Synastry Chart: Overlay Two Birth Charts and Read Them Together
Everyday "zodiac compatibility" compares only two Sun signs. Synastry overlays two complete charts: your Moon on their Sun, their Venus on your Mars. These aspects between two people are what astrology actually looks at when it talks about relationships. Cast both charts first, then read the key contacts below.
Free Synastry Chart
Enter both people's birth dates, birth times and birthplaces to get the cross-aspects between the two charts. The calculator picks out the contacts synastry values most (Sun–Moon, Venus–Mars, Moon–Moon) and reads each one, draws a bi-wheel, and shows which of your houses your partner's planets fall in. It works without birth times too; you just lose the houses and the Ascendant. Everything is calculated in your browser; nothing is uploaded.
Tropical zodiac, Placidus houses. Cross-aspect orbs: conjunction and opposition 6°; square and trine 5°; sextile 4°; plus 1° when the Sun or Moon is involved. Enter the local clock time on each birth certificate; daylight saving time is removed automatically from the birthplace.
Start with these contacts
Two charts usually have thirty-odd aspects between them, and they can't all matter equally. Traditional synastry starts with a handful of key contacts. The last column is our own calculation: the share of random pairs of people who have that contact within orb.
| Contact | Between | About | Random pairs with this contact |
|---|---|---|---|
| Sun–Moon | One person's Sun and the other's Moon | one person's goals meet the other's need for security | 46.7% |
| Venus–Mars | One person's Venus and the other's Mars | where the attraction comes from | 40% |
| Moon–Moon | Both Moons | how the two of you recharge | 27.8% |
| Venus–Venus | Both Venuses | taste and values | 21.8% |
| Sun–Sun | Both Suns | your two life directions | 26.8% |
| Mercury–Mercury | Both Mercuries | can you talk? | 22% |
| Saturn | One person's Saturn and the other's Sun, Moon or Venus | commitment and pressure | 84% |
| Ascendant | One person's Sun, Moon, Venus or Mars and the other's Ascendant | the first impression | 89.1% |
Charts of people born in Taipei between 1960 and 2005, paired at random, 2,100 pairs in all, calculated by our engine with our orbs.
As you can see, even the "good" contacts such as Sun–Moon and Venus–Mars appear in forty to fifty percent of random pairs; 77.3% of pairs have at least one of Sun–Moon, Venus–Mars or Moon–Moon. Saturn and Ascendant contacts are even more common, over eighty percent, because each covers several planets. So the question in synastry is not whether a contact exists but how tight the orb is, whether it is soft or hard, and which part of the two lives it touches.
Soft, hard and conjunctions
As in a natal chart, trines and sextiles are soft aspects, squares and oppositions hard aspects, and a conjunction depends on the planets. Between two people the meaning shifts slightly:
- Soft aspects are ease: you understand each other without much explanation, and your daily habits fit. A relationship full of soft aspects is comfortable, but may lack a little spark and the push that makes you both grow.
- Hard aspects are friction, and also attraction. Plenty of relationships get their spark from squares and oppositions: more arguments, but also more growth.
- Conjunctions bind two forces together: Venus conjunct Mars is strong attraction, Saturn conjunct the Moon can be security or pressure.
Two random charts average 15.3 soft aspects, 13.2 hard aspects and 5.3 conjunctions between them; the two kinds come in roughly equal numbers. There is no need to look for someone with "only soft aspects"; statistically that pairing barely exists.
House overlays: where the other person lands in your life
If both of you have a birth time, you can also see which of your houses the other person's planets fall in. Their Venus in your 7th house (partners) is often read as them feeling like "the one" at first sight; their Saturn in your 10th house (career) might be someone who puts pressure on you at work and also gives you direction. The calculator lists the houses that each person's Sun, Moon, Venus and Mars fall in for the other. If only one of you has a birth time, you can only read it one way.
A real example: Charles and Diana
King Charles III (November 14, 1948, 21:14, London) and Diana, Princess of Wales (July 1, 1961, 19:45, Sandringham) both have documented birth times. Their two charts have 34 aspects between them; the key contacts are:
| Contact | Aspect | Orb |
|---|---|---|
| Sun–Moon | Charles's Sun square Diana's Moon | 2.6° |
| Mercury–Mercury | Charles's Mercury trine Diana's Mercury | 3.8° |
| Saturn | Charles's Moon square Diana's Saturn | 2.6° |
| Ascendant | Charles's Venus sextile Diana's Ascendant | 2.0° |
Their Sun signs are Scorpio and Cancer, both water signs; by the zodiac compatibility chart that is the most harmonious pairing, a trine. But the contact synastry pays most attention to is Charles's Sun square Diana's Moon: one person's goals and the other's emotional needs blocking each other. There is also no major Venus–Mars or Moon–Moon aspect between them. Sun signs that look like a great match, a synastry with obvious tension: which shows why comparing Sun signs alone isn't enough. Of course, this is hindsight; the same charts in another couple could tell a completely different story.
A straight word: synastry shows how you relate, not how the story ends
Synastry overlays two birth charts and gives you a language for "where we click and where we get stuck". In our calculations, any two random people have thirty-odd aspects between their charts, and the "good" contacts such as Sun–Moon and Venus–Mars turn up in forty to fifty percent of pairs, so whether a single aspect is present says nothing about whether two people will get together or last. It is a fine way to understand your differences, but it cannot make the decision for you. Everything on this site is for reference and entertainment only.
See the same relationship through another system
BaZi compatibility looks at how two people's elements and day pillars complement each other, a different angle from synastry.
Synastry FAQ
Does synastry need both birth times?
Ideally, but it works without them. The Sun, Venus, Mars and the other planets barely move in a day, so most of the cross-aspects can still be calculated; what you lose is the Ascendant, Midheaven, houses and the Moon's exact position (it moves about 13° a day). The calculator flags any aspect where the Moon's position is uncertain.
Are more aspects between two charts better?
No. In our calculations two random people already average 33.9 aspects between their charts, and the count depends mainly on the orbs used. What matters is how tight the key contacts (Sun–Moon, Venus–Mars, Moon–Moon) are, and whether they are soft or hard.
How is synastry different from zodiac compatibility?
Zodiac compatibility compares only the two Sun signs; synastry compares every planet in two complete charts. In our test, Sun-sign pairings that "match" had only a couple more soft aspects between the charts than other pairings; see the zodiac compatibility chart.
Does a Sun–Moon square mean we're incompatible?
No. A Sun–Moon square means one person's goals and the other's emotional needs tend to miss each other, so you need to talk separately about "what I want" and "what I need". 46.7% of random pairs have a Sun–Moon contact, plenty of them squares and oppositions, and many long-lasting couples have one.
Which is more accurate, astrological synastry or BaZi compatibility?
They look at different things: BaZi compatibility asks whether two people's favorable elements complement each other and how their day pillars relate; synastry looks at the aspects between the planets of two charts. We offer both, so you can compare; see BaZi compatibility.