How Chinese zodiac compatibility works
The Chinese zodiac gives each year one of twelve animals, in a fixed order: Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog and Pig. Compatibility is read from where two animals stand in that cycle. Animals four years apart share a trio; certain pairs are secret friends; animals six years apart are opposites. Every pair falls into one of these bonds, or into none, which tradition reads as neutral: the people matter more than the animals.
The year runs from one Lunar New Year to the next, not from January to December. Someone born on January 20, 2000 is a Rabbit, because the Dragon year began on February 5, 2000. Charts that go by the calendar year get January and February birthdays wrong, so the calculator checks each date against the exact Lunar New Year of its year, worked out from the new moons and solar terms as the Chinese calendar defines them.
The trios of allies
The best matches are the four trios of animals four years apart: the Rat, Dragon and Monkey; the Ox, Snake and Rooster; the Tiger, Horse and Dog; and the Rabbit, Goat and Pig. Tradition reads each trio as sharing a temperament and a pace, so two animals of one trio understand each other without much effort. The circle below shows them as four triangles.
Secret friends, harms and clashes
Six pairs are called secret friends: the Rat and Ox, Tiger and Pig, Rabbit and Dog, Dragon and Rooster, Snake and Monkey, and Horse and Goat. Each pair is read as quietly supportive, each giving the other what it lacks. Six more pairs are called harms, read as prone to misunderstanding and hidden friction. The opposites, six years apart, are the clashes: two animals with opposite instincts that pull against each other.
The elements of the two years
Every year also carries one of five elements, Wood, Fire, Earth, Metal or Water, two years at a time. The elements of two birth years meet in one of two cycles. In the generating cycle, each element feeds the next: Wood feeds Fire, Fire makes Earth, Earth bears Metal, Metal gathers Water and Water nourishes Wood. In the controlling cycle, each holds the one after next in check: Wood parts Earth, Earth dams Water, Water puts out Fire, Fire melts Metal and Metal cuts Wood. The calculator says which applies to your two years.
Tendencies, not fate
Astrologers treat these bonds as tendencies. A clash is not a verdict and a trio is not a promise: Chinese astrology itself looks at the whole birth, the month, day and hour as well as the year, in the Four Pillars. For that deeper reading, the BaZi calculator gives each birth's four pillars, and for Western astrology's version of compatibility, with Sun, Moon, Venus and Mars signs, use the zodiac compatibility calculator.