Astrologer

Free Synastry Calculator

Enter two sets of birth details and get a real comparison: every major and minor cross-aspect computed between both charts with the Swiss Ephemeris, written up in plain English and delivered to your inbox. No account and no payment — just an email address to send it to. And no compatibility percentage: a single score collapses dozens of distinct contacts into one arbitrary number and deletes the part you actually wanted.

Compare two charts

What you need

Date, time, and place of birth for both people. Birth times matter more in synastry than in a single chart, because house overlays are usually the most revealing part of a comparison and houses depend entirely on the time. With only one time you can still read planet-to-planet aspects, which is worthwhile, but the overlay layer is unavailable.

What the free report returns

A written comparison built from the full cross-aspect set between both charts — where the two of you are strongest, the core challenges the contacts point to, and answers to your own questions about the relationship — in plain English rather than aspect notation. It is computed from both complete charts with the Swiss Ephemeris, not from the two sun signs, and it arrives by email. The interactive side of synastry — the biwheel, the aspect grid with orbs, house overlays in both directions, and a midpoint composite cast as its own chart — is part of the subscription workspace.

Which contacts to read first

Sun and Moon contacts between the charts come first — they describe whether two people fundamentally recognise each other, and Moon-to-Moon in particular governs whether their emotional needs are legible to one another. Then Venus and Mars contacts, which are the most consistent indicator of physical chemistry in the whole comparison. Then house overlays. Then Saturn, which brings duration and also restriction. Those four layers carry most of the meaning; the remaining hundred contacts are detail.

Why difficult contacts are not disqualifying

Comparisons dominated by trines are comfortable, and comfortable relationships are frequently the ones that quietly end, because nothing generates enough friction to require either person to change. Hard aspects between personal planets produce recurring conflict, and recurring conflict that both people engage with is what tends to hold relationships together over decades. A lot of long marriages have squares and Saturn contacts in them.

What synastry does not tell you

It does not tell you whether a relationship will work. It describes dynamics, not decisions, and two people determine the outcome. It also does not date anything: natal comparison is static, and timing in relationships comes from transits and progressions to both charts and to the composite.

Frequently asked questions

Is the synastry calculator free?

Yes. Comparing two charts at astrologerapp.org/free-synastry requires no account and no payment. The report is delivered by email, so an email address is needed. The interactive biwheel, aspect grid, house overlays, and saved comparisons are part of the subscription.

Do I need both birth times?

For house overlays, yes, since houses depend entirely on birth time. Without both times you can still read planet-to-planet aspects between the charts, but the overlay layer cannot be calculated.

Does it give a compatibility percentage?

No, deliberately. A single score collapses dozens of separate contacts using an arbitrary weighting, and different tools give the same couple different scores. The specific contacts are what carry the information.

What is the difference between synastry and a composite chart?

Synastry overlays two charts to show how two people affect each other. A composite is built from the midpoints of both charts and is read as the relationship’s own chart. Both are included here.

Which aspects matter most in synastry?

Sun and Moon contacts for fundamental recognition, Venus and Mars contacts for attraction, house overlays for the effect each person has on the other’s life, and Saturn contacts for duration and restriction.

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