Moon Phase Calculator

What is a Moon Phase Calculator?

A Moon Phase Calculator shows the phase of the moon for any given date. It uses astronomical algorithms to determine whether the moon is new, waxing, waning, full, or in a quarter phase based on the lunar cycle.

How to calculate moon phase manually (the formula)

The moon phase depends on the moon's position relative to Earth and the sun. Calculate the number of days since a known new moon (e.g., January 6, 2000), divide by the lunar cycle length (29.53058867 days), and use the fractional remainder to determine the phase. A remainder of 0 = new moon, 0.25 = first quarter, 0.5 = full moon, 0.75 = last quarter.

Example calculation

For July 6, 2026: approximately 9678 days since the reference new moon. 9678 ÷ 29.53 = 327.7. The fractional part 0.7 × 29.53 = 20.7 days into the cycle, which corresponds to a waning gibbous phase, nearing the last quarter.

Common mistakes

• Using a 28-day lunar cycle — The lunar cycle is approximately 29.5 days, not 28. Using 28 days produces incorrect results.
• Confusing waxing and waning — Waxing means the illuminated portion is increasing (toward full moon); waning means it is decreasing (toward new moon).
• Ignoring time zone effects — The exact phase depends on UTC. Local time can shift the phase by up to a day near the new or full moon.

Frequently asked questions