Moon Phase Calculator
Find the moon phase for any date, its illumination percentage, and the days until the next full moon.
By Konstantin Iakovlev · Updated September 2026 · Source: U.S. Naval Observatory — Phases of the Moon
Moon Phase
Waning Crescent
Illumination
6.5%
Lunar Details
| Phase | Waning Crescent |
| Illumination | 6.5% |
| Days Since New Moon | 27.1 |
| Days Until Full Moon | 17.2 |
| Lunar Cycle | 29.53 days |
Use the Moon Phase Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.
Disclaimer: This calculator is for informational purposes only. Results are estimates based on the information you provide and the assumptions described on this page.
How It Works
At a glance you can see the moon's phase on any date, how much of its face is lit, and how many days remain until the next full moon. Those details matter when you're timing outdoor plans, working in the garden, or just curious about what the sky is doing overhead. Illumination in particular shapes stargazing conditions.
Behind the result is an average-cycle approximation rather than a full orbital model. The calculator counts the days from a reference new moon at noon on January 6, 2000 to noon on your date, divides by an average lunar cycle of 29.53 days, and treats the remainder as the moon's age in days. The lit fraction is (1 − cos(2π × age ÷ 29.53)) ÷ 2, and the next full moon is taken to fall 14.77 days after each new moon, so that result is a count of days rather than a date and time.
Real lunar cycles vary in length, so an average cycle drifts. Checked against the U.S. Naval Observatory's phase times for 2025–2026, the calculator's moon age was off by up to 16.5 hours in U.S. time zones and its days-until-full-moon count by up to about 10 hours, enough to shift a phase label by a day near a changeover. Cloud cover and local weather can also hide what the numbers describe, so for an exact phase time, use the Naval Observatory's tables.
Example: Finding the Moon Phase for a Stargazing Trip
- 1 You're planning a stargazing trip for the evening of July 20, 2026, and want to know how much moonlight there will be. Enter 2026-07-20 as the date.
- 2 Moon age: from noon on January 6, 2000 to noon on July 20, 2026 is 9,691.96 days in a U.S. time zone on daylight saving time. That is 328 cycles of 29.53 days (9,685.84 days) plus 6.12 days, so the moon is 6.12 days old.
- 3 Illumination = (1 − cos(2π × 6.12 ÷ 29.53)) ÷ 2 = (1 − 0.2657) ÷ 2 = 36.7%. At 6.12 days the moon is under half lit and more than a day before first quarter (at 29.53 ÷ 4 = 7.38 days), so the calculator labels it Waxing Crescent. The next full moon is 14.77 − 6.12 = 8.65 days away, shown as 8.7.
- 4 The U.S. Naval Observatory puts the new moon on July 14 at 09:43 UTC and the full moon on July 29 at 14:36 UTC, which agrees with a waxing crescent on July 20. A moon about one-third lit will add some light to the evening sky; for the darkest skies, the next new moon is August 12, 2026 (17:37 UTC).
Source: U.S. Naval Observatory — Phases of the Moon · Last updated: September 2026
Frequently Asked Questions
How long is a complete moon cycle?
When is the next full moon?
Does the moon phase affect fishing?
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