Everyday Life Calculators

Moon Phase Calculator

Estimate the moon phase and illumination percentage for any date using the lunar cycle.

Moon phase —

Illumination —

About the Moon Phase Calculator

The moon cycles through eight phases roughly every 29.53 days (a synodic month), from New Moon to Full Moon and back again.

How it works: This calculator counts the days elapsed since a known reference new moon (January 6, 2000) and finds the remainder after dividing by the length of the lunar cycle, then maps that remainder to a phase name and estimated illumination percentage.

This is a simplified astronomical approximation good to within about a day — for precise viewing times, consult a dedicated astronomy source.