Why isn't there a fixed mathematical formula for conversion?
The Gregorian calendar is solar with an almost fixed length (365 or 366 days in a leap year), while the Hijri calendar is lunar with a variable length (354 or 355 days), which is why a simple mathematical equation isn't enough to convert accurately — this relies on the astronomical Umm al-Qura calendar table, the same reference used by trusted Hijri age and occasion calculators.
For precise religious occasions (the start of Ramadan, the eve of Eid), it's always best to verify with an official adopted announcement rather than relying solely on the astronomical calculation, since it may differ by one day from the actual moon sighting.
FAQ
How accurate is this converter?
It relies on the digital Umm al-Qura calendar, which is accurate for everyday practical use. It may differ by one day from sources that rely on actual moon sighting in some months, so for precise religious occasions it's best to confirm the date from an official source.
Why are some Hijri months 29 days and others 30?
A lunar month actually ranges between 29 and 30 days depending on the moon's cycle, and the Umm al-Qura calendar reflects this variation month by month instead of using a fixed length.
What if I enter a Hijri day that doesn't exist in that month (like day 30 in a 29-day month)?
The calculator automatically uses the actual last valid day of that month instead of ignoring the request or showing an error, with a note about this in the result.
Source
The Hijri calendar used for conversion: the astronomical Umm al-Qura calendar (islamic-umalqura), via the international Unicode standard (ICU) used in web browsers to set global calendars.
Related calculators
Last updated: July 2026
This converter's results are astronomical estimates, not a substitute for an official announcement for religious occasions.
Read the full disclaimer.