Guide · 6 min read
Synastry chart calculator: how to read a synastry chart
Synastry compares two birth charts to show how two people tend to meet, spark, soothe and stretch each other. Here is how a synastry chart calculator works, and how to read the result without getting lost.
By Love Compatibility Report · Updated
What is synastry?
Synastry is the branch of astrology that compares two birth charts. Your birth chart is a snapshot of where the Sun, Moon and planets were at the moment you were born. Lay your partner's chart over yours and you can see how their planets "talk" to your planets: which placements harmonize, which ones create heat, and which ones ask you both to grow.
Sun sign compatibility, the kind you see in magazines, uses just one point from each chart. Synastry uses all of them. That is why two Scorpios can have completely different relationships, and why a so-called mismatch on paper can feel like home in real life. Think of synastry as a conversation starter about how you love, not a pass or fail grade.
What you need for a synastry chart
- Both birth dates. Required. This places the Sun, Mercury, Venus, Mars and the slower planets.
- Both birth times, if you know them. The Moon moves about 13 degrees a day, so time sharpens its position. Time is also needed for the rising sign (ascendant).
- Both birth places, if you know the time. Place and time together give the ascendant and houses. The place is also used to convert local time to universal time with the correct historical time zone.
Don't know a birth time? You can still learn a lot. Most of the important relationship planets don't depend on the hour, and a good calculator will tell you when the Moon might be in one of two signs that day.
How a synastry chart calculator works
A synastry chart calculator does three jobs. First, it computes each person's planetary positions: the exact degree of the zodiac for each planet at the birth moment. Ours uses astronomy-engine, an open source astronomy library, and runs in your browser, so your birth data never leaves your device.
Second, it compares every relevant planet in chart A with every relevant planet in chart B and measures the angle between them. Certain angles, called aspects, are considered meaningful. Each aspect has an orb, a tolerance in degrees. A Venus at 10° Leo and a Mars at 12° Aries form a trine with a 2° orb: close, and therefore strong.
Third, it summarizes. Our calculator weighs each aspect by type and tightness and turns the whole pattern into a score from 0 to 100, with sub-scores for romance, communication, emotional connection and long-term potential. The same inputs always give the same result.
The planets that matter most in synastry
| Planet | What it shows in a relationship |
|---|---|
| Sun | Core identity, vitality and what each person admires and wants to be seen for. |
| Moon | Emotional needs, comfort, home life and how you care for each other on hard days. |
| Mercury | Conversation, humor, how you argue and how you make decisions together. |
| Venus | Affection, romance, taste and what makes each of you feel loved. |
| Mars | Desire, drive, physical chemistry and how you handle anger. |
| Jupiter | Generosity, optimism and where you help each other grow. |
| Saturn | Commitment, responsibility and the lessons that build lasting trust. |
| Ascendant | First impressions, instant chemistry and how you show up for each other day to day. |
The personal planets (Sun, Moon, Mercury, Venus, Mars) and the ascendant change quickly from person to person, so contacts between them are the most personal. Uranus, Neptune and Pluto move so slowly that whole generations share their signs, so we give them less weight.
The five major aspects
| Aspect | Angle (orb) | Feels like |
|---|---|---|
| Conjunction | 0° (8°) | Fusion. The two energies blend and amplify each other, for better and for more intense. |
| Sextile | 60° (4°) | Friendly opportunity. Easy rapport that grows when you both put in a little effort. |
| Square | 90° (6°) | Friction and chemistry. A growth edge that keeps things lively and asks for understanding. |
| Trine | 120° (6°) | Natural flow. Things just work, sometimes so easily that you forget to appreciate them. |
| Opposition | 180° (8°) | Magnetic polarity. You see yourself in the other person and balance each other out. |
We give the Sun and Moon two extra degrees of orb, as many astrologers do, because they are the brightest and most personal points in a chart.
How to read a synastry chart, step by step
- Start with the Sun and Moon. Sun–Moon contacts between charts are classic signs of ease: one person's core self meets the other's emotional needs. Moon–Moon contacts show whether your moods and comfort habits match.
- Look at Venus and Mars. Venus–Mars aspects describe attraction and romantic rhythm. Venus–Venus shows shared taste; Mars–Mars shows how your energy and tempers mix.
- Check Mercury. Mercury contacts reveal whether conversation flows, whether you get each other's jokes, and how you solve problems as a team.
- Find Saturn. Saturn aspects to personal planets often show up in couples who stay together. They can feel serious or even restrictive at first, and they also build loyalty and structure.
- Weigh by tightness. An aspect with a 1° orb speaks louder than one with a 7° orb. Read the tightest aspects first.
- Look for themes, not single lines. Three aspects pointing to great conversation matter more than one isolated square. Notice repetition.
- Read both directions. Your Venus to their Mars is not the same as their Venus to your Mars. Each direction describes who brings what to the relationship.
Squares and oppositions are not bad news
It is easy to see a square and panic. Please don't. Many long, happy couples have plenty of squares between their charts, because friction creates movement and attraction. A relationship with only trines can feel lovely but sleepy. Challenging aspects simply point to the places where you will learn the most about each other. Treat them as growth edges and talking points.
Synastry vs. sun sign and composite charts
Sun sign compatibility compares one point in each chart and is a fun, quick starting point. Synastry compares every planet and is far more personal. A composite chart is a third approach that averages both charts into one "relationship chart." If you are new to all of this, start with synastry: it is the easiest to relate to your real experience, and our birth chart compatibility guide walks you through the layers.
How our compatibility score works
Every aspect between your personal planets and your partner's planets (up to Saturn) adds to the score. Trines and sextiles add the most, conjunctions add a lot (a little more when Saturn is involved), oppositions add attraction, and squares subtract a little while adding to "chemistry." Tighter aspects count more. The sun sign elements add a small extra component, and a gentle curve keeps most couples between about 55 and 90, because no algorithm should tell you that a relationship you love scores 12. The score is a mirror for reflection, never a prediction.
Synastry FAQ
Is the synastry chart calculator free?
Yes. Calculating both charts, the main aspects between them and your compatibility score is free. An optional full written report, with every aspect explained, is a one-time purchase. There is no subscription.
Can I do synastry without a birth time?
Yes. Without a birth time you still get the Sun, Venus, Mars, Mercury and the outer planets accurately, and usually the Moon too. You lose the rising sign and houses, and the Moon is marked approximate on days it changes sign.
What is the most important aspect in synastry?
There is no single magic aspect. Many astrologers look first at Sun and Moon contacts for emotional ease, Venus and Mars contacts for attraction, and Saturn contacts for commitment. The overall pattern matters more than any one line.
Is synastry the same as a composite chart?
No. Synastry overlays two separate charts and looks at the aspects between them. A composite chart blends both charts into one, using midpoints, to describe the relationship as its own entity. They answer different questions and work well together.