Moon Phase Calculator

What is a moon phase calculator?

A moon phase calculator tells you which of the eight lunar phases the Moon is in on any date, how old the Moon is in days, and what percent of its disc is lit. Enter a date and the tool returns a named phase — New, Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Last Quarter or Waning Crescent — along with the Moon’s age and its illumination.

The tool above needs just one input: the date. It defaults to today, so you can read the current phase at a glance, or set any past or future date to look up a birthday moon, a planned event or an upcoming full moon. Everything runs instantly and privately in your browser; no date is sent to a server.

How does the moon phase calculator work?

The Moon takes one synodic month — the time from one new moon to the next, 29.530589 days — to run through its full cycle of phases. The calculator counts the whole days from a fixed reference new moon (6 January 2000) to your date, then takes the remainder over 29.53 days. That remainder is the moon’s age: how many days have passed since the most recent new moon.

In formula form:

moon age = (date − reference new moon) mod 29.53 days; illumination = (1 − cos(2π × age/29.53)) ÷ 2

A few terms worth defining:

• Moon age (days): days elapsed since the last new moon, from 0 up to about 29.5.
• Fraction: the age divided by 29.53 — a number from 0 to 1 describing how far through the cycle the Moon is.
• Illumination: the percent of the visible disc that is lit, found with (1 − cos(2π × fraction)) ÷ 2. It is 0 percent at new moon, rises to 100 percent at full moon, then falls back.

To name the phase, the fraction is split into eight equal slices. A fraction near 0 (or near 1) is a New Moon; near 0.25 is First Quarter; near 0.5 is Full; near 0.75 is Last Quarter; and the in-between slices are the crescent and gibbous phases.

The eight phases by moon age

This reference table maps the moon’s age to its phase and rough illumination. Because the cycle is just under 29.53 days, each of the eight slices spans roughly 3.7 days.

Phase	Approx. age (days)	Illumination	Trend
New Moon	0 (and ~29.5)	~0%	dark
Waxing Crescent	~3.7	~15%	growing
First Quarter	~7.4	~50%	growing
Waxing Gibbous	~11.1	~85%	growing
Full Moon	~14.8	~100%	peak
Waning Gibbous	~18.5	~85%	shrinking
Last Quarter	~22.1	~50%	shrinking
Waning Crescent	~25.8	~15%	shrinking

“Waxing” means the lit portion is growing toward full; “waning” means it is shrinking back toward new. The quarters sit at the halfway points where exactly half the disc is lit.

Examples

Here are worked examples you can reproduce with the calculator above. Each follows the same rule: take the moon’s age, divide by 29.53 to get the fraction, then apply the illumination formula.

Example 1 — moon age near 0 days (New Moon). A fraction of 0 gives illumination = (1 − cos(0)) ÷ 2 = (1 − 1) ÷ 2 = 0, so about 0 percent lit. This is a New Moon, the dark start of the cycle. The same applies near 29.5 days, where the cycle resets to a new moon.

Example 2 — moon age near 7.4 days (First Quarter). Here the fraction is about 0.25, so illumination = (1 − cos(π/2)) ÷ 2 = (1 − 0) ÷ 2 = 0.5, that is about 50 percent. The result is a First Quarter moon, with the lit half still growing.

Example 3 — moon age near 14.8 days (Full Moon). The fraction is about 0.5, so illumination = (1 − cos(π)) ÷ 2 = (1 − (−1)) ÷ 2 = 1, which is about 100 percent. This is a Full Moon, the brightest point of the cycle.

Example 4 — moon age near 22 days (Last Quarter). The fraction is about 0.75, so illumination = (1 − cos(3π/2)) ÷ 2 = (1 − 0) ÷ 2 = 0.5, about 50 percent again. But now the disc is waning, so the tool returns a Last Quarter moon rather than First Quarter.

Notice how the illumination is the same 50 percent at both quarters — the phase name is what distinguishes a growing (first quarter) moon from a shrinking (last quarter) one.

Common uses

• Stargazing and astrophotography: dark new-moon nights are best for faint deep-sky objects, while a full moon washes them out.
• Planning a full moon event: weddings, hikes, beach nights and rituals often aim for a bright full moon.
• Fishing and tides: many anglers track the lunar cycle because new and full moons drive the largest tidal swings.
• Gardening lore: lunar-calendar gardeners time planting and pruning to waxing or waning phases.
• Curiosity and content: look up the moon phase on a birthday, anniversary or any memorable date.

Tips and common mistakes

• Age and phase are not the same. The age is a number of days; the phase is the named slice it falls in. A 7.4-day moon is First Quarter, not “a quarter day old.”
• Don’t read 50 percent as one phase. Both quarters show 50 percent illumination — the waxing-versus-waning trend is what tells First Quarter from Last Quarter.
• New moon means dark, not invisible forever. At about 0 percent illumination the lit side faces away from Earth, so the Moon is effectively unlit, not gone.
• The cycle is 29.53 days, not 28. The popular “28-day” figure is a rounding; the synodic month used here is 29.530589 days.
• Time and place are not inputs. This tool gives a date-based global average, so it will not pinpoint moonrise for your city.

Limitations and notes

This calculator uses a simple, well-known astronomical model: the days since the reference new moon of 6 January 2000, reduced over the 29.530589-day synodic month, with illumination from a clean cosine curve. That makes it fast and accurate to within a day for everyday use, but it does not model the Moon’s slightly varying orbit, so a precise observatory ephemeris can differ by a few percent on illumination or shift a phase boundary by under a day. It reports a single global phase rather than a location-specific view, and it does not compute moonrise, moonset or the exact clock time of a phase. For planning and curiosity it is more than sufficient, and it runs entirely on your device.

For more birth-date and number tools, try our zodiac sign calculator, chinese zodiac calculator and numerology calculator, or browse the full astrology category.