Birth Chart Compatibility Calculator
Birth chart compatibility, or synastry, compares two natal charts to see how one person's planets interact with the other's. Enter both people's birth date, time and city below, and Astrodamus opens a free synastry chart: your chart inside, your partner's planets around it, and every aspect between you explained in plain English.
What a synastry chart shows
Synastry places two charts on one wheel. Your planets stay in the inner circle and your partner's planets form the outer ring, so you can see which of their planets fall into which of your houses and which angles connect the two sets of planets.
Those angles are aspects. A Sun of one person in trine to the Moon of the other is read as easy understanding; a square between Mars and Saturn as friction that needs patience. The app lists every major aspect between the charts, strongest first, with a short reading for each.
Synastry, composite and Davison charts
There are three classic ways to look at a couple. Synastry compares the two birth charts directly and answers the question of how each person affects the other. It opens from the form on this page.
A composite chart takes the midpoint between each pair of planets, so the composite Sun sits halfway between both Suns, and reads the result as the chart of the relationship itself. A Davison chart is a real chart cast for the moment and place halfway between the two births. Composite and Davison charts are part of the Pro plan in Astrodamus.
Key aspects in a compatibility chart
Start with the personal planets: Sun, Moon, Mercury, Venus and Mars. The aspects below are the ones astrologers usually check first; each link opens the full article with its synastry section.
- Sun Conjunct Moon
- Sun Trine Moon
- Sun Opposite Moon
- Sun Conjunct Venus
- Sun Square Saturn
- Moon Trine Venus
- Moon Trine Mars
- Moon Opposite Saturn
- Venus Conjunct Mars
- Venus Square Mars
- Venus Conjunct Saturn
- Venus Conjunct Pluto
How to read your compatibility
Look at both directions: your Venus to their Mars is a different story from their Venus to your Mars. Then weigh the overall picture rather than a single aspect. Harmonious contacts show where things flow easily; challenging ones show where you will need to talk things through. Most lasting couples have both.
Saturn contacts are worth special attention. Astrologers read them as commitment and responsibility, which can feel heavy at times but often hold a bond together over the years.
What you need to enter
The birth date, time and city of both people. The time matters for the Moon, which moves about 13° a day, and for the houses and Ascendant. If a time is unknown, enter 12:00 PM: planet aspects will be close, but house overlays and aspects to the Ascendant will not be reliable.
Frequently asked questions
Is zodiac sign compatibility the same as synastry?
No. Sign compatibility compares only the two Sun signs, which millions of people share. Synastry compares the exact positions of all the planets, the Ascendant and the houses in both charts, so it is specific to the two of you.
Can we check compatibility without birth times?
Yes, with limits. Enter 12:00 PM for an unknown time. Aspects between the Sun, Mercury, Venus, Mars and the outer planets stay accurate; the Moon can be off by several degrees, and house overlays and the Ascendant cannot be used.
Which aspects are best for a relationship?
Astrologers traditionally look for harmonious contacts between the Sun and Moon, Venus and Mars, and the Moon and Venus, along with conjunctions of personal planets. No single aspect makes or breaks a couple; the overall balance and how you both handle tension matter more.
Do difficult aspects mean the relationship won't work?
No. Squares and oppositions describe areas of friction and growth, not a verdict. Astrology describes tendencies, and what you do with them is up to you. Many strong partnerships have several challenging aspects.
Is the compatibility chart free?
Yes. The synastry wheel and the list of aspects between you open free from this page. The full written compatibility reading and its PDF come with the Basic plan or as a one-time report, and composite and Davison charts are part of Pro.
More free tools: Rising Sign Calculator · Solar Return · Aspects