Is the Umm al-Qura calendar lunar or solar?

Purely lunar: twelve lunar months of 354 or 355 days a year; in Saudi Arabia it operates alongside the Gregorian calendar for certain official purposes.

Why does the Umm al-Qura calendar sometimes differ from crescent sighting?

Because it is computed astronomically, not based on actual sighting; the rule ensures the moon is above Mecca’s horizon when a month begins, but the crescent may not be visible to the naked eye that first evening — so the religious sighting announcement can come a day or two later than the computed date.

Who uses the Umm al-Qura calendar?

It is Saudi Arabia’s official civil calendar, followed by several Gulf states, and the default Islamic calendar in Windows for Arabic locales; many Muslims worldwide rely on it to align their dates with official calendars.

Can it tell me when Ramadan starts in my country?

It gives the standard official Saudi date; the start of fasting in your country follows your local religious authority, which may rely on sighting. That is why this site’s tools offer a ±1/±2 day adjustment to match your local announcement.

Umm al-Qura Calendar: Rules and Sighting Differences

What is the Umm al-Qura calendar?

The Umm al-Qura calendar is the official Hijri calendar of Saudi Arabia for civil purposes, named after the official gazette “Umm al-Qura” in which it was first published. Its dates are computed today at the Institute of Astronomical and Geophysical Research of the King Abdulaziz City for Science and Technology (KACST) in Riyadh from modern astronomical theories of the sun and moon. Several Gulf states follow it officially, and it is the default Islamic calendar in Windows systems set to Arabic — which has carried its use to Muslim communities worldwide.

How is it computed today?

The rule in force since 1423 AH (15 March 2002) is: on the 29th day of the month, if both conditions below hold at Makkah, the next day is the first of the new month:

the geocentric conjunction (astronomical birth of the crescent) occurs before sunset;
the moon sets after the sun.

Otherwise the month runs to thirty days. The rule guarantees that a month never begins before the astronomical new moon and that the moon is above the horizon at the month’s start in Makkah — yet it remains purely computational: in about three quarters of cases the crescent is not visible to the naked eye on the month’s first evening from Makkah, since the moon being above the horizon does not mean it can be seen.

How the rules evolved over the decades

The Umm al-Qura rules were not fixed historically; they evolved through stages documented by researchers from the old official tables:

Before 1392 AH (1972 CE): no securely documented rule for the early period; some published studies point to requiring the crescent to stand at least 9° above the Meccan horizon after sunset, a rule the old official tables do not conclusively confirm.
1392–1419 AH (1972–1999 CE): if conjunction occurred before midnight GMT, the month began at the previous sunset — a lunar age of about 15 hours or less at month’s start; in the great majority of cases the crescent was invisible to the naked eye on the first evening.
1420–1422 AH (1999–2001 CE): the month began on the first evening the moon set after the sun at Makkah; two anomalous cases (Rajab 1421 and Shaban 1422) began before the astronomical new moon itself, prompting a rule change.
From 1423 AH (2002 CE): the current rule described above, designed so a month can never begin before the astronomical new moon.

Civil vs religious: why dates sometimes differ

Strictly speaking, the Umm al-Qura calendar is civil, regulating state affairs, documents, contracts and pensions. The months tied to worship — Muharram, Ramadan, Shawwal and Dhul-Hijjah — remain subject to crescent sighting: Saudi Arabia’s Supreme Court issues monthly calls to sight the crescent, and the announced start can differ from the computed date by a day or two based on sighting testimony. For this reason every tool on this site carries a ±1/±2 day adjustment to match the calendar announced in your country; see the methodology page for details.

Umm al-Qura on this site

The date converter, the Hijri age calculator and the month calendars on the Hijri year pages are all based on the complete official Umm al-Qura table (1300–1500 AH / 1882–2077 CE), so results match the official Saudi calendar, with an optional ±1/±2 day adjustment for those following their country’s sighting announcement.

Sources and references

Umm al-Qura University, Makkah: the official source of the Umm al-Qura calendar.
King Abdulaziz City for Science and Technology (KACST): the computing authority, through its Institute of Astronomical and Geophysical Research in Riyadh.
R. van Gent, Utrecht University — The Astronomical Rules Governing the Umm al-Qura Calendar: an academic study documenting the evolution of the rules with crescent-visibility statistics for each stage.

Frequently asked questions

Is the Umm al-Qura calendar lunar or solar?
Purely lunar: twelve lunar months of 354 or 355 days a year; in Saudi Arabia it operates alongside the Gregorian calendar for certain official purposes.
Why does the Umm al-Qura calendar sometimes differ from crescent sighting?
Because it is computed astronomically, not based on actual sighting; the rule ensures the moon is above Mecca’s horizon when a month begins, but the crescent may not be visible to the naked eye that first evening — so the religious sighting announcement can come a day or two later than the computed date.
Who uses the Umm al-Qura calendar?
It is Saudi Arabia’s official civil calendar, followed by several Gulf states, and the default Islamic calendar in Windows for Arabic locales; many Muslims worldwide rely on it to align their dates with official calendars.
Can it tell me when Ramadan starts in my country?
It gives the standard official Saudi date; the start of fasting in your country follows your local religious authority, which may rely on sighting. That is why this site’s tools offer a ±1/±2 day adjustment to match your local announcement.