Moon phase calculator

The moon cycles through its phases over one synodic month, an average of 29.53 days from one new moon to the next. To find the phase on any date, the calculator counts the days since a known new moon, divides by 29.53 to get the position in the cycle, and from that names the phase, gives the illuminated percentage and the moon's age in days, and finds the next new and full moon. This mean-lunation method is accurate to within a few hours of the true astronomical phase, which is plenty to name the phase and plan around the next full moon.

Enter any date to see the moon phase, how lit the moon is, how far through the cycle it is, and when the next new and full moon fall.

Date

Date

Show the moon phase

How this is worked out

The moon cycles through its phases over one synodic month, an average of 29.53 days from one new moon to the next.

days since a known new moon ÷ 29.53059 = number of cycles
the fractional part × 29.53059 = the moon's age in days
illuminated fraction = (1 − cos(2π × age ÷ 29.53059)) ÷ 2
reference new moon: 6 January 2000, 18:14 UTC

This is the standard mean-lunation method. It is accurate to within several hours of the true astronomical phase, which is enough to name the phase and find the next new and full moon. For precise rise and set times, HM Nautical Almanac Office publishes official lunar data.

For sunrise and sunset on the same date, use the sunrise and sunset calculator, or see all the calculators.

Method: standard 29.53-day synodic-month formula. For precise lunar rise and set times, HM Nautical Almanac Office publishes official data.