Birth Chart Compatibility (Synastry)
Synastry is the astrology of relationships — it lays two birth charts over each other to see where you harmonize and where you spark. Enter both birth dates (times sharpen the Moon and attraction reading) to compare your charts.
You
Them
Both charts are calculated in your browser — no birth details are ever sent anywhere.
What the calculator reads
Six connections between the two charts, in both directions — because who reaches for whom is part of the answer.
- Sun – Sun · Core selves
- Moon – Moon · Emotional worlds
- Sun – Moon · Their heart meets your identity
- Moon – Sun · Your heart meets their identity
- Venus – Mars · Attraction (your Venus–their Mars)
- Mars – Venus · Attraction (your Mars–their Venus)
How each aspect is scored
An aspect is the angle between two planets. Each one below counts when the two fall within its orb — the margin of error astrology allows around an exact angle.
| Aspect | Angle | Orb | What it reads as |
|---|---|---|---|
| conjunction | 0° | ±8° | fused and intense |
| sextile | 60° | ±5° | easy and encouraging |
| square | 90° | ±6° | charged with friction to work through |
| trine | 120° | ±8° | flowing and natural |
| opposition | 180° | ±7° | magnetic, a pull of opposites |
A square subtracts and an opposition still adds: friction is not the same as incompatibility, and the pull of opposites is real attraction. Only the square is scored as a cost, and even then a single one barely moves the total.
Element compatibility
Before any aspect is counted, the two Suns and the two Moons are compared by element. This is the backbone of long-term fit, so it carries the most weight in the score.
Same element · 3 points
You want the same things at the same speed — recognition, with little friction to grow against.
Complementary elements (Fire–Air, Earth–Water) · 2 points
One feeds what the other needs. The easiest long-term fit in the model.
Everything else · 0 points
Different tempos. Not doomed — this is the pairing that asks for translation rather than assuming.
How to read your score
Compatibility here weighs the element harmony of your Suns and Moons and the aspects between your core planets — especially the Venus–Mars connection that governs attraction. A high score means an easy natural flow; a lower one doesn't mean doomed, only that the match rewards patience and honest effort. The scale floors at 35 and caps at 99, because no two charts are hopeless and none are a guarantee. No chart decides a relationship — the two of you do.
Questions people ask
What is synastry in astrology?
Synastry lays two birth charts over each other and reads the angles between them. Where your planets fall against someone else's describes how the two of you actually meet — what feels effortless, what keeps catching, and where the attraction lives.
Do I need my exact birth time for a compatibility reading?
No, but it sharpens two things. The Moon moves about half a degree an hour, so without a time the emotional reading and the attraction aspects are approximate. Sun signs and the overall element picture hold either way. Leave the time blank and the calculator assumes midday.
What does the Venus–Mars connection mean?
Venus is what you are drawn to; Mars is how you pursue. Read across two charts, your Venus against their Mars, it is the classic marker of physical attraction — which is why this calculator reads it in both directions rather than once.
Is a low compatibility score bad?
No, and the scale is built to say so: it floors at 35, never zero. A low score means your elements run at different tempos, so the relationship rewards translating rather than assuming. Plenty of lasting couples score in the middle. No chart decides a relationship.
Are my birth details sent anywhere?
No. Both charts are calculated in your browser. Nothing you type is uploaded, stored, or logged.
Reading for one chart rather than two? The birth chart calculator maps every placement on its own, and the horoscope pages cover each sign in depth.
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