About the Moon Phase Calculator
The Moon goes through a cycle of phases every 29.53 days, from new, through first quarter, full and last quarter, and back to new. This calculator gives the phase for any date and hour in UTC. It shows the age of the Moon in days since the last new moon, the share of the disc that is lit, and approximate dates of the next new and full moons. Type a date and an hour in UTC and the moon phase appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Moon Phase Calculator
- 1Enter a date and an hour in UTC.
- 2Pick the units or options from the menus if the moon phase tool offers them.
- 3Read the moon phase in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the moon phase
People look up the phase for photography, fishing, gardening, observing, planning an event or a birthday, and just out of curiosity. Writers use it to get the Moon right in their stories.
The method uses the average length of the lunar month. The real Moon speeds up and slows down on its elliptical orbit, so the true time of a phase can differ from the average by up to about 14 hours. The result is therefore accurate to roughly a day.
How the moon phase is worked out
The age of the Moon is the time since a reference new moon (6 January 2000, 18:14 UTC), taken modulo the mean synodic month of 29.530588853 days. The lit share is (1 − cos(2π × age ÷ 29.53)) ÷ 2. At noon UTC on 4 October 2026 the Moon is about 22.8 days old, a last quarter moon 43 percent lit. The next new moon is about 11 October and the next full moon about 25 October. The phase at the reference moment, 6 January 2000, is a new moon.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Phase, age and light share
- Next new and full moon
- Any date from year 1
Mistakes to avoid with moon phase
For exact moments of new and full moons, use an astronomical almanac. The phase names here are based on the age: new moon within about 1.8 days of the cycle's start, then waxing crescent, first quarter, waxing gibbous, full, and the waning phases. The dates for the next phases are estimates.
On phone and computer
On a phone, type into the boxes one after another and read the moon phase underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the moon phase move. The moon phase calculator works the same on both. The moon is one part of a night-sky plan. Add the times with the Sunrise and Sunset Calculator, count days with Days Between Dates, and see what sign a date falls under with the Zodiac Sign Calculator.
Features
- Accurate to about a day
- Mean month 29.53 days
- Almanac for exact times
- Clear messages when the moon phase inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate moon phase?
The age of the Moon is the time since a reference new moon (6 January 2000, 18:14 UTC), taken modulo the mean synodic month of 29.530588853 days. The lit share is (1 − cos(2π × age ÷ 29.53)) ÷ 2. At noon UTC on 4 October 2026 the Moon is about 22.8 days old, a last quarter moon 43 percent lit. The next new moon is about 11 October and the next full moon about 25 October. The phase at the reference moment, 6 January 2000, is a new moon.
What is the formula for moon phase?
The age of the Moon is the time since a reference new moon (6 January 2000, 18:14 UTC), taken modulo the mean synodic month of 29.530588853 days. The lit share is (1 − cos(2π × age ÷ 29.53)) ÷ 2. The moon phase calculator applies it exactly and rounds only the displayed answer to six significant figures.
How long is a lunar month?
29.53 days on average, called a synodic month.
What does 43% lit mean?
That 43 percent of the face of the Moon, seen from Earth, is in sunlight.
Is the phase the same everywhere on Earth?
Yes, though in the southern hemisphere the Moon appears upside down and the waxing and waning sides are swapped.
Is this moon phase calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.