What BaZi is
BaZi (八字, "eight characters") is the birth chart of Chinese astrology, also called the Four Pillars of Destiny. It writes the moment of birth as four pillars, the year, month, day and hour, and each pillar as two characters of the sixty-cycle: a heavenly stem on top and an earthly branch below. The eight characters are read through the five elements, Wood, Fire, Earth, Metal and Water, and yin and yang, to describe a person's nature and the rhythm of their life.
The four pillars
The year pillar describes the family and the social background you were born into, and gives the animal most people know as their Chinese sign. The month pillar stands for parents and the season of your birth, and is where the luck pillars begin. The day pillar is you: its stem is your Day Master, and its branch is often read as your partner. The hour pillar covers children, later life and your private hopes. Tradition reads the chart from the day pillar outward.
Stems, branches and hidden stems
The ten heavenly stems are the five elements, each in a yang and a yin form: 甲 and 乙 are Wood, 丙 and 丁 Fire, 戊 and 己 Earth, 庚 and 辛 Metal, 壬 and 癸 Water. The twelve earthly branches are the twelve animals of the zodiac, each with an element of its own. Every branch also holds one to three hidden stems, the elements stored inside it: the Ox (丑) holds Earth, Water and Metal, for instance, so a chart has more going on than its eight visible characters show.
Your Day Master
The stem of the day pillar is the Day Master, the character BaZi reads as the self. Tradition gives each of the ten an image: yang Wood is the tall tree, yin Wood the vine and the flower, yang Fire the sun, yin Fire the candle flame, yang Earth the mountain, yin Earth the garden soil, yang Metal the sword and raw ore, yin Metal the jewel, yang Water the ocean and yin Water the rain and dew. Whether the chart supports or drains the Day Master, through the elements around it, is the first question a reading asks.
The five elements and the Ten Gods
The five elements feed and control one another in two cycles: Wood feeds Fire, Fire makes Earth, Earth bears Metal, Metal carries Water and Water feeds Wood; Wood parts Earth, Earth holds Water, Water puts out Fire, Fire melts Metal and Metal cuts Wood. The calculator counts the elements among your stems and branches. It also names each stem's relation to your Day Master, one of the Ten Gods. The Ten Gods come from whether a stem is the Day Master's own element, one it produces, one it controls, one that controls it or one that produces it, and whether the two share yin or yang. Each god stands for a part of life: the Direct Officer for order and status, the Eating God for creativity and enjoyment, Direct Wealth for earned income, and so on.
Luck pillars
After the four pillars come the luck pillars, ten-year periods that tradition reads as the changing weather of a life. They continue the sixty-cycle from the month pillar, forward or backward, and begin at an age set by the distance from the birth to the nearest solar term: every three days count as a year. The calculator lists eight, marks the one you are in now and says when each begins.
How the pillars are calculated
The year begins at Lichun, when the Sun reaches 315° along the ecliptic, and each month at the next of the twelve solar terms that follow, 30° apart. The calculator finds each term with the site's own ephemeris, the one behind every chart here. Days follow the unbroken sixty-day count, which has run through the Chinese calendar for over two thousand years and put 甲子 on October 1, 1949. The hour takes its branch from the two-hour period and its stem from the day's stem. The site's tests compare the calculator with the lunar-python calendar library for 300 random births from 1901 to 2098: every pillar agrees, under both day conventions, and so do the direction and order of the luck pillars, with the starting age within a month. With solar time switched on, the hours are read from the Sun's position over your birthplace (the equation of time included) instead of the clock.