⚭ Free Synastry Calculator — Compare Two Birth Charts
Umbriel's free synastry calculator compares two birth charts — every aspect between them with its exact orb, and where each person's planets fall in the other's houses. No account, no email. It draws the two charts as a bi-wheel rather than reducing them to a compatibility percentage.
- What you'll needBoth people's birth dates, times and places
- What you getEvery aspect between two charts with exact orbs, each person's planets in the other's houses, and the overlay drawn as a bi-wheel
- CostFree — no account required
Runs in your browser. Nothing to install, nothing to set up.
What synastry actually is
Synastry is chart comparison: laying one birth chart over another and reading what touches what. It is the oldest and most direct of the relationship techniques, and unlike a composite it keeps both people intact — you can always see who is doing what to whom.
Two things are being looked at. The inter-aspects are the contacts between one person's planets and the other's, and the tight ones do most of the work: a Moon on a Saturn at one degree describes something a ten-degree contact does not. The house overlays are where each person's planets land in the other's houses, which is often the more recognisable half — it describes the area of life each person shows up in for the other.
⚠️ A word about scores. Reducing two charts to a percentage requires inventing weights for every kind of contact, and no two systems weight them the same way, so the number tells you more about the calculator than the couple. Umbriel gives you the contacts and their orbs and leaves the judgement where it belongs. The house overlays also need both birth times — with only one, the aspects still stand but half the picture is missing, and Umbriel will say which half.
What's in your result
- Every inter-aspectBoth directions, each with its exact orb, so the tight ones are distinguishable from the incidental.
- House overlays both waysYour planets in their houses and theirs in yours — the two are different readings.
- The bi-wheelBoth charts drawn as one figure, inner and outer.
- The tightest contacts firstOrdered by orb rather than by planet, since orb is what decides which ones are doing anything.
- Composite and Davison alongsideTwo different ways of making a single chart for the pair, if you want them.
- No compatibility scoreDeliberately. The contacts are the information; a percentage is a guess wearing a number.
Common questions
What is synastry?
Comparing two birth charts to see how they interact — the aspects between them and where each person's planets fall in the other's houses. It is the standard technique for looking at a relationship astrologically.
Do we both need birth times?
For the aspects between your planets, no — those come from the dates. For house overlays, the Ascendant and anything involving the angles, yes, both times are needed. Umbriel shows what it can stand behind and names what is missing.
Which synastry aspects matter most?
Generally the tight ones to the personal planets and the angles: Sun, Moon, Venus, Mars, Ascendant and Midheaven. A contact within a degree or two says considerably more than one at the edge of orb, which is why Umbriel prints every orb.
Why does Umbriel not give a compatibility percentage?
Because producing one means inventing weights for every kind of contact, and no two systems agree on them. The number would look precise and mean very little. The contacts themselves are the useful part.
Is the synastry calculator free?
Yes — every aspect, both house overlays and the bi-wheel, with no account and no email.
Go deeper
Every one of these is a page in its own right, written out one at a time.
Related free tools
Umbriel offers every system here as a set of prompts to think with, never as a statement of fact about who you are. Take what's useful, leave the rest.