Enter your birth data above, then invite the other person or type in theirs. NatalChart.AI calculates both charts and compares them on two layers. The first is the five major aspects between your bodies and theirs, with synastry orbs of 6° for a conjunction, square, trine and opposition and 4° for a sextile, tighter than the natal 8° and 6°. The second is the house overlays: which of their houses your planets fall in, and which of yours theirs fall in. From the two layers each of the six life spheres gets a score from 1 to 10, and the reading names the aspects behind it. It works for friendship, family and collaboration as much as for romance. Compatibility is part of Premium; a person invited by a Premium member sees the shared reading free.
What is calculated
- Aspects between the two charts: one person’s body against the other’s, five major aspects, each with its orb in degrees. The 17 bodies and points of each chart take part.
- House overlays: which of their houses your planets fall in, and which of yours theirs fall in. These need a birth time for the person whose houses are used.
- Six sphere scores: Identity, Love, Body, Money, Home and Mind, each from 1 to 10, from a fixed rule that weighs the aspects (60%) and the overlays (40%) relevant to that sphere. The same two charts always give the same numbers.
- The reading, written afterwards from those numbers, naming actual aspects, for example “your Venus opposite their Saturn, orb 1.2°”.
Why the orb is named: a square 0°30′ from exact and a square 7°30′ from exact are both called a square, and they are not the same thing. The closer one is far stronger, so each aspect here comes with its orb in degrees. How orbs work
What it can do, and what it cannot
- It can show where two charts meet with ease and where with friction, sphere by sphere.
- It can name the contacts behind a score, so you can see why a number is what it is.
- It cannot say whether a relationship will last or whether two people should be together. A score is not a probability of anything.
- It does not calculate aspects to the Ascendant or Midheaven between the charts, minor aspects, a composite chart or a Davison chart.
- Without a birth time for one person, that person’s houses are undetermined, because houses depend on the minute of birth. The aspects between the charts still stand; anything about that person’s houses does not, and the API leaves those overlays out.
Why chart-grounded matters here
Generic AI compatibility tools usually run a sun-sign lookup or improvise plausible text from two names. Both are nearly useless: dozens of pairs of charts share the same two suns and play out completely differently in real life.
Synastry depends on calculation. The Venus-Mars aspect, the Saturn-Moon contact, whose Pluto falls in whose 8th house: these are calculated facts that change the whole picture. NatalChart.AI calculates them with Swiss Ephemeris and writes the reading from those calculations, as described in Chart-Locked AI Reading.
Frequently asked questions
What is synastry?
Synastry is the astrological practice of comparing two natal charts to study the relationship between them. It looks at inter-chart aspects (e.g. one person's Venus aspecting the other's Mars) and house overlays (which of your partner's houses your planets land in, and vice versa).
How is this different from sun-sign matching?
Sun-sign matching only compares two zodiac signs out of dozens of factors. Synastry compares the planets and houses of both charts, with the aspects between them and their orbs. The picture is far richer and far more specific to the two actual people.
What does NatalChart.AI's synastry actually compute?
The five major aspects between the bodies of one chart and the bodies of the other: conjunction, sextile, square, trine and opposition. The orbs are tighter than in a natal chart: 6° for a conjunction, square, trine and opposition and 4° for a sextile, against 8° and 6° natally. Then the house overlays: which of the other person's houses your planets fall in, and which of yours theirs fall in. From those two layers each of the six spheres gets a score from 1 to 10. Both charts are calculated separately with Swiss Ephemeris before any comparison runs. Aspects to the angles, composite charts and Davison charts are not calculated.
Can the AI tell me if we're compatible?
It can describe how the two charts interact: where there is natural ease, where there is friction, what each person tends to bring out in the other. It does not predict whether a relationship will last, and there is no yes or no verdict.
Do we both need exact birth times?
Inter-chart aspects work even without exact times for both people, but house overlays and angles (Ascendant, Midheaven) need exact times to be meaningful. The reading marks which factors are time-sensitive, and exact times sharpen the interpretation considerably.
Can I calculate synastry with a friend's chart?
Yes. Synastry is not only for romance: it works just as well for friendship, family and collaboration. Any two people with birth data can be compared, and the reading describes how the two charts meet rather than assuming a romantic frame.
Do both people need exact birth times?
An exact birth time sharpens the Moon, Ascendant, houses and the house overlays between the two charts. Without it, planet-to-planet aspects between the two charts still work and remain meaningful, but the house overlays are less certain. The reading flags which factors depend on birth time.
What is the difference between synastry and a composite chart?
Synastry compares person A's chart directly to person B's: the aspects between the two charts and the house overlays. A composite chart instead builds a single midpoint 'relationship chart' from both charts and reads it as one entity. NatalChart.AI currently supports synastry (two-chart comparison); it does not compute a composite chart.
Can AI tell whether we are compatible?
It maps strengths and friction across life spheres using the actual inter-chart aspects between the two charts, and shows a score from 1 to 10 for each sphere. The numerical score is calculated from defined chart factors and rules; it is not a probability of an event or a relationship outcome, and there is no verdict or soulmate guarantee. The AI explanation is a symbolic interpretation of those factors.