How Easter's Date Is Calculated: The Computus Explained

Western Easter is the first Sunday after the ecclesiastical full moon that falls on or after March 21. In 2026 that church full moon fell on Thursday, April 2, so Easter Sunday was April 5. The rule always produces a date between March 22 and April 25, and the method for working it out has a name of its own: the computus.

The rule sounds astronomical, but it runs on tables rather than on observations of the sky. That is why Easter sometimes lands a day or two away from where a skywatcher would expect it, and why Orthodox churches, using older tables on the Julian calendar, often celebrate on a different Sunday. The table below this guide lists Easter, Good Friday and Ash Wednesday for every year from 1990 to 2035.

The Easter rule, phrase by phrase

Each part of the rule has a precise meaning:

• “On or after March 21.” The church fixes the spring equinox on March 21 every year. The astronomical equinox currently falls on March 19 or 20 in US time zones, as our guide to equinoxes and solstices shows, but that does not change the calculation.
• “Ecclesiastical full moon.” This is a date taken from the church’s lunar tables, not a measured moment. It approximates the real full moon and can differ from it by a day or two. The first such date on or after March 21 is called the Paschal full moon.
• “The first Sunday after.” Easter must come after the Paschal full moon, never on it. If the Paschal full moon is itself a Sunday, Easter moves to the following Sunday.

That last point decided Easter 2025. The Paschal full moon fell on Sunday, April 13, 2025, so Easter was a week later, on April 20.

What the Council of Nicaea did and did not decide

The Council of Nicaea in 325 is often credited with the Easter rule. The council wanted churches to keep Easter on the same day as Rome and Alexandria, but it did not publish an explicit rule for finding that date. The March 21 equinox became attached to Nicaea through later tradition. Pope Gregory XIII’s 1582 calendar bull, for example, describes its goal as returning the equinox to March 21, where it said the council had placed it.

The working method came from Alexandria, where the date was tied to a 19-year lunar cycle. Dionysius Exiguus published a new Easter table in 525 using that reckoning, and two centuries later, in 725, the English scholar Bede described the Dionysian system in full. Gregory’s reform in 1582 kept the framework but corrected the tables.

How the church full moon is calculated

The computus combines three pieces.

The 19-year lunar cycle. Nineteen years contain almost exactly 235 lunar months: 19 × 365.2422 days is 6,939.6 days, while 235 months of about 29.5306 days come to 6,939.7 days. Moon phases therefore return to nearly the same calendar dates every 19 years. A year’s place in this cycle is its Golden Number: divide the year by 19 and add 1 to the remainder. For 2026, 2026 ÷ 19 leaves a remainder of 12, so the Golden Number is 13.

The epact. The epact is the age of the moon, in days, at the start of the year according to the church’s tables. Because 12 lunar months total about 354.4 days, roughly 11 days short of a calendar year, the epact usually advances by 11 from one year to the next. The 2026 epact is 11.

The Gregorian corrections. The 1582 reform added two adjustments that the older Julian tables lack. The solar correction shifts the lunar tables by one day whenever a century year skips its leap day, as 1700, 1800 and 1900 did. The lunar correction adds a day eight times in every 2,500 years to absorb the small error in the 19-year cycle itself.

Together these steps place the Paschal full moon somewhere from March 21 to April 18.

Why Easter falls between March 22 and April 25

Both limits follow directly from the rule:

• Earliest, March 22. The Paschal full moon can fall as early as March 21. If that day is a Saturday, Easter is the next day. This happened in 1818 and will not happen again until 2285.
• Latest, April 25. The Paschal full moon can fall as late as April 18. If April 18 is a Sunday, Easter must wait a full week. That happened in 1943 and will happen again in 2038.

That leaves 35 possible dates. In the 21st century the earliest Easter was March 23, 2008, and the latest will be April 25, 2038. Because the rest of the spring church calendar is counted from Easter, Mardi Gras (47 days before), Ash Wednesday (46 days before) and Good Friday (2 days before) move with it.

Worked example: Easter 2026 with the anonymous Gregorian algorithm

The table method can be compressed into integer arithmetic. The best-known version was sent to the journal Nature in 1876 by an anonymous correspondent in New York. It was later reprinted by Samuel Butcher, Spencer Jones and Jean Meeus, which is why it is also called the Meeus/Jones/Butcher algorithm. The Royal Observatory of Belgium publishes the same steps as a short computer routine.

