MasterTool

Moon Phase Calculator

Get the moon's phase name, illumination percentage and age for any date, plus the approximate dates of the next full and new moons.

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What this tool does

Get the moon's phase name, illumination percentage and age for any date, plus the approximate dates of the next full and new moons. A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.

How to use the Moon Phase Calculator

  1. Enter or select date (yyyy-mm-dd).
  2. Read the calculated result; change any measurement to compare alternatives.

Formula

age = (date at 12:00 UTC − new moon of 2000-01-06 18:14 UTC) mod 29.53058867 days (the synodic month); illumination = (1 − cos(2π × age / synodic)) / 2 × 100%
date
Date (YYYY-MM-DD)

A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.

Worked example

For moon phase calculator, the following measurements illustrate the exact method: Date (YYYY-MM-DD): 2026-10-07.

Inputs

  • Date (YYYY-MM-DD)2026-10-07

Result

  • Moon phaseWaning Crescent
  • Illumination (%)16.1
  • Moon age (days since new moon)25.65
  • Next full moon (approx.)2026-10-26
  • Next new moon (approx.)2026-10-11

Results explained

Moon phase
Moon phase from the formula above. A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.
Illumination (%)
Illumination (%) from the formula above. A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.
Moon age (days since new moon)
Moon age (days since new moon) from the formula above. A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.
Next full moon (approx.)
Next full moon (approx.) from the formula above. A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.
Next new moon (approx.)
Next new moon (approx.) from the formula above. A closed-form synodic approximation, not an ephemeris: phase names and illumination are typically accurate to about a day and a few percent. The entered date is evaluated at 12:00 UTC. Real full/new moon instants vary because the Moon's orbit is elliptical.

Frequently asked questions

The Moon repeats its phase cycle every synodic month — 29.53058867 days on average. The tool measures the time since a known new moon (January 6, 2000, 18:14 UTC), takes the remainder within the cycle as the moon's age, and maps the age to a phase name and illumination.

It is a mean-cycle approximation: the phase name is right for the date, and illumination is within a few percent, but the exact full and new moon instants can be off by several hours because the real orbit speeds up and slows down. Treat the next-full/new dates as accurate to about a day.

It is the share of the Moon's Earth-facing disc that is sunlit: 0% at new moon, about 50% at the quarter phases (slightly more than the geometric half near the quarters in this cosine model), and 100% at full moon.

The phase and illumination are identical worldwide at a given instant, but the lit side appears flipped — the waxing crescent lights the left side instead of the right when viewed from southern latitudes.

Enter today's date (the example default is fixed so the page stays stable). The age output counts days since the most recent new moon, and the cycle restarts at the next new moon date shown.