Zodiac Compatibility Calculator

How this calculator works

Choose two Sun signs for a comparison that runs in your browser, with no lookup. Or enter two birth dates from 1900 to today. We look up Sun, Mercury, Venus and Mars positions for each date, then compare the signs we can identify.

Both modes report the same three things: whether the elements match, pair or differ, which modalities are involved, and how many signs apart the two placements sit. Each combination comes with a modern symbolic reading and a question you can ask each other. Any two signs work, in any combination, friendships included. If you want the fuller picture behind either date, the free birth chart calculator takes a time and place. Single placement tools sit in the calculators index, the Big 3 calculator covers Sun, Moon and rising together, and there are background notes on every sign in the zodiac sign guides.

Elements, modalities and the distance between signs

Every sign belongs to one of four families: fire, earth, air and water. Aries and Leo share the fire element, which modern astrology associates with enthusiasm and action. Aries and Cancer combine fire and water. There we use the pairing to ask about acting quickly and checking feelings first. Neither example tells us how two particular people behave. Modality adds another layer, since cardinal, fixed and mutable signs mark the beginning, middle and end of each season.

Distance matters too. Ptolemy's Tetrabiblos sets out four configurations, sextile at 60 degrees, square at 90, trine at 120 and opposition at 180, and sorts them into harmonious and disharmonious groups. We count whole signs rather than measuring degrees, so Aries and Libra come out as opposite signs. The discussion prompts we attach to each spacing are our own modern application, not Ptolemy's writing. A same sign result means the two signs match, and it doesn't mean the planets are close enough in degrees to form a conjunction.

When a birth date leaves room for doubt

We allow extra room for the unknown birth time and time zone. If that allowance reaches a sign boundary, we show both possible signs and skip that comparison. It doesn't mean the planet changed signs that day. If you have a reliable birth time and place, you can check the placement in the free birth chart calculator.

The positions come from Astrology API, calculated for noon UTC on each date. Around that reference we allow 26 hours in either direction, a margin set generously so that an unknown birth time or timezone is still covered. While building this we independently checked how far the Sun, Mercury, Venus and Mars can move, using PyEphem samples across dates from 1900 to 2100.

This tool compares Sun, Mercury, Venus and Mars signs. It doesn't calculate Moon compatibility or exact aspects, and rising sign and houses need a birth time and place. That makes it a sign level comparison rather than a whole chart reading, and it isn't a synastry calculation. For those layers, see Moon sign compatibility and our guide to free synastry charts, or read Venus and Mars one at a time.

What the result can tell you

Astrology works here as a symbolic language for reflection rather than a validated test of relationships. Helgertz and Scott compared Sun sign pairing rules against Swedish register data on marriages and divorces from 1968 to 2001, and found no consistent support for the pairings they tested (Genus, 2020). That study covered Sun sign rules and marriage and divorce records only. It's still a fair reason to hold any zodiac verdict loosely.

It's also why there's no percentage here. The distance between two signs is countable, but turning it into a score means choosing weights, and those weights are a rubric someone picked. Different scoring rules can produce different results for the same signs. Read the themes, try a prompt, and let what you already know about the relationship carry more weight than the symbolism.

The twelve signs at a glance

Compatibility calculator questions