Computus (Latin for "computation") is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age.

In principle, the date of Easter is defined as the Sunday following the Full Moon following the Northward equinox (the so-called Paschal Full Moon).

But neither the vernal equinox nor the full moon are determined by astronomical observation. Instead, a theoretical model known as the Ecclesiastical Full Moon is used, and the vernal equinox is taken to be set on 21 March. The model assumes that 19 tropical years have the same duration as 235 synodic months (modern value: 234.997).

The computus is thus the procedure of determining the first Sunday after the first ecclesiastical full moon that falls on or after 21 March.

Since the 16th century, there have been differences in calculation in the Western and Eastern Churches. Eastern Orthodox Christians use an ecclesiastical full moon that occurs four to five days later than the western ecclesiastical full moon. The Roman Catholic Church since 1583 has been using the Gregorian Calendar to calculate the fixed date of 21 March, while Eastern Churches continue to use the Julian Calendar.