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Moon Phase Calculator

Use this free moon phase calculator to find the phase of the moon on any date. Pick a date — today is filled in by default — and the calculator shows the moon's age in days, its phase name, how much of its face is lit, and the dates of the next full moon and the next new moon, with a shaded moon diagram that matches the phase. It runs entirely in your browser, with no sign-up and nothing to install.

How it works

The moon does not shine by itself: we see it because sunlight bounces off its surface, and as the moon orbits the Earth, the angle between the sun, the moon and us changes, so a different amount of the moon's Earth-facing side is lit. That changing appearance is the lunar cycle, repeating every 29.53 days on average — the synodic month. The cycle starts at the new moon, when the moon sits between the Earth and the sun and its dark side faces us, so we see nothing. Over the next two weeks the lit part grows — a thin crescent, then a half-lit first quarter, then a swelling gibbous — until the full moon, when the whole face is lit. Then it shrinks back through the waning gibbous, the last quarter and the waning crescent, and the cycle begins again.

This calculator works out where the moon is in that cycle by counting days from a fixed reference point: the new moon of January 6, 2000 at 18:14 UTC. It takes the days between that reference and your chosen date, divides by 29.53, and keeps the remainder — the moon's age in days. An age near 0 means a new moon, near 7.4 days the first quarter, near 14.8 days the full moon, and near 22.1 days the last quarter. The calculator rounds the age into one of eight named phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent. "Waxing" means the lit part is growing, "waning" means it is shrinking, and a gibbous moon is more than half lit but not full.

Illumination, the percentage of the visible face that is lit, comes from the same age: 0% at new moon, 50% at each quarter, 100% at full moon, and back to 0% at the next new moon. The calculator uses the standard formula, illumination = (1 − cos(2π × age ÷ 29.53)) ÷ 2, as a percentage. A moon 10 days old is about 76% lit, and one 25 days old is about 20% lit — a thin crescent rising in the early morning. The shaded diagram draws the same idea: the bright part of the disc matches the phase for your date, with the lit limb on the right while the moon is waxing and on the left while it is waning, as it appears in the northern hemisphere.

The next full moon and next new moon dates come from simple subtraction. If the moon is 14.58 days old, the next full moon is about 0.18 days away — it is tonight — and the next new moon is about 14.95 days away. The calculator adds those remaining days to your chosen date and prints both dates, so you can plan around them: a full moon is the brightest night of the month and the worst time for stargazing, while a new moon is the darkest and best time to look for faint stars, meteor showers and the Milky Way. Photographers plan the same way, since moonlight changes the look of a night landscape.

One honest caveat: the result is accurate to about a day, not to the minute. The moon's orbit is an ellipse, not a perfect circle, so the real cycle varies by several hours from month to month — the synodic month is an average, not a fixed constant. Published moon tables use the moon's exact position and can pin a full moon to the hour; this calculator uses the 29.53-day average, which is more than good enough for planning a walk, a photograph or a school project, and it will always land on the right phase name and the right day for the full and new moons. If you need the exact minute, check an astronomical almanac as well.

Worked example

Take November 24, 2026. Count the days from the reference new moon of January 6, 2000 at 18:14 UTC to noon on November 24, 2026: about 9,819 days. Divide by the synodic month, 29.53 days, and the remainder is 14.58 days — the moon's age. That is almost exactly half of 29.53, so the moon is at the middle of its cycle, and the phase is a Full Moon.

The illumination formula gives (1 − cos(2π × 14.58 ÷ 29.53)) ÷ 2, which works out to 100.0% — the whole visible face is lit, as expected at full moon. The next full moon is only 0.18 days away, so it falls on the same day, November 24, 2026, and the next new moon is 14.95 days later, around December 9, 2026. Pick November 24, 2026 in the calculator — or use the preset — and it shows the same answers: Full Moon, moon age 14.58 days, 100.0% illuminated, with a fully bright disc in the diagram.

For the other end of the cycle, try November 8, 2026. The same calculation gives a moon age of 28.12 days, 1.42 days short of a full 29.53-day cycle, so the phase is a New Moon with only 2.2% illumination — practically invisible. The next new moon falls about 1.42 days later, on November 9, 2026, and the next full moon is about 16.18 days away, on November 24, 2026. Together the two dates show the whole rhythm of the month: dark on the 8th, full on the 24th.

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