Moon Phase Calculator: Which Moon Is It Tonight?
The Moon repeats its cycle about every 29.53 days. Counting from a known new moon gives a good estimate of tonight's phase without an astronomy app.
What is the Moon Phase Calculator?
Enter a date and the tool estimates the Moon's age and phase from the known new moon of 6 January 2000, using the average lunar cycle of 29.53 days. Moon phases here are estimates: the real cycle varies slightly, so the phase near a boundary may differ by a day from an astronomical table.
How to use it
- Enter a date as YYYY-MM-DD.
- Click Calculate Moon Phase.
- Read the phase, the Moon's age, and the approximate illumination.
Worked example
On 1 November 2026, the Moon is about 20.6 days old, in the last-quarter part of the cycle, about 60% illuminated.
How accurate is this estimate?
Good to within about a day. The real lunar cycle varies slightly, so phases near a boundary can differ from an astronomical table.
Frequently asked questions
What does Moon age mean?
How many days have passed since the last new moon, on a scale of 0 to about 29.53 days.
Why is illumination only approximate?
It is worked out from the Moon's age on a simple curve, not from the Moon's exact position.