Softonic

Moon Phase Calculator

Step 1 / 2 Date
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Pick any date and this calculator returns the moon phase, the moon’s age in days since the last new moon, and the percentage of the visible disc that is illuminated. It works the same for 1492, for tonight, or for a wedding you are planning three years out. The algorithm is anchored to the well-established new moon of 6 January 2000 at 18:14 UTC and steps through the 29.5306-day synodic cycle.

How to check a moon phase

  1. 1

    Choose the date

    Use the date picker for past, present or future dates. The calculator defaults to today in UTC.

  2. 2

    We compute lunar age

    The number of days between your date and the reference new moon is divided by the synodic period (29.530588861 days).

  3. 3

    Phase is assigned by age

    Age under 1 day is new moon; around day 7 it is first quarter; day 14-15 is full moon; day 21 is last quarter.

  4. 4

    Illumination is derived

    We apply (1 - cos(2π × fraction)) / 2 to get the visible percentage, matching the standard astronomical formula.

The eight phases of the moon

The moon does not actually change shape, what changes is the angle between Earth, Moon and Sun, which determines how much of the sunlit side we can see from here. A full lunar cycle (new moon back to new moon) takes 29.5306 days on average, which is why our calendar months do not line up with lunations.

The eight phases and what they look like

Phase Lunar age (days) Illumination What you see
New moon 0.0 – 1.0 0% Moon invisible, up with the sun
Waxing crescent 1.0 – 7.0 1–49% Thin sliver on the right (N hemisphere)
First quarter 7.0 – 8.0 50% Right half lit, visible afternoon/evening
Waxing gibbous 8.0 – 14.0 51–99% More than half, bulging to the right
Full moon 14.0 – 15.0 100% Whole disc lit, rises at sunset
Waning gibbous 15.0 – 21.0 99–51% Still mostly lit, now bulging to the left
Last quarter 21.0 – 22.0 50% Left half lit, rises around midnight
Waning crescent 22.0 – 28.5 49–1% Thin sliver on the left, dawn sky

Why this is an approximation

The true synodic month varies from about 29.18 to 29.93 days because the Moon’s orbit is elliptical and the Earth-Moon system is itself orbiting the Sun. For civilian use (farming, photography, planning a stargazing trip) the average-period approximation is accurate to within a few hours of the precise phase time. For astronomical work, use JPL DE421 ephemerides or the Meeus algorithm.

Practical uses

  • Astrophotography. The Milky Way disappears within a day of the full moon, plan deep-sky shoots around the new moon.
  • Fishing. Many anglers swear by the 3 days either side of new and full moons.
  • Wedding planning. A full moon rising at sunset makes for one of the cheapest light sources in photography.
  • Gardening. Biodynamic gardening schedules sowing around the moon; whether that works is another conversation.

Frequently Asked Questions

Within a few hours of the true phase for dates between roughly 1900 and 2100. Over centuries the error grows because the synodic period drifts slightly. For weddings, photography or stargazing, it is more than accurate enough.

The Moon’s phase is a single astronomical event that happens at the same instant everywhere, the only difference is whether it is night or day in your timezone. Anchoring to UTC avoids off-by-one errors near midnight.

Lunar age is how many days have passed since the last new moon (0 to 29.53). Illumination is the percentage of the visible disc lit by the Sun (0% at new, 100% at full). Age grows linearly; illumination follows a cosine curve, which is why the “quarter” phases look exactly half-lit.

Step the date forward a day at a time until the phase reads “Full moon”, or check a 29-day window from today. Future versions will surface upcoming phase dates directly.

No. The phase is the same worldwide at a given instant, so no location is requested or stored.

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