Moon Age Calculator

Tracking the moon with simple arithmetic

The moon completes a full cycle (new moon to new moon) in an average of about 29.53 days, called a synodic month. Starting from any known new moon date, the moon's current age in its cycle can be found with straightforward modular arithmetic.

Worked example

For July 26, 2026, measured against the January 6, 2000 reference new moon:

Moon Age = (Days Since Reference) mod 29.53058867 = 11.97 days old (Waxing Gibbous)

Moon Age Range	Phase
0 - 1.8 days	New Moon
1.8 - 9.2 days	Waxing Crescent to First Quarter
9.2 - 16.6 days	Waxing Gibbous to Full Moon
16.6 - 24.0 days	Waning Gibbous to Last Quarter
24.0 - 29.5 days	Waning Crescent back to New Moon

This approximation is accurate to within about a day for most dates, since it uses a fixed average synodic month length rather than accounting for the moon's slightly elliptical, variable-speed orbit.