moon on their Mars means; the contacts are listed and ranked, and reading them is a person’s work.

And it keeps nothing: two sets of birth details, used to compute, then forgotten. Nothing stored, nothing attached to an account, nothing written into a link — which matters more here than usual, since half the data is somebody else’s.

The three readings, side by side

They are not rivals and they do not have to agree. Synastry is the one to start with; the other two are what you reach for when the question is about the relationship itself rather than about two people.

Questions

Is it free? Do I need an account?

Free, no account, no limit.

Do you store the birth data?

No — neither person’s. Used to compute, then forgotten, and never put in a shareable address.

One of us does not know the exact birth time.

The planet-to-planet contacts are still sound, apart from the Moon if the time is very uncertain. What is lost is that person’s ascendant, Midheaven and houses — so the house overlays, which are a large part of a synastry reading, become guesses.

Why is there no compatibility percentage?

Because it would be made up. Weighting one aspect against another on a single scale is a judgement no method agrees on, and putting a number on it hides that.

Can I use it for friends, family or colleagues?

Yes. Synastry compares two charts; nothing in the technique is specific to romance, and parent-child or working pairs are read the same way.

What is the difference between a composite and a Davison chart?

A composite is arithmetic: the midpoint of each pair of positions, which does not correspond to any real sky. A Davison is a genuine chart cast for the midpoint moment and the midpoint location — a sky that actually happened.

Which chart goes on the inside of the wheel?

The first person’s. Swapping the two changes the drawing and the house overlays, but not the aspects themselves.

Our results differ from another site.

Usually the house system, or a time zone applied to one of the two births. Both are stated here so they can be compared directly.

Sources and further reading

• Synastry — the technique and its vocabulary.
• Aspects and orbs — why a tight square outranks a loose trine.
• House systems — what the overlays depend on.