Free Synastry Calculator — Birth Chart Compatibility & Aspects

A synastry calculator compares two people's birth charts to reveal relationship dynamics. Enter both birth dates, times, and places to see planet positions, inter-chart aspects, house overlays, and a compatibility summary — calculated instantly in your browser, completely free.

Unlike sun-sign match tests, this tool computes real planetary positions with a professional astronomy engine, then builds the full picture: a transparent compatibility score, a bi-wheel chart, house overlays, and written interpretations. No signup, no email, no watermark — your birth data never leaves your device.

Please note: this tool is for entertainment, reflection, and traditional astrology purposes. Astrology is not science, and results are interpretive — not a basis for relationship, financial, or health decisions.

100% FreeNo Signup Real Astronomy EngineBi-Wheel Chart Full Aspect GridHouse Overlays PDF ExportPrivate — In Your Browser

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Person A

Name

Birth date

Birth time (24h)

I don't know the exact time

Without birth time: Ascendant, MC, and houses are excluded, and the Moon may be off by up to ~7°.

Birth location — type to search Select birth city

Offsets use standard time. If the birth occurred during daylight saving time, adjust the offset by +1 (or use manual entry).

Enter location manually

Latitude

Longitude

UTC offset

West longitudes are negative. Use the standard-time offset (+1 if DST was active).

Person B

Name

Birth date

Birth time (24h)

I don't know the exact time

Without birth time: Ascendant, MC, and houses are excluded, and the Moon may be off by up to ~7°.

Birth location — type to search Select birth city

Offsets use standard time. If the birth occurred during daylight saving time, adjust the offset by +1 (or use manual entry).

Enter location manually

Latitude

Longitude

UTC offset

West longitudes are negative. Use the standard-time offset (+1 if DST was active).

Include minor aspects (quincunx 150°, semi-sextile 30°)

✓ Synastry chart calculated — explore the tabs below.

✎ Edit Birth Details

What Is Synastry in Astrology?

Synastry is the art of overlaying two natal (birth) charts to study the dynamics of a relationship. Instead of asking "what is a Leo like?", synastry asks "how does her Mars relate to his Venus?" — comparing the exact planetary positions of two people to find conjunctions, trines, squares, and other inter-chart aspects, plus house overlays showing where each person's planets activate the other's life areas. It's one of the oldest techniques in traditional astrology and remains the standard method astrologers use for relationship readings.

How to Use This Synastry Calculator

1. Enter Person A's birth details — date, time as precisely as you know it, and birthplace. The city search covers 100+ world cities; use manual entry anywhere else.
2. Do the same for Person B. Don't know an exact time? Tick "I don't know the exact time" — the tool works honestly with planets only.
3. Press "Calculate Synastry Chart." The first run downloads a small astronomy library (~100 KB); after that, everything computes on your device.
4. Read the tabs: Summary & Score first, then Aspect Grid, Bi-Wheel Chart, House Overlays (both times needed), and Interpretations.
5. Download the PDF to save or share the full report.

How the Calculation Works

This tool uses the open-source astronomy-engine ephemeris, accurate to arcminutes across 1700–2200. Your local birth date, time, and UTC offset are converted to Universal Time, then the geocentric ecliptic longitude (adjusted to the tropical zodiac of the birth date) is computed for the Sun through Pluto, plus the Mean Node via the standard mean-lunar-node formula. When a birth time is known, the Ascendant and MC are derived from Local Sidereal Time and latitude with the classic spherical formulas. Aspects are angular separations within an orb — 8° for conjunctions/oppositions when the Sun or Moon is involved, 7° for squares/trines, 5° for sextiles, 2° for minors, tightening by 1–2° without luminaries. The compatibility score is a fully published weighting model — click "How scoring works" in the results to see every number.

Reading the Aspect Grid

Each row shows one inter-chart connection: Person A's point · aspect · Person B's point, its exactness percentage, and its nature in words — harmonious (trine, sextile: ease), challenging (square, opposition, quincunx: friction that produces growth), neutral-dynamic (conjunction: fusion whose tone depends on the planets), or minor. Rows are sorted strongest-first. In astrological tradition, grids dominated by personal-planet links (Sun, Moon, Mercury, Venus, Mars) feel more "personal" than those driven only by slow outer planets.

What the Five Major Aspects Mean

Aspect	Angle	Glyph	Traditional reading
Conjunction	0°	☌	Fusion — the two energies blend and act as one; powerful, tone set by the planets.
Sextile	60°	⚹	Opportunity — easy flow that rewards a little initiative.
Square	90°	□	Friction — tension that demands action; classic "growth through challenge."
Trine	120°	△	Harmony — natural, effortless support between the two points.
Opposition	180°	☍	Polarity — a see-saw of attraction and difference seeking balance.

Synastry vs Composite Charts

Synastry keeps both birth charts whole and studies the aspects between them — "how do these two people interact?" A composite chart mathematically blends the two charts into one midpoint chart, treating the relationship as its own entity — "what is this relationship like?" This tool performs true synastry (inter-chart aspects, overlays, bi-wheel); the planet pairs it highlights — Sun-Moon, Venus-Mars — are the same ones composite astrologers weigh most heavily.

Twin Flames, Soulmates & Synastry

In astrology communities, certain inter-chart links are popularly associated with the twin-flame and soulmate ideas: Sun-Moon conjunctions or trines, Venus-Mars contacts, nodal connections, and strong 7th-house overlays. These are tradition and community language, not measurements — two people can share all of them and part ways, or share none and build a beautiful life. Treat this calculator as a mirror for reflection and conversation, never as proof of destiny.

