Calculation notes
Four Pillars calculators differ in a few conventions, so the same birthday can give different results. These are the conventions this calculator uses.
What the calculation is based on
- Solar terms (year and month pillars): a table of solar-term instants computed astronomically (1899–2101), checked against two independent astronomical calculations to within 2 minutes for every term.
- Day pillar: the continuous 60-day cycle, checked for every date 1900–2100 against another calendar library.
- Birth time: historical standard time and daylight saving from the IANA time-zone database.
- Lunar calendar: the Korean lunar calendar (KASI, 1900–2050), with leap months.
- Ten Gods, hidden stems, 12 stages: the standard tables; hidden stems in the order residual · middle · main qi.
No person or AI adjusts the results. Where calculators use different conventions, this page says so, and the result screen tells you when it matters.
When the year changes (year pillar)
The year starts at the instant of 立春 (Start of Spring), not on 1 January or the lunar new year.
When the month changes (month pillar)
Months change at the instants of the twelve 節 solar terms, not on the first lunar day. A solar term is the same instant everywhere on Earth, so births abroad are compared with that instant. If the birth time is within 30 minutes of a term, the result screen tells you (because of possible error in the recorded time).
Time basis (hour and day pillars)
- Births in Korea (default): by the Korean standard-time clock with hours changing at :30 (Zi hour 23:30–01:29 …), because Korean standard time (135°E) runs about 30 minutes ahead of the sun in Korea. Former offsets and daylight saving are handled automatically.
- Births abroad (default): true solar time from the city’s longitude and the equation of time (Zi hour 23:00–00:59 …).
- Which basis is right depends on the school. For example, 1:20 pm in Seoul is the 午 hour with the Korean 30-minute convention and the 未 hour on the clock as is. Whenever the basis changes the hour or day pillar, the result screen shows each result.
- In Advanced settings you can use true solar time or the recorded clock time for Korean births too, and enter the time zone and longitude yourself.
Zi hour (子時) and the date
The default is the unified Zi hour: the next day begins when the Zi hour begins (11:30 pm in Korea). With the split Zi hour the day changes at midnight, and only the hour stem of the Zi hour before midnight follows the next day. If the two give different day pillars, both are shown.
Daylight-saving changes
If you enter a clock time that did not exist when daylight saving began, you are asked to check it. On days when the same clock time occurred twice, the earlier one (daylight time) is used.
Unknown birth time
No hour pillar or hour-related values are produced. Year, month and day pillars are taken at noon; if a solar term changed that day, both candidates are shown.
Luck cycles
- Forward for a yang-year male or yin-year female, backward otherwise (the year pillar counts from 立春).
- The starting age is the number of days from birth to the next (forward) or previous (backward) solar term divided by 3, rounded. If truncation would give a different number, that is shown too.
- Annual luck is the year pillar of each year (from 立春).
Zi Wei Dou Shu
- Time basis (default: standard-time hours): births in Korea use the Korean standard time (UTC+9) clock with no longitude correction. Hours change on odd o’clock (子 23:00–00:59, 巳 09:00–10:59 …). Clock times from the 1954–61 UTC+8:30 period and from daylight saving are converted to UTC+9. Births abroad use precise true solar time from the birthplace longitude and the equation of time. On the result screen you can switch to precise true solar time, Korean time with a 30-minute shift, or the clock as is (Korean lunar calendar); if the chart changes, you are told.
- Lunar date and leap months: the default uses the Chinese lunar calendar (120°E). A leap month keeps its own number. For births from day 16 of a leap month, the result if it counted as the next month is shown too.
- Star placement: the Life and Body palaces start from 寅, move forward by the birth month, then the Life palace moves back and the Body palace forward by the birth hour. Palace stems follow 五虎遁; the element pattern (五行局) is the 納音 of the Life palace. 紫微 is placed from the birth day and the pattern number, the other 13 main stars by the 紫微/天府 spacing rules, and the minor stars (左輔, 右弼, 文昌, 文曲, 地空, 地劫, 天魁, 天鉞, 祿存, 擎羊, 陀羅, 天馬, 火星, 鈴星) by their birth month, hour or year rules.
- Transformations (生年四化), in 祿·權·科·忌 order: 甲 廉貞 破軍 武曲 太陽; 乙 天機 天梁 紫微 太陰; 丙 天同 天機 文昌 廉貞; 丁 太陰 天同 天機 巨門; 戊 貪狼 太陰 右弼 天機; 己 武曲 貪狼 天梁 文曲; 庚 太陽 武曲 太陰 天同; 辛 巨門 太陽 文曲 文昌; 壬 天梁 紫微 左輔 武曲; 癸 破軍 巨門 太陰 貪狼. Some schools use other tables for 庚 and 壬 years.
- Brightness (廟旺得利平陷): the 「諸星廟旺利陷表」 of 《紫微斗數全書》 vol. 2; where vol. 3 grades a cell differently (天機 in 辰, 文曲 in 寅) both are shown, e.g. ‘利/旺’. Comparison with an original facsimile edition is not yet complete, and some cells differ from other calculators.
- Decades, annual and monthly charts: decades run from the Life palace, forward for yang-year men and yin-year women and backward otherwise, the first starting at the pattern number (East Asian age); before it the screen says “before the first decade (童限)”. The annual chart uses that year’s stem and branch: the palace of the year’s branch is the annual Life palace and the year’s stem gives the annual transformations. Annual age is East Asian reckoning from the solar birth year. Monthly charts (流月) start month 1 at the annual Life palace and move one palace per month by default; you can switch to the Dou Jun (斗君) method on the result screen (schools differ).
- Without a birth time no chart is drawn.
Why Four Pillars and Zi Wei use different hours
- For the Four Pillars hour, the usual practice divides hours by where the sun is at the birthplace, so births in Korea get a 30-minute correction by default (the difference between the 135°E standard-time meridian and Korea’s actual longitude); births abroad use true solar time.
- For Zi Wei, both practices are in use: with and without a longitude correction. The default here uses no correction (the Korean standard time clock as is, hours changing on odd o’clock), and the corrected chart is one switch away on the result screen. Which is right depends on the school.
- So the two tabs can pick different hours for the same birth time. Example: Cheonan, 1984-12-06 09:15 is 08:45 after the 30-minute correction, the 辰 hour (07:30–09:29) for the Four Pillars, and 09:15 standard time, the 巳 hour (09:00–10:59), for Zi Wei.
Natal chart (Western astrology)
- Tropical zodiac, geocentric. Ephemeris: astronomy-engine (VSOP87-based), 1900–2099.
- Houses: Placidus (default), Whole Sign or Equal.
- Compared with an independent ephemeris (PyEphem), planets and Moon are within 0.005° and ASC and houses within 0.01°. Bodies within 0.05° of a sign boundary are marked ‘cusp’.
- Without a birth time only noon positions are shown, with no ASC, MC or houses.
Copy for AI
The copied text contains the calculated values and the conventions used, with a note asking the AI not to recalculate. The AI does the interpreting; this service does not write interpretations.
Input range and limits
- Solar 1900–2100, lunar 1900–2050.
- Lunar dates before 1912 are unverified. The lunar tables use 120°E (UTC+8) to start months up to 1911 and 135°E (UTC+9) from 1912. Eight months in 1900–1911 start a day apart depending on the meridian; for those, both solar dates and day pillars are shown. If you know your solar birthday, please enter that.
- Time-zone records before 1970 for some regions depend on the accuracy of the international time-zone data. If your birth record differs, correct it in Advanced settings.
- City positions are city-centre coordinates. If you need the exact longitude of the hospital, enter it yourself.