Moon Phase Calculator
Calculate the Moon's phase, illumination percentage, lunar age, and upcoming phase timing for any calendar date.
Run tool
This online lunar calculator provides calendar-based estimates of lunar phases and illuminated surface percentages for any target day. When planning night sky viewing, outdoor photography, or general date research, enter a calendar date in standard YYYY-MM-DD format, or leave the date field blank to calculate values for the current UTC day.
The tool models the lunar cycle using a mean synodic-month calculation anchored to a known baseline new moon. For each evaluated date, the output identifies which of the eight standard phase names applies: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, or Waning Crescent. Alongside the phase category, you receive the estimated illuminated fraction rounded to three decimal places, the illumination percentage displayed to one decimal place, the approximate age of the current lunar cycle in days, and estimated counts of days remaining until the subsequent new moon and full moon.
Instructions
- Select or enter a target date formatted as YYYY-MM-DD in the date field.
- Leave the date field empty if you want to inspect the phase for today in UTC.
- Submit the date to view the estimated lunar phase, illumination percentage, and days until the next new or full moon.
Examples
- Entering 2024-01-11 displays New Moon with near 0.0% illumination, an early lunar cycle age, and estimated day counts to the next major phases.
- Entering 2024-08-19 displays Full Moon with an illuminated fraction near 1.000 and 100.0% surface illumination.
FAQ
What happens if I do not enter a date?
When the date field is left blank, the calculator automatically runs using the current date in UTC.
Which date format is supported?
The date field requires a standard four-digit year, two-digit month, and two-digit day formatted as YYYY-MM-DD. Dates with alternative formatting or invalid calendar days will not parse.
Is this calculation equivalent to high-precision astronomical ephemeris data?
No. This tool provides calendar-date approximations based on a standard synodic month cycle and does not replace ephemeris-grade or topocentric observation models.
What phase categories does the calculator report?
The tool classifies the date into one of eight categories: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, or Waning Crescent.