About this tool
Moon Phase Calculator is a free, in-browser tool that tells you the lunar phase for any date — its name, an emoji, the illuminated percentage of the disc, and the moon's age in days. Pick a date and it computes instantly on your device with nothing uploaded.
It converts the date to a Julian Day number, then measures the days elapsed since a known new moon at Julian Date 2451550.1 (6 January 2000), reduced modulo the synodic month of 29.53058867 days to get the moon's age. Illumination is (1 − cos(2π × age ÷ 29.53)) ÷ 2, expressed as a percentage, and the age is mapped into eight named phases from New through Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Last Quarter and Waning Crescent.
Use it to plan stargazing, photography or a moonlit walk. It is a mean-phase model, so the actual moment of a phase can differ by a few hours from ephemeris tables, but the phase name and illumination are accurate for everyday planning.
Frequently asked questions
How is the moon phase computed?
The date becomes a Julian Day number; subtracting the reference new moon (Julian Date 2451550.1, 6 Jan 2000) and taking the remainder modulo the synodic month (29.53058867 days) gives the moon's age, which is then classified into a named phase and matched with an emoji.
What does the illumination percentage mean?
It is the fraction of the moon's visible disc that is lit, from 0% at new moon to 100% at full moon. It is calculated as (1 − cos(2π × phase fraction)) ÷ 2 × 100, where the phase fraction is the moon's age divided by the synodic month.
Which time zone does it use?
The phase is computed for midnight on the date you select. Because the moon moves through roughly 12–13% of a cycle per day, the named phase is stable for the whole day even though the exact age shifts slightly across time zones.
How accurate is it?
This is a mean synodic model, not a full ephemeris, so the precise timing of new and full moons can be off by a few hours. The phase name, emoji and illumination are reliable for planning stargazing, photography or events.
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