Two charts, compared
Synastry measures the angles between one person's planets and another's — where you meet easily, where you grate, and what each brings that the other doesn't. The grid below is the whole of it, free, with nothing kept afterwards.
First chart
Second chart
Neither set of details is stored, and nothing is sent anywhere but the arithmetic. Both charts are computed and then forgotten — the second person didn't ask to be in a database.
Both charts first. A birth time makes the Moon and the angles exact, but the comparison works without one.
How to read the grid
Each cell is an aspect between a planet of theirs and a planet of yours. Conjunctions fuse two functions together; trines and sextiles make things easy; squares and oppositions are the friction — which is not the same as the trouble, and is usually what keeps two people interested.
Weight matters more than count. Two exact aspects between Suns, Moons or Venus say far more than a dozen wide ones between outer planets, which everyone born in the same few years shares with each other anyway.
There is no score here on purpose. Compatibility percentages are invented — they read well and they mean nothing, and a grid you can actually read is more use than a number somebody made up.
Questions
What is a synastry chart?
A comparison of two birth charts. Rather than reading one chart on its own, synastry measures the angles between one person's planets and the other's — so it describes what happens between two people rather than what either is like alone.
Do I need birth times for synastry?
No, but they help. Without a time the Moon can be out by several degrees and neither chart has houses or a rising sign, so aspects involving the Moon or the angles become approximate. Every other planet-to-planet aspect is reliable either way.
Is the synastry calculator free?
Yes. The comparison is arithmetic and costs nothing to run, so it is free and needs no account. Only a written interpretation costs anything, and it is entirely optional.