In each step, “÷” means whole-number division (drop the remainder) and “mod” means the remainder after division. For the year Y = 2026:

Step	Formula	2026 value
a	Y mod 19	12
b	Y ÷ 100	20
c	Y mod 100	26
d	b ÷ 4	5
e	b mod 4	0
f	(b + 8) ÷ 25	28 ÷ 25 = 1
g	(b − f + 1) ÷ 3	20 ÷ 3 = 6
h	(19a + b − d − g + 15) mod 30	252 mod 30 = 12
i	c ÷ 4	6
k	c mod 4	2
l	(32 + 2e + 2i − h − k) mod 7	30 mod 7 = 2
m	(a + 11h + 22l) ÷ 451	188 ÷ 451 = 0
month	(h + l − 7m + 114) ÷ 31	128 ÷ 31 = 4
day	((h + l − 7m + 114) mod 31) + 1	4 + 1 = 5

Month 4, day 5: Easter Sunday 2026 was April 5.

Each variable stands for a piece of the church rule:

• a is the year’s place in the 19-year cycle; a + 1 = 13 is the Golden Number.
• b, d, f and g apply the Gregorian solar and lunar corrections for the century.
• h counts the days from March 21 to the Paschal full moon. With h = 12, the church full moon was April 2, a Thursday.
• e, i, k and l handle the weekday. The value l + 1 is the number of days from the Paschal full moon to the next Sunday. Here l + 1 = 3, and three days after Thursday, April 2 is Sunday, April 5.
• m is a rare correction for two special cases in the tables, which would otherwise push the Paschal full moon too late. Between 1900 and 2100 it equals 1 only in 1954, 1981, 2049 and 2076.

A second check with 2027 gives a = 13, h = 1 and l = 5. The Paschal full moon is March 21 + 1 = Monday, March 22, and l + 1 = 6 days later is Sunday, March 28, 2027. You can see both years in the 2026 calendar and the 2027 calendar.

From the 2026 result, Good Friday was April 3 and Ash Wednesday was February 18, 46 days before Easter Sunday.

Western Easter and Orthodox Easter

Eastern Orthodox churches use the same basic definition but apply it with the Julian calendar and the older lunar tables, without the 1582 corrections. They do this even in countries whose civil calendar is Gregorian. Since 1900 the Julian calendar has run 13 days behind, so the Orthodox March 21 equinox corresponds to April 3 on a modern calendar. The older lunar tables have also drifted, and they put the church full moon a few days later than the Western tables do.

From 1900 through 2099, Orthodox Easter either matches Western Easter or comes one, four or five weeks later, and it always falls between April 4 and May 8 on the Gregorian calendar. In the 41 years from 2000 through 2040, the two coincide 13 times.

Year	Western Easter	Orthodox Easter
2024	March 31	May 5
2025	April 20	April 20
2026	April 5	April 12
2027	March 28	May 2
2028	April 16	April 16

The shared date in 2025 fell in the 1,700th anniversary year of the Council of Nicaea. Proposals to unify the dates have not taken hold. In 1923 the Greek Orthodox Church agreed to use the astronomical full moon at the meridian of Jerusalem, but the system saw only sporadic use in the 1920s. A 1997 World Council of Churches consultation in Aleppo proposed a similar astronomical method, and no church has adopted it.

The cycles also differ in length. The Julian Easter sequence repeats every 532 years, the product of the 19-year lunar cycle and the 28-year Julian weekday cycle. The Gregorian corrections stretch the full Gregorian sequence to 5,700,000 years.

Checking an Easter date yourself

For any year, a quick verification takes four checks:

1. Confirm the result is a Sunday.
2. Confirm it lies between March 22 and April 25.
3. Find the Paschal full moon (March 21 + h) and confirm Easter is the first Sunday strictly after it.
4. Count back 2 days for Good Friday and 46 days for Ash Wednesday, and forward 49 days for Pentecost.

Passover often falls within days of Easter, but it follows the Hebrew calendar and its own rules, so the two can separate by about a month in some years. The church full moon and the real one also drift apart slightly, and our guide to moon phases explains the astronomical cycle the tables approximate. For the calendar reform behind the modern tables, see the history of the Gregorian calendar.