Free · private · calculated on your device

Free synastry calculator

Compare two complete birth charts with cross-chart aspects and house overlays. Free and private, with no invented compatibility percentage.

Synastry compares the calculated placements of two people. It can show an angle between one person’s Moon and the other person’s Mercury, for example, without reducing a relationship to a single score. Both charts remain visible as distinct sets of positions.

This calculator needs JavaScript to keep the calculation on your device. Your details are not submitted anywhere. The instructions, method and related guides below remain available without JavaScript.

Person 1

Birth dateRecorded local birth time

IANA time zone

Use the place’s time zone, such as Europe/Warsaw or America/Sao_Paulo. Keep the recorded clock time; do not adjust for summer time yourself.

If the clock time occurred twice

One fixed download; the selected country or city is never sent in a request.

Birthplace latitudeBirthplace longitude

House system

Person 2

Birth dateRecorded local birth time

IANA time zone

Use the place’s time zone, such as Europe/Warsaw or America/Sao_Paulo. Keep the recorded clock time; do not adjust for summer time yourself.

If the clock time occurred twice

One fixed download; the selected country or city is never sent in a request.

Birthplace latitudeBirthplace longitude

House system

Place data: GeoNames geographical database,CC BY 4.0. Snapshot retrieved 27 August 2026. The cities15000 directory does not include every settlement; manual coordinates and time zones remain available.

No name, email or account is needed. Your details stay on your device. Use the clear button to remove entered details and results.

How to use this tool

Enter the recorded birth details for each person, including the correct birthplace time zone. Choose a house system explicitly. Use only details you are entitled to use; no names are required and the inputs stay in this page’s browser session.

How to read the result

The aspect table pairs the first chart with the second, including contacts between the same planet in different charts. The house overlays answer a different question: which houses in one chart contain the other chart’s positions? These are traditional comparison frameworks, not measurements of relationship quality.

Method and limits

This exact-chart comparison requires both birth times. Uncertain inputs affect the Ascendant and house overlays especially strongly. A chart does not determine consent, safety, affection or whether a relationship should continue. No percentage or prediction is generated.

The calculation uses Astronomy Engine 2.1.19 and, where applicable, the project’s offline time-scale and house geometry modules. Mathematical positions and traditional interpretation remain separate.Read the full methodology.