Free Compatibility Calculator

This calculator compares two charts by synastry aspects and a composite midpoint chart, then turns those factors into a transparent relationship index. The score is a model output, not a verdict on the relationship.

Synastry can describe ease, friction, attraction, timing, and repeating themes. It cannot prove compatibility, consent, mutuality, emotional safety, long-term success, spiritual status, or whether a relationship should continue.

Synastry is the astrological technique of overlaying two natal chartsto see how one person's planets aspect the other's. It maps where attraction, harmony, tension, and growth show up in a relationship, not as a single score but as a detailed map of how two people affect each other.

Synastry is different from a composite chart, which calculates the midpoint of each pair of corresponding planets to create a single chart representing the relationship itself as a third entity. Synastry preserves both individuals and shows the dynamic between them: how Person A experiences Person B, and vice versa. Our calculator computes all major and minor aspects between both charts and produces an overall compatibility index from chart positions built on the JPL DE440s planetary ephemeris via ANISE.

The foundation of any synastry reading is cross-aspects between the five most personal points: Sun, Moon, Venus, Mars, and the Ascendant. Sun-Moon contacts can describe how two people's core identities and emotional needs meet; a conjunction or trine here often feels like instant mutual recognition. Venus-Mars aspects drive romantic chemistry, with the conjunction being one of the most commonly cited physical-attraction indicators in synastry.

Moon-Moon and Moon-Venus aspects are often read for emotional fit: how comfortable and understood each person feels day to day. Saturn aspects are less glamorous but are often read for structure, accountability, and endurance. Challenging Saturn contacts (square, opposition) can indicate restriction or criticism, but they do not prove a bond will last.

The lunar nodes add a layer of depth. A personal planet conjunct the partner's North Node suggests the planet person catalyzes the other's growth, and these connections often feel unusually familiar or fated-feeling. South Node contacts can bring instant familiarity and comfort but can become stagnant if the couple only leans on what's easy. Moon or Sun conjunct a partner's North Node is one of the most frequently cited aspects in relationship astrology, especially when the rest of the chart supports it.

The compatibility index above summarizes part of the everyday texture of a relationship. Two narrower lenses go deeper. The karmic synastry calculator tracks karmic-style indicators that some astrologers read as pre-existing relational patterns (South Node contacts, sect-aware Saturn, Chiron, Vertex axis, Hellenistic Lot of Eros and Lot of Marriage, plus 8th and 12th house overlays). For Vertex synastry specifically, the Vertex synastry guide walks through every contact type with interpretive depth, and the Vertex calculator returns your single-chart Vertex sign and degree.