Synastry Chart Calculator: Instant Compatibility Score

Two birth dates. Two cities. One compatibility score — plus a full wheel chart, every planet placement, and a list of aspects with interpretations telling you exactly what that Venus-Mars trine actually means. That’s what this synastry chart calculator does in about two seconds, without sending a single byte to anyone’s server.

Enter Your Birth Data and Read the Results

The form is split into two cards — Partner A and Partner B. Each one takes a name (optional, but it makes the results more fun to read), a birth date, a birth time with AM/PM toggle, and a city. Start typing a city name and an autocomplete will suggest matches from a built-in database of major world cities. If your hometown isn’t listed, click “Enter coordinates manually” and punch in latitude, longitude, and UTC offset directly.

Birth time matters for the Ascendant calculation. If you don’t know it, check the “Unknown time” box and the calculator defaults to noon. You’ll still get all ten planet positions — Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto — just without the rising sign. Hit Calculate, and the results panel slides in with an animated compatibility gauge, category breakdowns, and three tabs worth of astrological detail.

Check Your Compatibility Breakdown Before the Aspects

The overall score is a weighted percentage based on how many harmonious versus challenging aspects exist between the two charts. But the number alone doesn’t say much — the category bars underneath are where things get interesting. Four dimensions are scored independently: Emotional (driven by Moon, Venus, and Neptune contacts), Intellectual (Mercury, Jupiter, Uranus), Physical (Mars, Venus, Sun), and Spiritual (Neptune, Pluto, Jupiter, Moon).

A couple scoring 78% overall but 42% on the Physical bar tells a very different story than a couple with 78% across all four. The scoring weighs aspects involving personal planets (Sun, Moon, Venus, Mars) more heavily than outer-planet contacts, and tighter orbs count for more than wide ones. A Sun conjunct Moon at 1° carries significantly more weight than the same aspect at 7°.

Explore the Chart, Placements, and Aspect Cards

Three tabs organize the results. The Chart tab renders an SVG synastry wheel — Partner A’s planets sit in the outer ring, Partner B’s in the inner ring, and aspect lines cross the center in blue (harmonious) or red (challenging). You can download this as a high-resolution PNG or print it directly from the page.

The Placements tab shows every planet position for both partners side by side, with zodiac glyphs color-coded by element — fire signs in red, earth in green, air in blue, water in teal. Each entry lists the sign, degree, and arcminute.

The Aspects tab is the deep cut. Every inter-chart aspect gets its own card showing the two planets involved, the aspect type (Conjunction, Sextile, Square, Trine, or Opposition), the exact orb, and a written interpretation explaining what that particular connection means for the relationship. A Sun-Moon conjunction gets a different read than a Mercury-Mercury square, and the text reflects that. Filter the list by planet or by type if you want to focus on, say, only Venus aspects or only challenging contacts.

Use the Filters to Find What Actually Matters

Not every aspect carries the same weight in relationship astrology. The filter dropdowns above the aspect cards let you isolate specific planets — handy when you want to know what your Moon is doing in someone else’s chart without scrolling past fifteen Jupiter-to-outer-planet contacts. You can also toggle between harmonious and challenging aspects to get a quick read on where the friction lives versus where things flow. The aspect count badges at the top of the results — showing totals for harmonious, challenging, and mixed — give you the ratio at a glance.

All your birth data stays on your machine. The planetary positions are computed using simplified astronomical algorithms running entirely in the page — no API calls, no external servers, no data collection. The last five calculations are saved locally so you can revisit them, and a “Copy Summary” button puts the full compatibility breakdown on your clipboard as plain text.