Synastry Chart Calculator
A synastry chart lays two people's birth charts over each other and lists every angle between one person's planets and the other's. Enter both birth dates, times and places in the synastry calculator to see the bi-wheel, the contacts sorted by closeness, and where each person's planets fall in the other's houses.
What synastry compares
Synastry is the astrology of relationships, and it works by comparison rather than by blending. Each person keeps their own natal chart. The calculator then measures the angle between every planet of the first chart and every planet of the second: your Venus to their Mars, your Moon to their Sun, and so on. These are called inter-aspects or synastry aspects.
It also places each person's planets in the other's houses, called house overlays. If your Sun falls in their seventh house, astrologers read it as you lighting up their area of partnership. House overlays need both birth times; inter-aspects need only the dates, unless the Moon is involved.
Synastry describes themes, not outcomes. It does not predict whether a relationship will last, and it works the same way for friends, family and colleagues as for couples.
How synastry works, with an example
Say your Sun is at 10 degrees Leo and your partner's Moon is at 12 degrees Aries. Counting along the zodiac, the two points are 122 degrees apart, which is 2 degrees from an exact trine (120). The calculator lists this as "Sun trine Moon, orb 2 degrees" and shows the reading for that contact. Astrologers traditionally read a Sun-Moon trine between two charts as an easy fit between one person's sense of self and the other's emotional needs.
Contacts are sorted by orb, closest first, because a contact within a degree or two is usually given more weight than one at the edge of its range. Every contact shows its exact orb, and the default orbs are listed on the method page.
What astrologers look at first
- Sun and Moon contacts: identity meeting emotional needs, read in both directions.
- Venus and Mars contacts: attraction, affection and desire.
- Contacts to the Ascendant: first impressions and physical ease, only reliable with exact birth times.
- Saturn contacts: read as commitment and seriousness, and also as pressure or restraint.
- Repeated themes: when the same idea appears two or three ways (for example, Venus to Mars in both directions), astrologers give it more weight than a single contact.
Squares and oppositions are not bad news. They are read as friction that can be lively or growth-giving, and many long relationships show plenty of them.
Synastry, composite and Davison
Synastry keeps two charts side by side. Two other methods build one chart for the relationship itself: the composite chart, made from the midpoints of each pair of planets, and the Davison chart, cast for the moment and place halfway between the two births. Many astrologers read synastry for how two people affect each other and a composite or Davison chart for the relationship as a third thing.
Common synastry mistakes
- Judging compatibility by Sun signs. Two Sun signs say very little; the contacts between full charts are where synastry starts.
- Trusting a single percentage. Any compatibility score has to weight aspects somehow, and those weights are a choice, not a measurement. Reading the contacts themselves is more honest.
- Guessing birth times. Without times, leave out house overlays and contacts to the Ascendant, and treat Moon contacts with care.
For a tarot view of the same question, try the love tarot spread or the soulmate tarot spread.
Questions
How do I make a synastry chart?
Enter both people's birth dates, times and places in the calculator above. It draws both charts in one wheel and lists every aspect between them, closest first, with the house overlays.
Is a synastry chart free?
Yes. The calculator is free, needs no account and runs in your browser; neither person's birth data is sent to our server.
What is the difference between synastry and a composite chart?
Synastry compares two separate charts planet by planet. A composite chart combines them into one chart using the midpoint of each pair of planets, and is read as the chart of the relationship itself.
Can I do synastry without birth times?
Yes, partly. Planet-to-planet contacts work from dates alone, except that the Moon may be uncertain. House overlays and contacts to the Ascendant or Midheaven need both birth times.
What are the best aspects in synastry?
Astrologers often point to Sun-Moon, Venus-Mars and Moon-Venus contacts, especially trines, sextiles and conjunctions. But no single aspect makes or breaks a relationship, and astrology cannot predict how one will go.
Does synastry work for friends?
Yes. The method is the same for any two people. The readings for each contact are written so they apply to friendships and family as well as couples.
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