Accuracy & Unknown Birth Times

Planetary longitudes are computed with professional-grade astronomy, typically within an arcminute. Two caveats: time zones — the tool uses standard-time offsets and asks you to add an hour when daylight saving applied (historical DST rules vary) — and the unknown-time case: without a time, Ascendant, MC, and houses are genuinely unknowable, and the Moon (moving ~13°/day) can drift up to about 7°. The tool tells you plainly which limitations apply rather than hiding them.

Is Astrology Scientifically Proven?

Honestly: no. Controlled studies have not found evidence that charts predict personality or outcomes, and astrology is not accepted as a science. Yet millions find it useful as a language for reflection and conversation — a symbolic framework like tarot or the I Ching. This tool is honest in both directions: the astronomy is real and precise; the meanings are interpretive tradition. Use it to think and talk, not to decide.

Why Use This Synastry Calculator?

🛰 Real calculations, not sun-sign fluff

Professional astronomy engine, arcminute precision — full birth chart compatibility, not just "Leo + Libra."

🆓 Genuinely free, no gates

No signup, no email, no paywall, no watermark, unlimited charts — the whole report, instantly.

💬 Interpretations included

Balanced plain-English meanings for key aspects — most free tools leave you a bare grid.

🎡 Bi-wheel + house overlays

The classic two-ring chart drawn live on canvas, plus house-by-house overlays both ways.

📄 PDF & text export

Download the full report — positions, aspects, score, interpretations — to keep or share.

🔒 Private by design

Birth data is processed entirely in your browser — nothing is sent, stored, or shared.

Popular Use Cases

Couples exploring compatibility

See where you flow and where you challenge each other — fuel for an honest conversation.

Friends, roommates & family

Synastry isn't only romantic — Mercury and Moon contacts say a lot about living together.

Astrology students practicing

A free sandbox for aspect reading: real positions, full grids, interpretations to check against.

Twin-flame community exploration

Check the placements the community talks about — on real calculated charts.

Writers developing characters

Build two charts, read the friction and chemistry, steal the dynamics for your story.

Pro Tips for Better Synastry Reading

1. An exact birth time from a birth certificate beats memory — it unlocks the Ascendant, houses, and overlays.
2. Compare Sun, Moon, Venus, and Mars first — tradition gives them the core of any relationship reading.
3. Squares aren't "bad" — hard aspects create the friction that keeps two people growing.
4. Look for patterns, not single aspects — one Venus-Saturn square means little; four Saturn contacts is a theme.
5. Never make relationship decisions from a score alone — real relationships are built, not calculated.

Common Mistakes to Avoid

• Judging the whole relationship from one hard aspect — the full pattern is the story.
• Ignoring unknown-time limitations — without a time, houses and overlays are guesses; the tool hides them for a reason.
• Confusing synastry with composite — overlay vs blended midpoint chart; different questions.
• Treating the 0–100 score as a verdict — it's one transparent weighting model for reflection.

Synastry FAQ

It takes two people's birth data, computes each natal chart, and compares them: which of Person A's planets form conjunctions, trines, squares, sextiles, and oppositions with Person B's, where those planets fall in each other's houses, and what those connections are traditionally interpreted to mean. It's the standard astrological method for studying compatibility beyond sun signs.

Enter Person A's date, time, and place in the left card, Person B's in the right (city search covers 100+ cities; manual entry elsewhere), then press Calculate Synastry Chart. Tick "I don't know the exact time" if needed. Results open in five tabs; Download PDF keeps a copy.

Yes — completely. No signup, no email, no paywall, no watermark, and no limit on charts. The full feature set — astronomy engine, bi-wheel, overlays, interpretations, exports — is open to everyone. Paid-gated synastry charts aren't more accurate, just less shareable.

Tick "I don't know the exact time." The calculator then honestly excludes the Ascendant, MC, and all house overlays, and flags the Moon as approximate (it moves ~13°/day, so it can be off by up to ~7°). All planet-to-planet aspects, the score, the bi-wheel, and interpretations still work. An exact time from a birth record makes a real difference.

Tradition puts Sun (identity), Moon (emotional needs), Venus (affection), and Mars (desire) at the center, with Mercury (communication) close behind — which is why the score weights them highest. Saturn contacts color longevity themes; the slow outers mostly matter when exactly touching personal planets.

80–100 "very strong connection," 60–79 "strong," 40–59 "mixed — growth through friction," 20–39 "challenging dynamics," 0–19 "few major connections." Two caveats: the score is one transparent weighting model (weights are published in the tool), and plenty of thriving couples score "mixed" — hard aspects are growth material, not failure marks.

Synastry overlays two complete charts and studies the aspects between them — how two individuals interact. A composite merges both charts into one midpoint chart — the relationship as its own entity. This tool performs true synastry; composite calculation is a different technique and not included.

Planetary longitudes come from the open-source astronomy-engine ephemeris — accurate to arcminutes from 1700–2200. Practical caveats: add +1 to the standard UTC offset yourself when daylight saving applied, and note the lunar point uses the mean node formula (labeled Mean Node; differs from the "True Node" by under 2°). The astronomy is precise; interpretations remain tradition.

No. Birth details are entered and processed entirely inside your browser's JavaScript — nothing is transmitted to a server, saved, or shared, and there are no accounts. Refresh the page and the data is gone. (That describes this tool's handling; it isn't a blanket security claim about your device or network.)

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🔒 Privacy: birth data is entered and processed entirely in your browser — nothing is sent, stored, or shared. ⚠️ For entertainment, reflection, and traditional astrology purposes — astrology is not science, and results are interpretive, not a basis for relationship, financial, or health decisions.