How does the Moon Phase Calculator work out the phase?

It measures the time elapsed since a reference new moon and divides by the mean synodic month of 29.530588 days. The fractional position in that cycle maps directly to the phase name and the illuminated fraction of the disc.

When is the best moon phase for stargazing?

The nights around a new moon. With no moonlight the sky is at its darkest, which is when faint targets like the Milky Way core and meteor showers are most visible. A full moon is bright enough to wash out all but the brightest stars.

Why is a lunar month 29.5 days rather than 27.3?

The moon orbits Earth in 27.3 days, but Earth moves along its own orbit during that time, so the moon needs roughly two extra days to return to the same alignment with the sun — and it is that alignment that produces the phases.

Does the moon phase depend on where I am on Earth?

The phase itself is the same everywhere, because it is the moon-sun geometry. What changes with location is orientation — the southern hemisphere sees the disc flipped — and the local date at the moment the phase peaks.

How accurate are the next full moon and new moon dates?

Within a few hours, because the calculator uses the mean cycle length rather than a full lunar theory. That is more than enough to plan photography or fishing trips, though almanacs quote the exact minute.

Moon Phase Calculator

Find the moon phase for any date — name, illumination and moon age — plus the next full and new moon and a whole-month phase calendar.

The Moon Phase Calculator runs entirely in your browser. The dates you check are processed on your device and never uploaded.

Open the Sunrise & Sunset Calculator

About Moon Phase Calculator

Moon Phase Calculator places any date inside the 29.53-day synodic cycle — the interval from one new moon to the next — and names the phase it lands on: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter or waning crescent. Alongside the name you get the moon's age in days, the illuminated fraction of the disc, and the dates of the next new and full moon. A month grid shows the whole cycle at a glance, which is the view photographers, stargazers and anglers actually plan from: a full moon washes out the Milky Way, while the nights around a new moon are the darkest of the month.

Features

Phase name, emoji and waxing or waning direction for any date
Illuminated percentage of the lunar disc
Moon age in days since the last new moon
Dates of the next new moon and next full moon
Whole-month calendar with a phase icon on every day
Works for past and future dates alike
Based on the astronomical synodic cycle, not a lookup service
Everything computed on your device

How to use the Moon Phase Calculator

Pick a date — today is preselected
Read the phase name, illumination and moon age
Check the next full and new moon dates for planning
Scan the month grid to see how the cycle unfolds

Example

Input

2026-08-15

Output

🌑 New Moon · 2% illuminated · 29.2 days old · next full moon Aug 28

The days around a new moon are the best for astrophotography.

Common errors & troubleshooting

The phase shown differs by a day from a newspaper listing. — Phase boundaries fall at exact instants that depend on time zone. A moon that turns full at 01:00 UTC is still “waxing gibbous” the previous evening in the Americas — both statements are right.
The illumination percentage never quite reaches 100%. — Exactly 100% happens only at the instant of full moon. A calendar-day reading a few hours away will show 99% — that is precision, not error.
The crescent seems to point the wrong way. — Hemisphere matters. From the southern hemisphere the moon appears flipped, so a waxing crescent lights the left edge instead of the right.
Dates far in the past look slightly off against historical records. — The calculator uses a mean synodic month, so a small drift of a few hours accumulates over centuries. For calendar planning within decades it is well within a day.

Frequently asked questions

How does the Moon Phase Calculator work out the phase?: It measures the time elapsed since a reference new moon and divides by the mean synodic month of 29.530588 days. The fractional position in that cycle maps directly to the phase name and the illuminated fraction of the disc.
When is the best moon phase for stargazing?: The nights around a new moon. With no moonlight the sky is at its darkest, which is when faint targets like the Milky Way core and meteor showers are most visible. A full moon is bright enough to wash out all but the brightest stars.
Why is a lunar month 29.5 days rather than 27.3?: The moon orbits Earth in 27.3 days, but Earth moves along its own orbit during that time, so the moon needs roughly two extra days to return to the same alignment with the sun — and it is that alignment that produces the phases.
Does the moon phase depend on where I am on Earth?: The phase itself is the same everywhere, because it is the moon-sun geometry. What changes with location is orientation — the southern hemisphere sees the disc flipped — and the local date at the moment the phase peaks.
How accurate are the next full moon and new moon dates?: Within a few hours, because the calculator uses the mean cycle length rather than a full lunar theory. That is more than enough to plan photography or fishing trips, though almanacs quote the exact minute.

Related tools

Sunrise & Sunset Calculator — Sunrise, sunset, solar noon, twilight and golden hour for any date and location.
Date Calculator — Add or subtract years, months, weeks and days from a date to find the resulting date.
Day of the Week Calculator — Find the day of the week for any date, past or future.
Age Calculator — Calculate exact age in years, months and days as of any date, plus totals in weeks, days, hours and next birthday.
Date Duration Calculator — Calculate the duration between two dates in years, months, weeks, days and business days.
ISO Week Number Calculator — Find the ISO-8601 week number for any date, and the date range for a given week.