Synastry Chart Calculator
Synastry overlays your birth chart on someone else's to see exactly where your planets support each other and where they create friction. Enter both of your birth details below and this calculator draws every aspect your planets make to theirs, so you can see the actual pattern between you instead of a generic compatibility score.How it works ↓
Fill in the details above and compute to see your result here.
Enter both birth details to see synastry, composite, and Davison charts for any two people.
What synastry actually compares
Synastry does not blend your charts into one. It keeps both intact and measures the angles your planets make to each other's. If your Venus sits three degrees from their Mars, that is a synastry aspect: a real geometric relationship between two placements that exist independently of each other.
This is different from a composite or Davison chart, which each derive a single new chart for the relationship itself. Synastry stays two-sided by design, which is why it is the technique most people mean when they ask "are we compatible."
Which contacts carry the most weight
Not every aspect matters equally. Contacts between the personal planets (Sun, Moon, Venus, Mars, Mercury) describe daily compatibility: how you talk, what you want, how you show affection. Contacts to the outer planets (Jupiter, Saturn, Uranus, Neptune, Pluto) tend to describe deeper, slower-moving dynamics (growth, restriction, disruption, idealization, transformation) that surface less often but can dominate the relationship when they do.
Contacts to the Ascendant, Midheaven, and other angles matter most when both of you have a reliable birth time, since the angles move roughly one degree every four minutes and shift the whole picture if either birth time is off.
Why houses matter as much as aspects
An aspect tells you two planets are in contact; the house one person's planet falls into on the other's chart tells you where that contact plays out. Someone's Venus landing in your seventh house touches partnership directly. The same Venus landing in your sixth house shows up more through daily routine and work than romance.
This calculator overlays each of you onto the other's house wheel, so you can read both directions, where their planets land in your houses, and where yours land in theirs.
What synastry can't tell you without a birth time
Synastry's planet-to-planet aspects hold up without a birth time, because sign and planetary positions only depend on the date. Anything involving houses or angles (where their Venus lands in your chart, whether their Saturn touches your Ascendant) needs both of your birth times to be accurate.
If either birth time is estimated or unknown, read the planet-to-planet aspects with confidence and treat the house placements as approximate.
Synastry, composite, and Davison, three different questions
These three two-person techniques are often confused because they use the same two birth charts, but they answer different questions. Synastry keeps both charts separate and asks how you affect each other. A composite chart merges you into one symbolic chart and asks what the relationship itself looks like as its own entity. A Davison chart finds the real midpoint moment and place between your two birth times and casts an ordinary chart for that actual event.
Most astrologers read synastry first, since it is the most direct answer to "how do we interact," then move to composite or Davison for a picture of the relationship as its own thing.
Reading your compatibility score
The score on this page is a simple summary, not a substitute for reading the aspects yourself. It weighs harmonious contacts (trines, sextiles) positively and tense ones (squares, oppositions) negatively across every cross-aspect this calculator finds, then normalizes the result to a 0-100 scale.
A low score does not mean an unworkable relationship, and a high one does not guarantee an easy one, tense aspects often describe exactly what draws two people together. Use the score as a starting point for the cross-aspect grid, not as the final word.
Frequently Asked Questions
What is synastry?
Synastry overlays two natal charts to see how each person's planets aspect the other's, the traditional relationship-compatibility technique.
Do we both need exact birth times for synastry?
No, planet-to-planet aspects work from birth date alone. You only need birth times if you want the house placements and angle contacts to be accurate.