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 give a score out of 100 for love, sex, communication, friendship and marriage. Each pair of signs then gets a reading of 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, from the Aries guide to the Pisces guide.

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.

We calculate each position at noon UTC using Swiss Ephemeris, which is built on NASA JPL planetary and lunar ephemerides. To cover the unknown birth time and time zone, we allow 26 hours either side of that noon and check whether the planet could have crossed a sign boundary within that window. We use the planet's actual speed on that date when available, with a conservative fallback otherwise. Our calculation sources and precision notes explain where the data comes from and how precise it is.

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. To match two birthdays by the Moon's phase instead, the moon phase soulmate calculator shows how your two birth moons fit together.

How the compatibility score works

We give points for the distance between the two signs, their elements, their modalities and their ruling planets, using the same traditional rules for every pair. A trine or sextile starts high and a square or quincunx low. Each area weighs the four differently, so an opposition scores high for sex and lower for friendship. The overall score is the average of the five areas.

With two birth dates, each area reads the planets tied to it: love and marriage read your Venus signs, sex your Mars and Venus signs, communication and friendship your Mercury signs, and every area except sex reads your Sun signs too. Astrology is a symbolic language, and research hasn't confirmed Sun sign pairing rules: Helgertz and Scott tested them against Swedish marriage and divorce records from 1968 to 2001 and found no consistent support (Genus, 2020).

Compatibility guides by sign

Start with a sign hub, then open the pairing you want to explore. Each guide has a preset calculator, a practical example and a date-sample count showing how Venus and Mercury can differ within the same Sun pairing.

The twelve signs at a glance

Compatibility calculator questions