Moon Phase Calculator

How to use the moon phase calculator

1. Pick a date between 1801 and 2200. The default is today.
2. Read the phase name, how much of the moon is lit, the moon's age in days and whether it is waxing or waning.
3. Use the table of the next four main phases to plan ahead.
4. Copy or print the result if you need it.

Formula

The constants are the mean new moon of January 2000 and the mean synodic month from Jean Meeus, Astronomical Algorithms, chapter 49. The date is taken at noon UTC.

The full moon of March 25, 2024

What the numbers mean

The moon takes about 29.53 days to go from one new moon to the next. This is the synodic month. The tool measures how far into that cycle a date falls, which is the moon's age, and turns it into a position in the cycle. At 0 days the moon is new, at about 7.4 days it is at first quarter, at about 14.8 days it is full and at about 22.1 days it is at last quarter.

The illuminated fraction is how much of the visible face is lit. It is 0 at new moon, about 50 percent at the quarters, and nearly 100 percent at full moon. The eight phase names (new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent) each cover one eighth of the cycle, so the name "Full Moon" covers about 3.7 days around the exact full moon.

How accurate it is

This tool uses the mean length of the month. The real moon moves faster when it is closer to Earth and slower when it is farther away, so the true time of a phase can differ from the mean time by up to roughly half a day or more. The dates in the next phases table can therefore be a day off. The illuminated percent uses a smooth curve and can differ by a few percent from the true value.

Your location does not change the phase, and the phase is the same all over the world on a given moment. The date is evaluated at noon UTC, so in some time zones the phase can differ slightly from what you see in the evening. For a full look at your sky on the same day, add sunrise and sunset times.

Good uses

People look up the moon phase for a birthday or event, for fishing and gardening plans, for photography and for stargazing. Dark nights near the new moon are best for faint objects, and a bright full moon rises near sunset. Because the cycle is about 29.5 days, the same phase falls about 11 days earlier each year on the calendar. To count days between two dates, use the days between dates calculator.

Assumptions and limits

• The calculation uses the mean synodic month of 29.530588861 days and a mean new moon in January 2000.
• The date is evaluated at 12:00 UTC. The tool does not use your time zone or location.
• Phase dates can be about a day different from the true values, and the illuminated percent is approximate.
• Dates from 1801 to 2200 are accepted.

Formulas reviewed October 10, 2026. See the calculation methodology for how SumPanda rounds, tests and sources its formulas.

Frequently asked questions