Free Compatibility Calculator

Synastry is the astrological technique of overlaying two natal charts to see how one person's planets aspect the other's. It reveals 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 activate 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 score using NASA's ANISE toolkit for arc-second precision.

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 reveal how well two people's core identities and emotional needs mesh — a conjunction or trine here often feels like instant mutual recognition. Venus-Mars aspects drive romantic chemistry, with the conjunction being the strongest single indicator of physical attraction in the chart.

Moon-Moon and Moon-Venus aspects show emotional compatibility — how safe and understood each person feels day to day. Saturn aspects are less glamorous but essential for longevity: Saturn trine Venus or Moon adds commitment, stability, and staying power. Challenging Saturn contacts (square, opposition) can indicate restriction or criticism, but even these often correlate with lasting bonds.

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 — these connections often feel fated. South Node contacts 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 long-term partnerships.

Beyond standard synastry aspects, many astrologers look for Vertex contacts between two charts. The Vertex is a sensitive angle derived from the intersection of the prime vertical and the ecliptic; unlike the Ascendant or Midheaven, it operates largely below conscious awareness. When a partner's personal planet (Sun, Moon, Venus, Mars, or Mercury) falls conjunct your Vertex, the meeting tends to feel destined rather than chosen. Many people describe Vertex contacts as encounters that arrived unexpectedly and changed the course of their lives.

The Anti-Vertex, directly opposite the Vertex, marks the point where you consciously initiate contact and bring your own will into the relationship. Strong Vertex contacts in synastry are not guarantees of lasting love, but they are among the clearest indicators of a connection that carries karmic or fated weight. To find your Vertex sign and degree, use the free Vertex calculator, then read the Vertex synastry guide for a full breakdown of what each contact means.