🌙 Moon Phase Calculator
By ToolNimba Editorial Team · Updated 2026-06-19
Pick a date to see the moon phase, illumination and the moon's age in days.
This moon phase calculator tells you the phase of the Moon on any date you choose. Pick a date and you will see the named phase (such as New, First Quarter or Full), the percent of the disc that is lit, and the Moon age in days since the last new moon, along with a matching emoji. It works for past, present and future dates, so you can plan a stargazing night, check a birthday, or look back at a memorable evening.
What is the Moon Phase Calculator?
The Moon does not make its own light: it reflects sunlight, and the shape we see depends on the angle between the Sun, Earth and Moon. As the Moon orbits Earth, that angle steadily changes, so the lit portion grows from a thin crescent to a full disc and then shrinks again. One full round of this cycle, from one new moon to the next, is called a synodic month and lasts about 29.53 days on average. This tool divides that cycle into the eight phases most people recognise.
The method is straightforward. We measure how many days have passed since a known new moon (the reference used here is 2000-01-06), then take the remainder after dividing by 29.53059 days. That remainder is the Moon age, the number of days into the current cycle. Dividing the age by the cycle length gives a phase fraction from 0 to 1, where 0 and 1 are new, 0.25 is first quarter, 0.5 is full and 0.75 is last quarter. The illuminated percentage is found from the cosine of that fraction, which is why a full moon reads close to 100% and a new moon close to 0%.
This is a mean-phase model, meaning it uses the average length of the lunar month. The Moon orbit is slightly elliptical, so the real new and full moments can run a little ahead of or behind the average, typically by up to about a day. That is plenty accurate for knowing whether a night will be dark or moonlit, or which phase falls on a given date, but for an exact eclipse or precise rise and set time you should consult a detailed astronomical almanac.
When to use it
- Checking what moon phase falls on a future date for stargazing, astrophotography or a camping trip when you want dark skies.
- Finding the phase of the Moon on a birthday, wedding day or other memorable date in the past.
- Planning around the full moon for tides, night hikes, or simply to catch the brightest moonrise of the month.
- Teaching the lunar cycle, where students enter dates and watch the phase and illumination change across a month.
How to use the Moon Phase Calculator
- Pick the date you want to check using the date field, or press Use today for the current date.
- Read the named phase and its emoji in the first result box.
- Check the illumination percentage to see how much of the disc is lit.
- Look at the Moon age to see how many days into the 29.53-day cycle the date falls.
Formula & method
Worked examples
You want the moon phase for 25 January 2024.
- Days since the reference new moon (2000-01-06) = 8784.24 days
- Moon age = 8784.24 mod 29.53059 = 13.66 days
- Phase fraction = 13.66 ÷ 29.53059 = 0.4624
- Nearest eighth: 0.4624 x 8 = 3.70, rounds to 4 = Full Moon
- Illumination = (1 - cos(2 x pi x 0.4624)) ÷ 2 x 100 = 99%
Result: Full Moon 🌕, about 99% illuminated, moon age 13.7 days
You want the moon phase for 4 July 2026.
- Days since the reference new moon = 9675.24 days
- Moon age = 9675.24 mod 29.53059 = 18.74 days
- Phase fraction = 18.74 ÷ 29.53059 = 0.6345
- Nearest eighth: 0.6345 x 8 = 5.08, rounds to 5 = Waning Gibbous
- Illumination = (1 - cos(2 x pi x 0.6345)) ÷ 2 x 100 = 83%
Result: Waning Gibbous 🌖, about 83% illuminated, moon age 18.7 days
The eight principal moon phases, their emoji and rough illumination
| Phase | Emoji | Phase fraction | Illuminated | Trend |
|---|---|---|---|---|
| New Moon | 🌑 | 0.00 | 0% | Start of cycle |
| Waxing Crescent | 🌒 | 0.125 | About 25% | Growing |
| First Quarter | 🌓 | 0.25 | 50% | Growing |
| Waxing Gibbous | 🌔 | 0.375 | About 75% | Growing |
| Full Moon | 🌕 | 0.50 | 100% | Peak |
| Waning Gibbous | 🌖 | 0.625 | About 75% | Shrinking |
| Last Quarter | 🌗 | 0.75 | 50% | Shrinking |
| Waning Crescent | 🌘 | 0.875 | About 25% | Shrinking |
Common mistakes to avoid
- Expecting almanac-exact timing from a mean model. This tool uses the average 29.53-day cycle, so the moment of a new or full moon can differ from a published almanac by up to about a day. It is great for the phase and illumination on a date, but not for second-precise eclipse times.
- Confusing the quarter phases with a quarter lit. First Quarter and Last Quarter are named for being a quarter of the way through the cycle, not for being 25% lit. At both quarters the disc is actually half illuminated.
- Assuming the phase is the same worldwide at one instant. The phase fraction is nearly identical everywhere at a given moment, but because of time zones the calendar date can differ, and the Moon appears flipped or rotated depending on your latitude and hemisphere.
- Reading illumination as brightness. Percent illumination is the share of the visible disc that is lit, not how bright the Moon looks. A full moon is far more than twice as bright as a half moon because of how sunlight scatters off the lunar surface.
Glossary
- Synodic month
- The time from one new moon to the next, averaging about 29.53 days. It is the cycle this calculator divides into phases.
- Moon age
- The number of days that have passed since the most recent new moon, from 0 up to about 29.5.
- Phase fraction
- The Moon age divided by the synodic month, a number from 0 to 1 that marks how far through the cycle a date falls.
- Illumination
- The percent of the Moon visible disc that is lit by the Sun, ranging from near 0% at new moon to 100% at full moon.
- Waxing and waning
- Waxing means the lit part is growing toward full; waning means it is shrinking back toward new.
Frequently asked questions
What moon phase is it today?
Press Use today and the calculator fills in the current date, then shows the phase, the percent illuminated and the Moon age in days. The result updates instantly, so it always reflects the date you have selected.
How does the moon phase calculator work?
It counts the days since a known new moon (2000-01-06), divides by the average lunar cycle of 29.53059 days, and takes the remainder as the Moon age. That age maps to one of eight phases, and a cosine of the phase fraction gives the percent illumination.
How accurate is the result?
It uses the mean length of the lunar month, so the exact moment of a new or full moon can be off by up to roughly a day compared with a detailed almanac. For knowing the phase, illumination and Moon age on a date it is reliable.
What is the difference between first quarter and a half moon?
They describe the same appearance from a different angle. First Quarter is a quarter of the way through the cycle, and at that point exactly half the visible disc is lit, so people often call it a half moon.
Why is the Moon lit at all if it makes no light?
The Moon shines only by reflecting sunlight. The phase we see is the part of the sunlit half that faces Earth, which changes as the Moon orbits and the Sun, Earth and Moon shift their alignment.
Can I check moon phases for past or future dates?
Yes. The calculator works for any date, past or future, because it is based on a fixed reference new moon and a steady cycle length. Enter a birthday, a historic event or a date years ahead to see its phase.