1. What Is a Moon Age Calculator?
A moon age calculator gives the moon’s age on a date: the number of days since the most recent new moon. The age runs from 0 at new moon to about 29.53 at the next new moon, and it determines everything else about the moon that night — the phase name, how much of the disc is lit, and how far away the next new and full moons are.
Moon age is not the moon’s age in years; the moon is about 4.5 billion years old and no calculator here will tell you otherwise. It is the age of the current lunar cycle, the number astronomers and tide tables have used for centuries to say where the moon stands in its month.
Everything runs in your browser — the date you enter is never sent anywhere.
2. How to Use This Calculator
Enter a date, or leave it blank for today. Any past or future date works, because the cycle simply repeats every 29.53 days.
Select Calculate Moon Age and the results panel shows the headline figure — the phase name — alongside the moon’s age in days, the illuminated percentage, and the next full and new moon dates. The note below adds the previous new moon, so the full neighbourhood of the cycle is visible. Reset clears the date; Copy result puts a one-line summary on your clipboard.
3. How the Moon’s Age Is Calculated
The method needs two constants: a reference new moon and the synodic month, the average time from one new moon to the next. This calculator uses the new moon of 6 January 2000, 18:14 UTC (Julian Day 2451550.26) as the anchor, and 29.530588853 days as the synodic month — both standard astronomical values.
For any date, the Julian Day is computed (days since noon on 1 January 4713 BC, the astronomers’ day count), the difference from the anchor is taken, and the remainder after dividing by 29.530588853 is the moon’s age in days. The phase follows from the age bands in the table on this page, and the illumination follows from the geometry: (1 − cos(2π × age ÷ 29.53)) ÷ 2, which is 0% at new moon, 50% at the quarters and 100% at full moon.
The next new moon is the date plus (29.53 − age) days, and the next full moon is the date plus the distance to the half-cycle point (14.77 days), wrapped around the cycle. Both are rounded to whole days, because this is a date tool, not an ephemeris.
4. Worked Examples
Example 1 — 20 October 2026. The moon’s age is 8.62 days: a First Quarter moon, 63.0% illuminated. The previous new moon was 11 October 2026, the next full moon is 26 October 2026, and the next new moon is 10 November 2026.
Example 2 — the day after the anchor. On 7 January 2000, the day after the reference new moon, the age is 0.24 days — a New Moon, barely begun, about 0.1% illuminated. This is a useful check: the day after a new moon must give a small age, and it does.
Example 3 — a full moon. On 26 October 2026 the age is 14.62 days, inside the Full Moon band (12.92 to 16.61 days), with illumination near 100% — consistent with Example 1’s next-full-moon date.
5. Limits & Notes
This is an approximation, and the page says so wherever the numbers appear. The real synodic month varies between about 29.27 and 29.83 days because the moon’s orbit is elliptical, so a fixed 29.53-day cycle drifts from the true phase by up to about half a day over time. For naming the phase and finding the nearest full moon, that is ample; for eclipse prediction or precise astronomy, use a proper ephemeris.
The age is computed for 00:00 UTC on the chosen date. The moon ages about a day per day, so the age at your local evening can differ by up to half a day from the figure shown, and the date of a new or full moon can fall on the neighbouring calendar day in some timezones. The phase name is rarely affected, because the bands are several days wide.
Finally, a plain statement on a subject where folklore runs deep: there is no scientific basis for claims that moon phases change human behaviour, health or sleep. This calculator reports the astronomy only.
7. Frequently Asked Questions
What is the moon’s age?
The number of days since the most recent new moon. It runs from 0 at new moon to about 29.53 at the next new moon, and it determines the phase and illumination.
How is the moon’s age calculated?
From a reference new moon (6 January 2000, 18:14 UTC) and the average synodic month of 29.530588853 days: the days since the reference, divided by the cycle, leave a remainder, and that remainder is the age.
How accurate is it?
Within about half a day of the true phase. The real synodic month varies between about 29.27 and 29.83 days because the moon’s orbit is elliptical, so a fixed 29.53-day cycle drifts slightly. It is ample for naming the phase and finding the nearest full moon, but not for eclipse prediction.
What is illumination?
The percentage of the moon’s visible disc that is lit: 0% at new moon, 50% at the quarters, 100% at full moon, computed from the age by (1 − cos(2π × age ÷ 29.53)) ÷ 2.
What are the eight phases?
New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent, in that order, with the quarter boundaries at 1/4, 1/2 and 3/4 of the cycle.
When is the next full moon?
Enter a date and the calculator gives the next full moon (the half-cycle point, 14.77 days after the new moon) and the next new moon, rounded to whole days.
Why does my astronomy app show a slightly different age?
Because the moon ages through the day: this calculator computes the age at 00:00 UTC on the chosen date, while an app shows the age at the current time in your timezone. The difference is at most about a day, and the phase name rarely changes.
Does the moon phase affect sleep or behaviour?
There is no scientific basis for claims that moon phases change human behaviour, health or sleep. This calculator reports the astronomy only.
Can I check a date in the past or future?
Yes. Any date works, because the cycle repeats every 29.53 days; the same calculation runs backwards and forwards from the reference new moon.
What is the difference from the birthday moon phase tool?
That tool gives the moon phase on a birth date; this tool gives the moon’s age and phase for any date, with the next new and full moons. The method is the same, the question is different.
Is this calculator free?
Yes. The Moon Age Calculator is free on Age Calculator Lab, with no sign-up.
8. Methodology & Data Integrity
The anchor (new moon 6 January 2000, 18:14 UTC, Julian Day 2451550.26) and the synodic month (29.530588853 days) are standard astronomical values. Ages are computed for 00:00 UTC on the chosen date, and next new and full moons are rounded to whole days.
Worked examples on this page were produced by the calculator’s own engine and cross-checked against the lunar calendar: 20 October 2026 gives age 8.62 days (First Quarter, 63.0% lit, next full moon 26 October 2026, next new moon 10 November 2026); 7 January 2000 gives 0.24 days; 26 October 2026 gives 14.62 days (Full Moon).
The approximation and its limits are stated on the page: the true synodic month varies, the figure is for 00:00 UTC, and no behavioural or health claims are made for moon phases.
