Moon Phase Calculator
The moon's phase, age and lit fraction for any date, from the standard 29.53059-day cycle — with the approximation labelled, because it is one.
Example — Moon phase on January 21, 2000Full moon — 14.7 days into its cycle, about 100% lit
How we got it — the example, step by step
- The moon's cycle (new moon to new moon) is 29.53059 days. Count from a known new moon: 6 January 2000, 18:14 UTC.
- Days since then, divided by 29.53059, leaves a remainder of 14.74 days — the moon's age on that date.
- The lit fraction follows (1 − cos(2π × age ÷ 29.53059)) ÷ 2 ≈ 100%.
Check it backwards
Half-way through a 29.53-day cycle (day ~14.8) is the full moon by definition of the cycle — and day 14.7 lands on it.
Questions people also ask
How accurate is it?
Good to about a day. The real moon's orbit is elliptical, so exact new and full moments shift by hours either way; for an eclipse or a launch, use an ephemeris, not this page. For 'is it roughly a full moon this weekend', this is the right tool.