Synastry Calculator
Synastry compares two full birth charts — not just two sun signs — to read a relationship the way an astrologer does. Enter both sets of birth details to see your compatibility score and the real cross-aspects firing between your charts, free.
Birth times are optional but sharpen the read — especially anything involving the fast-moving Moon. Without a time, that person’s chart is estimated from noon.
What is synastry?
Synastry is the astrology of relationships. Where a sun-sign compatibility quiz lines up two birthdays, synastry overlays two entire birth charts and studies how they interact — planet by planet, angle by angle. It’s the same method an astrologer uses when a couple sits across from them: not “you’re a Leo and they’re a Scorpio,” but “your Moon sits right on their Venus, and your Mars squares their Saturn.” Those specific contacts are what make one pairing feel effortless and another feel like a magnet you can’t put down.
That’s why two people with the “wrong” sun signs can be wildly compatible, and two with textbook-perfect signs can fizzle. The sun is one planet out of ten, plus the Moon, the Rising, and the houses. Synastry reads the whole conversation. This calculator computes both full charts, finds the cross-aspects between them, and translates the strongest ones into plain language — so you can see why you click, not just a number.
Cross-aspects: where the chemistry lives
A cross-aspect is the angle between one of your planets and one of theirs. Your Venus at a certain degree, their Mars a precise number of degrees away — that gap is the aspect, and its size decides the flavour. The closer the angle is to exact (the smaller the orb), the more loudly that connection runs. A Moon–Moon trine that’s 0.4° from exact will be felt far more than a loose 5° one. The most personal contacts — Sun, Moon, Venus, Mars, and the Ascendant — carry the most emotional weight, which is why this tool surfaces those first.
Fusion — two planets sit together, blending into one shared force.
Ease — a flowing, effortless harmony that comes for free.
Opportunity — a friendly opening that rewards a little effort.
Friction — tension that's uncomfortable but keeps the bond charged.
Polarity — a magnetic push-pull that mirrors and balances you.
Adjustment — an odd-angle fit you keep quietly recalibrating.
Broadly, conjunctions, trines, and sextiles are the easy, attraction-building angles, while squares and oppositions create the tension that gives a relationship friction, growth, and heat. A great match isn’t one with zero challenge — it’s one where the harmonious contacts give you a soft place to land and the hard ones keep things interesting.
Why birth times sharpen the read
You can run synastry with just two birth dates, and the slower planets — Venus, Mars, Jupiter, and out — will be accurate, since they barely move across a single day. But two of the most important pieces of compatibility need an exact time. The Moon, which governs emotional needs and comfort, can move up to about 13° in a day, so its sign and its aspects can shift without a time. The Rising sign and the houses need a time entirely. If you have both birth times, add them — the score and the connections get noticeably more precise.
Free vs. the full report
This free calculator gives you the headline: your compatibility score, a one-word verdict, both people’s Big Three, and the five strongest cross-aspects with a plain-English meaning for each. The full synastry report goes the rest of the way — every cross-aspect between your charts, the complete aspect grid, your love / communication / values / passion breakdown, and the full written read. Save it, and Lune tracks it as a daily, transit-aware compatibility as the real sky moves over both of you.
Synastry FAQ
What is synastry?
Synastry is the branch of astrology that compares two people's full birth charts to read a relationship. Instead of just lining up two sun signs, it overlays both charts and looks at how every planet in one person's chart angles to every planet in the other's. Those angles — the cross-aspects — are where the real chemistry, friction, and attraction live. It's the same technique a professional astrologer uses to read a couple, and it works for romance, friendship, family, or business.
What are cross-aspects?
A cross-aspect is the angle between one of your planets and one of theirs — for example, your Venus trine their Mars, or your Moon square their Saturn. Conjunctions (0°) fuse two energies, trines (120°) and sextiles (60°) flow easily, while squares (90°) and oppositions (180°) create the tension that makes a bond feel alive. The tighter the orb — how close the angle is to exact — the louder that connection runs between you.
Do we need exact birth times?
Not to get a result, but they sharpen it. Without a time, your charts are calculated from noon, which is exact for the slower planets (Venus, Mars, and outward) but approximate for the fast-moving Moon and leaves out the Rising sign entirely. Since the Moon is central to emotional compatibility, adding both birth times gives you the most accurate, complete read. We'll flag your result as approximate if either time is missing.
Is the synastry calculator free?
Yes. Enter two birth dates (and times and cities, if you have them) and get your compatibility score, the verdict, both people's Big Three, and your five strongest cross-aspects — free, with no signup and no card.
What's in the full synastry report?
The free tool shows your headline score and your top five connections. The full report unlocks every cross-aspect between your charts, the complete aspect grid, and your love, communication, values, and passion breakdown — plus the full written read of the relationship and the option to save it for daily, transit-aware compatibility as the sky moves.
