Free Online Moon Phase Calculator
Find the Moon Phase for Any Date
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The moon cycles through its full set of phases roughly every 29.5 days — a period astronomers call the synodic month — and calculating which phase falls on any given date requires knowing precisely how many days have elapsed since a known reference new moon and where that lands within the current cycle.
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Last Quarter
Moon age: 23.2 days · 39% illuminated
Calculated from the synodic month (~29.53 days) using a known reference new moon — accurate to within roughly a day for any given date.
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Features
- Runs entirely in your browser
- Privacy-first — your data is never uploaded
- Real-time, instant results
- 100% free, no sign-up required
- Works on desktop, tablet, and mobile
- No installation needed
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About Moon Phase Calculator
The moon cycles through its full set of phases roughly every 29.5 days — a period astronomers call the synodic month — and calculating which phase falls on any given date requires knowing precisely how many days have elapsed since a known reference new moon and where that lands within the current cycle.
This calculator uses a known reference new moon (January 6, 2000) and the precise average length of the synodic month to compute the moon's age and phase for any date you enter, showing the phase name, its emoji symbol, the moon's exact age in days, and its percentage of illumination.
Because it's based on a precise astronomical constant rather than an approximation, it stays accurate to within about a day for any date — past, present, or years into the future — making it useful for planning stargazing, photography, or simply satisfying curiosity about a specific date's moon phase.
How it works
- Enter a date. Select any date using the date picker — defaults to today.
- See the moon phase. View the phase name, emoji, and moon age in days.
- Check illumination percentage. See what percentage of the moon's visible face is illuminated.
Examples
Checking today's moon phase
Input
Date: today
Output
e.g. Waxing Gibbous 🌔, Moon age: 11.2 days, 78% illuminated