A Synastry Calculator compares two people's birth charts to analyse romantic and interpersonal compatibility. Synastry is the branch of astrology concerned with how two people's planetary energies interact — which connections create harmony, attraction, and understanding, and which create friction, challenge, or intensity.
In synastry, the planets of one person's chart are overlaid onto the other's. When one person's Venus aligns with another's Mars, it often creates strong romantic attraction. When Saturn from one chart falls on the other's Moon, it may indicate a serious, stabilising but potentially restricting bond. These interplanetary contacts tell the story of the relationship.
Synastry is widely used in relationship counselling, self-awareness, and understanding why some connections feel fated while others require conscious effort. It does not predict relationship success or failure — instead, it illuminates the dynamics at play and the growth opportunities in the connection.
How the Formula Works
Each person's planetary positions are calculated individually using mean orbital elements. The synastry score is derived by measuring the angular distance (aspect) between each pair of planets and applying a weighted scoring system based on aspect type (conjunction, trine, square, opposition, sextile) and planetary importance.
Conjunction (0°) — planets merge energy, very strong, can be positive or challenging
Trine (120°) — harmonious flow, natural ease and compatibility
Sextile (60°) — supportive, cooperative energy requiring some effort
Square (90°) — tension and friction that drives growth and action
Opposition (180°) — polarity and attraction, can complement or conflict
Orb = allowed deviation from exact aspect (usually ±8° for major aspects)
How to Use This Calculator
Enter Person 1's date of birth
Enter Person 2's date of birth
Click Calculate to run the synastry comparison
Review the planet comparison table and four compatibility score bars
Read the written aspect interpretations for the most significant planetary connections
Tips & Best Practices
Focus on the Moon and Venus connections first — they matter most for emotional and romantic compatibility
Challenging aspects (squares, oppositions) are not bad — they often create the most growth and passion
A chart with mostly harmonious aspects but no tension can feel comfortable but lack excitement
Synastry works best read alongside a composite chart, which shows the relationship as its own entity
Important Limitations
Uses birth dates only (no exact time), so Moon signs are approximate — Moon changes sign every 2.5 days
Simplified scoring system — a professional astrologer would weigh aspects more nuancedly
Does not account for house overlays, which require exact birth times and locations
Compatibility scores are indicative only — real relationships depend on communication, values, and effort