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.
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.
Common questions
How does the moon phase calculator work?
It counts the days since a known new moon and divides by the synodic month of 29.53 days to find how far through the cycle the moon is on your date. From that position it names the phase, gives the illuminated percentage and the moon age in days, and finds the next new and full moon.
What is a synodic month?
A synodic month is the average time from one new moon to the next, about 29.53 days. It is the cycle the calculator uses to work out the phase. It is slightly longer than the moon's orbit around the Earth because the Earth also moves around the Sun during that time.
What does the illuminated percentage mean?
The illuminated percentage is how much of the moon's disc looks lit from Earth. It is 0% at new moon, about 50% at the first and last quarter, and 100% at full moon, rising and falling smoothly through the cycle.
How accurate is the moon phase result?
The mean-lunation method is accurate to within a few hours of the true astronomical phase, which is enough to name the phase and plan around the next full moon. For precise lunar rise and set times, HM Nautical Almanac Office publishes official data.
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Editor, Dates & Times
Oliver Mackman is the named editor of Dates & Times and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every result to cite the underlying date maths and the official UK source behind it, such as the gov.uk Bank Holidays API.
Last reviewed: 12 June 2026