How to read a synastry chart
Start with the closest aspects. The calculator sorts them by orb, the distance from an exact angle, and astrologers give the tightest ones the most weight: a conjunction 0°40′ from exact is read as stronger than one 5° off. Then look at which planets are involved. Contacts between the Sun, Moon, Mercury, Venus and Mars, the personal planets, describe how two people get on day to day. Contacts to Jupiter and Saturn are read as growth and commitment. Uranus, Neptune and Pluto move so slowly that they describe generations more than couples, so they count mainly when they touch the other person's personal planets.
The tone comes from the angle. A conjunction (0°) merges two planets. Trines (120°) and sextiles (60°) are read as easy: the planets help each other. Squares (90°) and oppositions (180°) are read as tense: the planets pull in different directions, which astrologers describe as friction and also as attraction. No chart is all one or the other. In our count of 200,000 random pairs of birth dates, every pair had at least one easy aspect and at least one tense one.
Read both directions. Your Venus on their Mars is not the same contact as their Venus on your Mars, and a pair can have one without the other. The calculator lists each aspect as "your planet, their planet", so the two directions show up as separate lines.
Sun, Moon, Venus and Mars between two charts
Most readings start from four planets. The tradition reads each of them the same way it does in a single chart, and synastry asks how one person's version meets the other's.
Sun and Moon
One person's Sun on the other's Moon is the contact astrologers mention first. The Sun is the self and its plans, the Moon is emotional need, so a conjunction or trine is read as "you see me and I feel looked after". It is common: 55% of our random pairs had some Sun–Moon aspect in one direction or the other. A square or opposition is read as two daily rhythms that need talking through.
Moon and Moon
Two Moons in aspect describe emotional weather: whether you need closeness, space, noise or quiet at the same moments. 33% of our pairs had a Moon–Moon aspect. Without birth times this one is the least certain, because the Moon moves about 13° a day.
Venus and Mars
Venus is what a person finds lovable and how they show affection; Mars is desire, pace and temper. One person's Venus in aspect to the other's Mars is the classic sign of physical attraction. 43% of our pairs had one, in at least one direction. Squares and oppositions here are often called the "hottest" contacts, with arguments as part of the chemistry; trines and sextiles are read as attraction that is easy to live with.
Venus and the Sun or Moon
Venus touching the other person's Sun is read as admiration, Venus to the Moon as tenderness and care. These are the contacts behind the common question about a woman's Venus on a man's Moon: the calculator does not split them by gender; it reads whose planet is whose.
Each Venus–Mars aspect has its own page, with a two-date check and our numbers: conjunction · sextile · square · trine · opposition · the five side by side.
For one planet at a time, our pages on Venus signs, Mars signs and Moon signs explain what each sign brings to love.
Worst synastry aspects and red flags
Astrologers who use the phrase "worst aspects" usually mean squares and oppositions from Saturn or Pluto to the other person's Sun, Moon or Venus. Saturn is read as weight and duty, so a hard Saturn contact can feel like criticism or restriction; Pluto is read as intensity and control. Mars square Mars is the usual answer for a pair that fights.
Two numbers from our count put these warnings in proportion. Saturn made a major aspect to one of the other person's personal planets in 97% of random pairs, so a Saturn contact on its own describes almost every couple. And the average pair had 10 tense aspects next to 11 easy ones. One hard aspect is normal and does not make a red flag. Astrologers look for a pattern: several hard contacts on the same planet, with few easy ones to balance them.
House overlays: their planets in your houses
With both birth times, the calculator also places each person's planets in the other person's houses. This is called a house overlay, and it says which area of your life the other person touches. Their planets in your 7th house are read as partnership and commitment (see Venus in the 7th house), in the 5th as romance and fun, in the 8th as intimacy and shared money, in the 12th as something private, sometimes secret, sometimes deeply spiritual.
Overlays need an accurate time. Houses turn once a day, so a birth time off by an hour can move a planet into the next house. Without a time we show aspects only, which is the part of synastry that does not depend on the hour. Our houses page explains each of the twelve.
Which orbs we use, and why
An orb is how far an angle may be from exact and still count. There is no single standard, and that is the main reason two synastry calculators list different aspects for the same couple. Astrodienst, for example, allows 10° for the four main aspects and 6° for the sextile in a single chart by default. Many astrologers use tighter orbs between two charts, because a wide contact between strangers' planets is felt less than one inside a single chart.
Our rule: the more personal of the two planets sets the orb. With the Sun or Moon involved we allow 8° (6° for a sextile); with Mercury, Venus or Mars, 6° (4°); for everything else, including the ascendant and midheaven, 4° (3°). We leave out aspects between two outer planets (Uranus, Neptune, Pluto): they move so slowly that these aspects depend on the two birth years, so they describe a generation, not a couple. Each line in the result shows its orb and the limit it was measured against, so you can apply a stricter rule yourself.
| The more personal planet of the pair | Conjunction, square, trine, opposition | Sextile |
|---|---|---|
| Sun or Moon in the pair | 8° | 6° |
| Mercury, Venus or Mars (no Sun or Moon) | 6° | 4° |
| Jupiter to Pluto, ascendant, midheaven | 4° | 3° |
Synastry vs composite chart
Synastry keeps two charts and compares them. A composite chart makes one new chart out of two: for each planet it takes the midpoint between the two people's positions, so your Sun at 10° Cancer and theirs at 20° Virgo give a composite Sun at 15° Leo. Astrologers read synastry as how two people experience each other, and the composite as the relationship treated like a third person with its own character.
