Moon Phase Calculator
Choose a date and see what moon phase it is, with estimated illumination percentage.
How to use the calculator
- Choose the date you want to see the moon phase for.
- The calculator shows which of the eight moon phases it is, and what portion of the moon is illuminated.
How is the moon phase calculated?
The calculation starts from a known reference new moon (January 6, 2000) and the average length of a synodic month (the time from new moon to new moon, about 29.53 days), and works out how far into this cycle the chosen date falls.
Frequently asked questions
How accurate is the calculation?
Tested against a known, publicly confirmed full moon date, the calculation was accurate to within a couple of hours — more than good enough to know roughly what moon phase it is on a given day, but not intended for precise astronomical use (like planning an observation to the minute).
Why doesn't the time of day match?
The calculator only uses the date, not the time, and calculates the phase at midnight UTC for the chosen date — the moon phase changes continuously throughout the day.
What does illumination percentage mean?
How much of the moon's disk (as seen from Earth) is lit by sunlight at the given time — 0% at new moon, 100% at full moon.