Easter Calculator
Easter Sunday calculator for any year 1-9999 AD using the Computus algorithm. Supports Gregorian and Julian calendars for Western and Eastern Orthodox Easter.
About This Calculator
The Easter Sunday Calculator computes the exact date of Easter for any year from 1 to 9999 AD using the classic Computus algorithm. It supports both the Gregorian calendar (used by Western Christian churches) and the Julian calendar (used by Eastern Orthodox churches), making it useful for planning liturgical calendars, holidays, and family gatherings worldwide.
The algorithm, known as the Anonymous Gregorian Computus, works through a series of modular arithmetic steps using the Golden Number (19-year Metonic cycle), solar cycle corrections, and century-based adjustments. The result is always a Sunday between March 22 and April 25 (Gregorian) or the equivalent Julian date for Orthodox calculations.
Understanding the Computus Algorithm
The Computus algorithm uses 12 intermediate variables (a through m) derived from the input year. Variable a represents the Golden Number (year mod 19), variables b through g handle century corrections for the Gregorian reform, h determines the ecclesiastical full moon date, L finds the first Sunday after it, and m ensures the date falls in the correct month. The final month (3 for March, 4 for April) and day are computed from h, L, and m.
Regional Notes
India: Easter is observed by Christian communities nationally. Both Western (Gregorian) and Eastern Orthodox (Julian) dates are relevant depending on the church denomination. The calculator supports both calendars.
United States: Easter is a widely observed Christian holiday with no fixed date. The US follows the Gregorian calendar for Easter, with the date computed using the standard Western Computus formula. Many schools and businesses close on Good Friday and Easter Monday.
United Kingdom: Easter Monday and Good Friday are public holidays in the UK. The Church of England follows the Gregorian Computus, and Easter dates are published years in advance for planning school terms and bank holidays.
Frequently Asked Questions
What is the Computus algorithm for calculating Easter?
The Computus is the mathematical algorithm used to determine the date of Easter Sunday. Easter falls on the first Sunday after the first ecclesiastical full moon following the vernal equinox (March 21). The Anonymous Gregorian algorithm uses variables a through m derived from the year to compute the exact month (March or April) and day. The algorithm incorporates the 19-year Metonic cycle, the solar cycle, and the Gregorian calendar correction factors.
Why does the date of Easter change every year?
Easter is a moveable feast because it is tied to the lunisolar calendar rather than the fixed solar calendar. It is defined as the first Sunday after the first full moon on or after the vernal equinox (March 21). Since the lunar cycle is approximately 29.5 days and the solar year is 365.25 days, Easter can fall anywhere between March 22 and April 25 in the Gregorian calendar. This variability distinguishes it from fixed-date holidays like Christmas.
What is the difference between Gregorian and Julian Easter dates?
Western Christian churches (Catholic and Protestant) use the Gregorian calendar for Easter calculations, while many Eastern Orthodox churches use the older Julian calendar. The Julian Computus formula uses h = (19a + 15) mod 30 instead of the Gregorian correction that accounts for century leap-year rules. As a result, Orthodox Easter often falls on a different Sunday than Western Easter, sometimes weeks later. In 2025, for example, both calendars agree on April 20, but in 2024 Western Easter was March 31 while Orthodox Easter was May 5.
What is the earliest and latest possible date for Easter?
In the Gregorian calendar, Easter can fall as early as March 22 and as late as April 25. March 22 is extremely rare -- the last time was in 1818 and the next will be in 2285. April 25 is also rare, occurring last in 1943 and next in 2038. The most common dates for Easter fall in early to mid-April. The Julian calendar used by Eastern Orthodox churches has a slightly different range due to the 13-day offset between the calendars.
How is the Golden Number used in the Easter calculation?
The Golden Number is the year's position in the 19-year Metonic cycle, which describes the alignment of lunar and solar calendars. It is calculated as a = year mod 19, ranging from 0 to 18. This number is a critical input to the Computus algorithm because it determines the phase of the moon for any given year. The term Golden Number dates back to medieval computus tables where the number was written in gold in church calendars.
What is the significance of the vernal equinox in Easter dating?
The vernal (spring) equinox is fixed on March 21 for ecclesiastical purposes, regardless of the astronomical equinox. Easter must occur on the first Sunday after the first full moon on or after this date. This convention was established by the First Council of Nicaea in 325 AD to standardize Easter dating across all Christian churches and to ensure it remained in spring. The ecclesiastical full moon dates are determined by tables rather than astronomical observation.
How does the calculator handle years before 1582?
The Gregorian calendar was introduced in October 1582. For years before 1582, the Gregorian Computus provides the date Easter would have been if the Gregorian calendar had been in use (proleptic Gregorian). The Julian Computus provides the historical Julian date as it would have been calculated at the time. Users studying historical Easter dates should select the Julian calendar for pre-1582 dates to match contemporary records, or Gregorian for modern retrospective calculations.
Can I calculate Easter for multiple years at once?
The calculator shows Easter dates for the 11-year window centered on your input year (5 years before and 5 years after), displayed in the Date Distribution bar chart. For a specific year, the result section shows the exact Easter Sunday date along with the complete Computus variable breakdown. You can bookmark or share any calculation by copying the URL, which automatically saves the input year and calendar selection.