What it is
Umm al-Qura is a lunar Hijri calendar that is calculated in advance, so the start of every month is known years ahead. It is the calendar used for civil purposes in Saudi Arabia, such as dates on official documents written in Hijri. It is computed astronomically with criteria tied to Makkah at the Institute of Astronomical & Geophysical Research of King Abdulaziz City for Science and Technology (KACST); its current calculation rules were standardised in 1423 AH (2002).
How a month starts
A lunar month begins after the new moon, when the crescent becomes visible. Umm al-Qura determines this by calculation rather than observation: criteria involving the moment of conjunction and the times of sunset and moonset at Makkah are checked on the 29th day; if they are met, the next day starts the new month, otherwise the month completes 30 days. The criteria have been revised over the decades.
The result: months of 29 or 30 days and years of 354 or 355 days. Both 1447 and 1448 AH have 355 days.
Why it can differ from moon sighting
The start of Ramadan, Shawwal and Dhu al-Hijjah is announced after moon sighting, because these months are tied to worship. Sighting on the evening of the 29th does not always match the calculation, so Ramadan or Eid can begin a day before or after the calendar date. The calendar is for planning and transactions; the announcement is final for these months.
Example: by Umm al-Qura, Ramadan 1448 AH starts Monday 8 February 2027 and Eid al-Fitr falls on Tuesday 9 March 2027. The countdown shows these with a “subject to moon sighting” label.
Umm al-Qura in our tools
We do not maintain the tables ourselves: we use the data in the ICU library used by modern browsers, covering 1300–1600 AH (roughly 12 November 1882 to 25 November 2174), and we test the round trip for every day of that range on every update. Outside it we use the arithmetic Hijri calendar and label results approximate. See the Methodology.
The arithmetic (tabular) Hijri calendar
The tabular calendar is a fixed rule: odd months 30 days, even months 29, with a day added to Dhu al-Hijjah in 11 of every 30 years. It is simple and good for estimates but does not follow the actual moon, so it can differ from Umm al-Qura by a day or two.
In practice
- Civil transactions and documents: Umm al-Qura.
- Ramadan and Eid: the official announcement.
- Old documents: the date written on the document.
Browse any full year in the Hijri Calendar.
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