They answer different questions, and many astrologers look at both. Synastry is the one that shows who triggers what in whom, which is why it is the usual first step. The calculator above lists your composite positions under the result; for the full composite chart with houses, aspects and a Davison chart to compare, use our composite chart calculator.
Synastry vs zodiac sign compatibility
Sign compatibility compares two Sun signs, which gives 78 possible pairs. Synastry compares every planet of both charts by exact degree, so two Aries–Scorpio couples can look nothing alike. Sign compatibility is a fair first look, and our zodiac compatibility calculator does it with the Sun, Moon, Venus and Mars of both people. Synastry is the full version: all ten planets, the ascendant and midheaven, exact orbs, and houses when the birth times are known.
Synastry without birth times
You can still get most of a synastry chart from two birth dates. The Sun and the planets from Mercury to Pluto move slowly enough that the hour rarely changes their aspects. Three things do need the time: the ascendant, the midheaven and the houses. The Moon is in between. It moves about half a degree an hour, so without a time we place it at local noon and mark every Moon aspect as approximate; on a day the Moon changed sign we also show the second sign.
To check a single chart first, with its houses and the Moon's exact degree, use the birth chart calculator.
What a typical synastry chart looks like
We drew 200,000 random pairs from every birth date between 1930 and 2025 and ran each pair through the same calculator and orbs as above, with the planets at noon UTC and no birth times.
| In a random pair of birth dates | Result |
|---|---|
| Aspects between the two charts (average) | 24.3 |
| Of them: conjunctions · sextiles · squares · trines · oppositions | 3.3 · 4.7 · 6.5 · 6.5 · 3.2 |
| Some Sun–Moon aspect, either direction | 55.0% |
| Some Venus–Mars aspect, either direction | 42.6% |
| Venus–Mars conjunction, trine or sextile | 27.3% |
| Moon–Moon aspect | 32.8% |
| Personal planets conjunct within 1° | 12.4% |
| Saturn to a personal planet of the other | 96.7% |
Pairs of dates, not real couples, and no ascendant or midheaven. The shares describe what synastry looks like by chance, which is the baseline any one couple is compared against.
Need a second chart to practice on? Our celebrity birth charts list birth data with a verified time, such as Beyoncé's, ready to enter above.
Questions about synastry
What is a synastry chart?
It is a comparison of two birth charts. Each person's planets are placed by sign and degree, and the angles between one chart's planets and the other's are listed as aspects. Astrologers use it to describe how two people affect each other in love, friendship or work.
How do I make a synastry chart for free?
Enter both birth dates in the calculator at the top of this page, with birth times and places if you know them. It is free, has no sign-up and runs in your browser; nothing you type is sent anywhere.
What are the hottest synastry aspects?
In the tradition, one person's Mars to the other's Venus, especially the conjunction, square and opposition, followed by Mars to the Moon and Mars to the ascendant. 43% of random pairs in our count had a Venus–Mars aspect in at least one direction, so it is common.
What are the worst synastry aspects?
Astrologers usually name squares and oppositions from Saturn or Pluto to the Sun, Moon or Venus, and Mars square Mars. Nearly every pair has some hard contact, though: Saturn touched a personal planet of the other person in 97% of our random pairs. One hard aspect is normal; a cluster on one planet is what astrologers flag.
How common is an exact conjunction in synastry?
Less common than it feels when you find one. In our count, 12% of random pairs had a conjunction within 1° between personal planets (the Sun, Moon, Mercury, Venus or Mars of each person). Within the wider orbs we use, a typical pair had about 3 conjunctions of any kind.
How many aspects is normal in synastry?
With our orbs the average was 24 and the middle pair had 24. The fewest we saw was 5 and the most 43. Other calculators use other orbs, so their counts differ; compare the closest aspects rather than the total.
What is the difference between a synastry chart and a composite chart?
Synastry compares two separate charts. A composite chart builds one chart from the midpoints of both people's planets and is read as the relationship itself. Synastry shows how each of you affects the other; the composite describes the couple as a unit.
Does synastry work for friends and family?
Yes. The same aspects are read for any two people. For friendship astrologers look more at the Sun, Moon and Mercury than at Venus and Mars, and for parents and children at the Moon and Saturn.
Can synastry predict if a relationship will last?
No. Astrology is a symbolic tradition, and synastry describes styles that meet, not outcomes. Couples with "easy" charts break up and couples with "hard" ones stay together; the chart is a set of topics to talk about, not a verdict.
Sources
- Astronomy Engine: the open-source ephemeris library behind our planet positions, dates and statistics
- Jean Meeus, Astronomical Algorithms (Willmann-Bell, 1991): formulas for the ascendant, midheaven, mean node and mean Lilith
- GeoNames: city coordinates for the calculator (CC BY 4.0)
- How we create our readings: our own calculation of sign dates, positions and the 1930 to 2025 statistics
Astrology is a symbolic tradition, not a science, and nothing here predicts how a relationship will go. We explain what the tradition says and show the calculation. Positions come from the astronomy-engine ephemeris in the tropical zodiac; time zones from the IANA database. More in How we work.