Moon Phase Calculator
Pick a date. The default is today.
This result is an estimate only. Double-check important figures before relying on them.
How it is calculated
- Moon age = days since a known new moon, repeated every 29.53 days
- Phase is the eighth of the cycle the age falls in
- Lit share = (1 − cos(2π × age ÷ 29.53)) ÷ 2
For a countdown to a date, use the days until calculator.
Worked example
13 January 2025:
- Full moon, about 100% lit
- Next new moon around 29 January
Why your number may differ
The average-month method does not follow the moon’s changing speed, so the date of a phase can be a few hours to half a day out.
Frequently asked questions
How accurate is this?
It uses the average length of the lunar month, so phase dates can be off by up to about half a day. It is fine for planning, not for astronomy.
Is this the same as the Islamic crescent sighting?
No. Hijri months begin by official moon sighting (rukyah) or calculation (hisab), which differs from this simple phase model.
Why does the moon look different from the percentage?
The percentage is the lit part facing Earth. Haze and the moon's tilt change how it looks.