Moon Phase Calculator
Find the moon phase and how much of it is lit for any date, plus the next full and new moon.
About the Moon Phase Calculator
The phase depends on the angle between the Sun and Moon seen from Earth. This uses the low-precision lunar theory from Jean Meeus's Astronomical Algorithms, accurate to within a few tenths of a percent in illumination and typically within an hour or two for phase times. A full cycle (synodic month) averages 29.53 days.
The formula
Illuminated fraction k = (1 + cos i) ÷ 2, i = phase angle
Worked example
Example: Waning crescent = 14.1% lit
- Input: Date: 15 March 2026.
- Formula: Illuminated fraction k = (1 + cos i) ÷ 2, i = phase angle
- Result: Waning crescent = 14.1% lit
- Illumination: 14.1%
- Elongation: 315.8°
Questions people ask
Is the phase the same worldwide?
Yes, though the date can differ across time zones and the image is flipped in the southern hemisphere.
Why isn't every month 29.53 days?
The Moon's elliptical orbit makes individual cycles vary by several hours.