Free Synastry Chart Calculator

A synastry chart is the core tool of relationship astrology. It works by overlaying two people’s birth charts and analysing the geometric angles—called aspects—that form between one person’s planets and the other’s. These inter-chart aspects reveal where two people naturally connect, where they challenge each other, and where growth potential lies.

Unlike sun-sign compatibility (which only considers one of your ten planets), synastry examines the full picture: how your Mercury communicates with their Venus, whether your Moon feels at home near their Saturn, and what happens when your Mars meets their Jupiter. The result is a nuanced map of the relationship that goes far deeper than “Are Aries and Libra compatible?”

Our calculator above computes precise planetary positions using VSOP87 astronomical theory, then identifies every inter-chart aspect within standard orb tolerances. The result is a professional-grade synastry analysis—completely free.

Not all synastry aspects carry equal weight. Astrologers pay closest attention to connections between the personal planets—Sun, Moon, Mercury, Venus, and Mars—because these shape the day-to-day experience of a relationship.

Sun-Moon aspects are often considered the foundation of compatibility. When one person’s Sun (core identity) connects harmoniously with the other’s Moon (emotional needs), there is a deep sense of belonging and mutual understanding. Explore the dynamics of individual signs through our zodiac sign guides.

Venus-Mars aspects speak directly to romantic and physical chemistry. A Venus-Mars conjunction or trine often indicates strong mutual attraction, while a square creates passionate but sometimes volatile desire.

Mercury connections determine how well you communicate. Mercury-Mercury trines suggest effortless conversation, while squares can lead to recurring misunderstandings that require conscious effort to resolve.

Saturn aspects add staying power. Though often experienced as challenging, Saturn connections between charts indicate the kind of seriousness and commitment that builds lasting partnerships. Check our compatibility guides for sign-level insights.

Start with the Overview tab, which shows compatibility scores and highlights the five strongest inter-chart aspects. These are the connections that define the relationship’s core themes.

The Charts tab displays both birth chart wheels side by side so you can visually compare planetary placements. Look for planets that fall in the same signs or houses—these areas of overlap create shared focus.

The Synastry Aspects tab lists every inter-chart aspect sorted by tightness of orb (tighter orb = stronger influence). Green “harmonious” badges indicate ease, red “challenging” badges indicate growth areas, and purple “neutral” badges (conjunctions) can go either way depending on the planets involved.

Finally, the Planet Positions tab gives you raw data for both charts—useful if you want to explore specific placements through our planets in signs pages.

Synastry and composite charts answer different questions. Synastry shows how two individuals interact with each other—“What happens when my Venus meets your Mars?” It preserves each person’s individuality and maps the dynamic between them.

A composite chart calculates the midpoints between both charts to create a single new chart that represents the relationship itself as a separate entity. It answers: “What is this relationship about? What is its purpose?”

Most astrologers use both: synastry for understanding how two people experience each other, and the composite for understanding the relationship as a whole. Our synastry guide dives deeper into these concepts.