Composite chart vs synastry
Synastry keeps the two birth charts as they are and lists the angles between your planets and theirs, while a composite merges them into one chart. Synastry covers things like your Venus on their Mars or their Moon on your Sun, and it is about what one person stirs up in the other. A composite chart drops the two separate charts and keeps only the midpoints, so it describes neither of you. Astrologers read it as the relationship's own character: what it is for, how it feels from inside, and where it works or strains.
The two answer different questions, which is why many astrologers look at both. A pair can have lively synastry and a quiet composite, or the reverse. Our synastry chart calculator does the first; this page does the second. When people search for a "compatibility chart" they usually mean synastry, since it compares two people; the composite is the chart of the couple.
How to read a composite chart
Read it like a birth chart, but for the couple. Start with the composite Sun: its sign and house describe what the relationship is for, the thing that keeps the two of you building it. The composite Moon is the emotional climate, what feels like home when you are together. Venus shows what you enjoy as a pair and how affection moves between you; Mars shows how you act as a team and how you fight.
Then look at the houses, if you have both birth times. Planets in the composite 7th house are read as a relationship that is about partnership itself; in the 5th, about fun and romance; in the 10th, a couple that is seen together in public or works together; in the 4th, one built around home and family. Last come the aspects inside the composite. Contacts between the Sun and Moon are read as how well the purpose and the feelings of the relationship agree, and Saturn on the Sun, Moon or Venus as the place where it asks for patience and commitment.
Composite vs Davison chart
A composite averages the positions of the planets, while a Davison chart averages the moments and places of the two births. Robert Hand set out the midpoint composite in his 1975 book Planets in Composite. Ronald Davison proposed his chart in 1977, and it casts a real chart for that halfway moment and halfway point on the map. The Davison has real planets that sit on the ephemeris, so transits to it can be timed; the composite is a symbolic chart that exists only as midpoints.
The two often disagree. In our count, the Davison Sun fell in the same sign as the composite Sun in 48% of random couples, and the Davison Moon matched the composite Moon in 43%. Neither is the "right" one; astrologers who use both read the composite for what the relationship is like from inside and the Davison as a chart for timing. The calculator above shows the Davison under the result, next to the composite, so you can compare sign by sign.
Odd things a composite chart can do
A composite chart can put Venus and Mercury farther from the Sun than any birth chart allows. In a real chart Venus never gets more than about 47° from the Sun and Mercury never more than about 28°, because both orbit inside Earth's orbit. In a composite they can. Across 200,000 random couples we ran, composite Venus sat more than 47° from the composite Sun in 10.5% of pairs, and composite Mercury more than 28° away in 5.2%. Astrologers read such placements as they stand. They follow from the method, so the calculator is not making a mistake.
The other quirk comes from the midpoint itself. Every pair of positions has two midpoints, 180° apart, and the composite uses the one on the shorter arc. When two planets are almost opposite, the shorter arc barely wins, and a few degrees in either birth chart can flip the composite planet to the other side of the zodiac. In 5.4% of our random couples the two Suns were within 10° of opposite, the zone where the composite Sun is the least certain. The calculator flags it when it happens.
Composite chart without birth times
Two birth dates are enough for most of a composite. A few hours either way hardly moves the Sun, Mercury, Venus, Mars or the outer planets, so their midpoints hold. The Moon is the exception: it covers roughly 13° in a day, so the composite Moon can be off by several degrees, and without times we take local noon and label it approximate. The ascendant, midheaven and houses need both birth times. We derive them from the midpoint of the two midheavens at the average latitude of the two birthplaces, which keeps the angles of the chart consistent with each other; taking the midpoint of the two ascendants instead can leave the composite ascendant and midheaven in impossible positions.
Questions about composite charts
What is a composite chart?
A single chart made from two people's birth charts. Each planet is placed at the midpoint between its positions in the two charts, and the result is read as the chart of the relationship.
How can I calculate my composite chart?
Use the form near the top: two birth dates, plus times and cities for the houses. Nothing is sent anywhere; it runs in your browser. By hand, take each pair of planets and find the halfway point on the shorter arc: 4° Aries and 26° Gemini meet at 15° Taurus.
Is a composite chart the same as a compatibility chart?
No. A compatibility chart, usually called synastry, sets two birth charts side by side and lists the aspects between them. A composite fuses them into a single chart. The first is about two people meeting; the second is about the relationship they make.
What is a Davison chart?
A relationship chart cast for the midpoint in time and place between two births, introduced by Ronald Davison in 1977. Unlike the composite it is a real chart, so it has real planet positions and can be used for timing.
What does the composite Sun mean?
In the tradition, the purpose of the relationship: what it is built around and what keeps it going. Its sign gives the style and its house, with both birth times, gives the area of life.
Do I need birth times for a composite chart?
For the planets, no. For the composite ascendant, midheaven and houses, you need both birth times; without them the calculator shows the planets and marks the Moon as approximate.
Can a composite chart predict if a relationship will last?
No. A composite describes what a relationship is like, and astrology, a symbolic tradition, makes no forecast about how long it lasts. A tense composite is read as a list of things to work on.
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
Written by The Zodiac Atlas editorial team with the help of AI and edited by a person. Astrology is a symbolic tradition, not a science, and nothing here predicts how a relationship will go. Positions come from the astronomy-engine ephemeris in the tropical zodiac; time zones from the IANA database. More in How we work.