Easter Date (Computus) Calculator
Western Easter Sunday is the first Sunday after the ecclesiastical full moon falling on or after 21 March. Rather than tracking the real moon, the church fixes the full moon with a table, so Easter can be computed exactly from the year alone. This calculator uses the Anonymous Gregorian Computus, the most widely cited closed-form method, to return the precise month and day of Gregorian Easter for any year you enter. It is deterministic with no estimation: the same year always yields the same date, which can fall anywhere from 22 March to 25 April.
Computus (Anonymous Gregorian) algorithm
a = Y mod 19
b = floor(Y / 100), c = Y mod 100
d = floor(b / 4), e = b mod 4
f = floor((b + 8) / 25), g = floor((b - f + 1) / 3)
h = (19a + b - d - g + 15) mod 30
i = floor(c / 4), k = c mod 4
L = (32 + 2e + 2i - h - k) mod 7
m = floor((a + 11h + 22L) / 451)
month = floor((h + L - 7m + 114) / 31)
day = ((h + L - 7m + 114) mod 31) + 1
Month 3 is March and month 4 is April. The procedure is exact for the Gregorian calendar and returns the same result every time for a given year.
Easter dating context
- Western Easter always lands on a Sunday between 22 March and 25 April inclusive.
- The "full moon" used is the ecclesiastical full moon from a fixed table, not the astronomical moon.
- This tool gives Gregorian (Western) Easter; Orthodox Easter uses the Julian Computus and often differs.
- The 19-year Metonic cycle underpins the lunar part of the calculation.
- The day-of-year output uses the Gregorian leap-year rule, so leap years shift it by one after February.
Easter date: frequently asked questions
How is the date of Easter calculated?
Western Easter is the first Sunday after the ecclesiastical full moon that occurs on or after 21 March. The ecclesiastical full moon is a fixed table value, not the astronomical moon. The standard method is the Computus, commonly implemented as the Anonymous Gregorian algorithm, which uses only the year to produce the exact month and day of Easter Sunday.
What range of dates can Easter fall on?
In the Gregorian calendar, Easter Sunday can fall on any date from 22 March to 25 April inclusive. The earliest (22 March) and latest (25 April) extremes are rare. Most years it lands in early to mid April.
Does this calculate Orthodox Easter?
No. This tool computes Western (Gregorian) Easter as observed by Roman Catholic and most Protestant churches. Eastern Orthodox churches use the Julian calendar for their Computus, which usually gives a different date. This calculator implements the Gregorian method only.
Is the algorithm exact?
Yes. The Anonymous Gregorian Computus is an exact, deterministic algorithm: there is no rounding or estimation. Given a four-digit year it returns the precise calendar date the Western church assigns to Easter Sunday for that year.
Why does Easter move each year?
Because it is tied to a lunar cycle (the ecclesiastical full moon) laid over the solar calendar. The lunar month is about 29.5 days and does not divide evenly into the year, so the full moon falls on a different solar date each year, and the following Sunday moves with it.
Official sources
- U.S. Naval Observatory: Astronomical Applications Department.
- National Institute of Standards and Technology: Time and Frequency Division.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.