Birth chart compatibility: synastry and aspect calculator
Compare two known zodiac positions, check the angle and orb, and learn what a synastry chart can and cannot say about a relationship.
Check one cross-chart aspect
Enter positions from charts using the same zodiac. This tool compares one pair; it does not generate birth charts. Entries stay in this page, with no account or saved history.
How to use the tool
Take one planetary position from each chart, such as Venus at 29° Pisces and Mars at 1° Aries. Select each sign and enter its degrees. Keep a note of which bodies you compared: this tool calculates geometry and does not infer their identity. If your source uses degrees and minutes, convert minutes to a fraction of a degree: 15°30′ becomes 15.5°.
Worked example: the zodiac wraps around
Pisces starts at 330°, so 29° Pisces is 359°. Aries starts at 0°, so 1° Aries is 1°. The direct difference is 358°, but the smaller angle is 360° − 358° = 2°. With an orb of 3° this is classified as a conjunction; with an orb of 1° it is outside the limit. Try the example button, then change the orb.
Exactly what is calculated
Each sign contributes its starting longitude: Aries 0°, Taurus 30°, and so on. Add the entered degrees, calculate the smaller separation from 0° to 180°, and find the nearest of 0°, 60°, 90°, 120° and 180°. A match requires the deviation to be no greater than your orb. The default 3° is a teaching setting, not a universal standard. Minor aspects, planetary motion and applying or separating status are not calculated.
Five major angles, one transparent rule
| Aspect | Exact angle | A reflection question, not a prediction |
|---|---|---|
| Conjunction | 0° | Where do our priorities overlap, and where do we assume too much similarity? |
| Sextile | 60° | What small opportunity for cooperation could we choose to act on? |
| Square | 90° | Which difference needs a concrete agreement instead of a fixed label? |
| Trine | 120° | What feels easy, and what still needs attention rather than being taken for granted? |
| Opposition | 180° | How can we make room for two perspectives without requiring either person to disappear? |
Why two Sun signs are not a full compatibility analysis
Synastry compares positions between two charts; a same-sign label loses the degrees and ignores other placements. Two planets at opposite ends of one sign can be almost 30° apart. Conversely, planets in neighbouring signs may be close across a boundary. A single aspect is only one entry in a chart comparison, not an overall verdict.
Data quality comes before interpretation
Use the same zodiac convention for both charts. Keep each person’s birth-time uncertainty visible, especially for the Moon, Ascendant and houses. If an entered position is uncertain, the aspect may be uncertain too. This tool accepts supplied longitudes; it cannot verify birth records, calculate missing planets, or establish house overlays.
Synastry, composite and house overlays
Synastry compares the two sets of natal positions. A composite uses a separate construction to represent a relationship chart; it is not simply the list of cross-chart aspects. House overlays examine where one chart’s positions fall within the other chart’s houses. The calculator here handles only one angular comparison, not either of those additional methods.
Make the reading useful outside astrology
Choose one reflection question below and answer it separately before comparing notes. Replace “you always do this because of your sign” with one recent observation and a specific request. Agree on a small action and a date to revisit it. Shared values, consent, practical circumstances and behaviour provide information a chart cannot measure.
Method, sources and limits
Astrodienst’s partner-chart FAQ documents chart comparison and differing orb conventions. Our angle formula, numerical examples and conversation questions are original explanatory material. Astrology is a cultural and symbolic practice, not an established scientific test of relationship success. Reviewed 8 November 2026.
Astrodienst · Sources and method
Does a trine guarantee a good relationship?
No. A geometric label does not establish trust, shared values or how people behave. The questions are optional conversation prompts.
Does a square mean we should separate?
No. Do not use an aspect to decide whether to stay, leave or excuse harmful behaviour. Real circumstances and your boundaries matter.
Why does another calculator show a different aspect?
Check exact degrees, zodiac convention, selected aspects and orb. A stricter orb can remove a match without changing the underlying angle.
Can I calculate full synastry from two birthdays here?
No. This tool needs two already calculated positions. It does not calculate a full chart from birth dates, times and places.