How is current moon phase calculated?

Using days since a known new moon divided by the lunar cycle length (about 29.53 days).

Why does the lunar cycle have a fraction of a day?

The Moon's orbital mechanics don't align to a clean whole number of days.

Does moon phase affect tides?

Yes — new and full moons produce larger "spring tides," quarter moons produce smaller "neap tides."

Moon Phase Calculator - CalculatorLabs.net

Moon Phase Calculator

Find the moon's phase and illumination for any date.

What Is the Moon Phase Calculator?

The Moon Phase Calculator estimates the age of the moon (days since the last new moon) and its approximate phase and illumination percentage for any given date, based on the mean 29.53-day lunar synodic cycle.

Formula Used

Moon Age = (Date − Reference New Moon) mod 29.53059 days

This uses a fixed known new moon reference point and the average synodic month length, so results are approximate (within about a day) rather than precise ephemeris calculations.

How to Use This Calculator

Select any date, past or future.
Click Calculate to see the moon's age, phase, and illumination percentage.

Worked Example

For a date roughly 15 days after a new moon, the moon age is about 15 days, which falls near a Full Moon phase with close to 100% illumination.