Moon Phase Calculator

Find the moon phase for any date

Pick a date to see the Moon’s phase, how much of its disc is lit (illumination), and the Moon’s age in days since the last new moon. The calculator uses the mean synodic month — the average 29.53-day cycle from one new moon to the next — measured from a known reference new moon. Everything runs in your browser, so it works offline and nothing you enter is sent anywhere.

The eight phases of the Moon

The lunar cycle moves through eight phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent. “Waxing” means the lit part is growing toward full; “waning” means it is shrinking back toward new. The first and last quarter are the moments when exactly half the disc is lit.

FAQ

How accurate is this calculator? It uses the average length of the lunar cycle, so the phase and illumination are accurate to within a few hours — more than enough to know the phase on any given day. It does not model the small month-to-month variations in the Moon’s orbit.

What does illumination mean? It is the percentage of the Moon’s visible disc lit by the Sun: 0% at new moon, 50% at the quarters and 100% at full moon.

What is the Moon’s age? The number of days since the last new moon. A full cycle lasts about 29.5 days, so the age resets to zero at each new moon.

Why might the phase differ from my calendar by a day? Phases are computed in UTC at midday. Near a timezone boundary the exact moment of a new or full moon can fall on the previous or next day in your local time.