Free tool
Synastry calculator
Every aspect between two birth charts, with real degrees instead of sun sign tables. The computation runs entirely in your browser: no account, no server, nothing stored.
How to read the result
The calculator compares ten bodies of one person against ten of the other and checks every pair for the five major aspects: conjunction (0°), sextile (60°), square (90°), trine (120°) and opposition (180°). A contact counts when the angle falls within its tolerance, the orb. The smaller the displayed orb, the more exact and the more tangible the contact.
Trines and sextiles read as flowing, squares and oppositions as challenging; conjunctions stay neutral, since their character depends on the planets involved. What matters is the pattern, not any single contact. A Saturn square is not a verdict; it is a place where commitment and constriction sit close together.
The same conventions as the app
Orbs are a question of school, so we disclose ours. The calculator uses exactly the graduated orbs of the Astraleria engine, documented on the accuracy page:
| Contact | Orb | Sextile |
|---|---|---|
| Sun or Moon involved | 8° | 6° |
| Personal point involved (Mercury, Venus, Mars) | 6° | 4.5° |
| Two slow movers (Jupiter to Pluto) | 4° | 3° |
Positions are geocentric apparent ecliptic longitudes of date, accurate to about one arcminute. The Ascendant, Midheaven and house overlays are deliberately absent here: they need the exact birth place, and a website without a server does not ask for one. In the app they are part of synastry.
Frequently asked questions
What is synastry?
Synastry overlays two birth charts and measures the angles between one person's planets and the other's. Every angle inside its tolerance (the orb) is a contact. No single zodiac sign decides anything; the pattern of contacts does.
Do I need the exact birth time?
For most contacts the date is enough: the Sun, Mercury, Venus, Mars and the slow planets move at most about one degree per day. Only the Moon travels around 13 degrees per day; without a birth time the calculator uses 12:00 noon and flags Moon contacts as uncertain (on average plus or minus 6 to 7 degrees).
Why doesn't the calculator ask for a birth place?
A birth place would require a location database and therefore a server. For planetary longitudes the time zone is enough, and that is what we ask for, resolved historically including daylight saving time on the birth date. The Ascendant, houses and house overlays need the exact place; the app computes those, on your device.
How accurate are the positions?
The calculator uses geocentric apparent ecliptic longitudes of date, accurate to about one arcminute. That is the same order of accuracy the app's engine documents against Swiss Ephemeris on the accuracy page. With orbs of 3 to 8 degrees, the margin is more than a hundredfold.
Which orbs does the calculator use?
Exactly the app's: contacts involving the Sun or Moon get 8 degrees, contacts involving a personal point (Mercury, Venus, Mars) 6 degrees, contacts between two slow movers 4 degrees; sextiles get three quarters of those values. Compare the app and this calculator and you will see the same contacts.
What does the 'generational' badge mean?
Uranus, Neptune and Pluto stay in one sign for years. An aspect between the outer planets of two people born close in age therefore describes the generation more than the couple. The calculator shows these contacts but labels them.
Is my birth data stored or transmitted?
No. The computation runs entirely in your browser; there is no server call and nothing is stored. If you share a result, the inputs live behind the # sign of the link, and that part of a URL never leaves the browser.