How to Convert Hijri Dates to Gregorian (and Back)
Converting between the Hijri and Gregorian calendars is not a matter of adding a fixed number, because the two calendars have years of different length. Here is how it is done, from a quick estimate to an exact conversion.
Quick estimate of the year
The Hijri year is about 354.37 days and the Gregorian year about 365.24 days, a ratio close to 32:33. That gives two handy formulas:
- Hijri year ≈ (Gregorian year − 622) × 33 ÷ 32
- Gregorian year ≈ Hijri year × 32 ÷ 33 + 622
For 2027: (2027 − 622) × 33 ÷ 32 ≈ 1448.9, so most of 2027 falls in 1448 AH, and the year 1449 begins in June 2027.
Exact conversion with the Julian Day Number
Converters do not calculate from one calendar straight into another. They first turn a date into a Julian Day Number (JDN), a continuous count of days used by astronomers, and then read that number back in the target calendar. 16 July 622, the first day of the Hijri era in the tabular calendar, is JDN 1,948,440.
For the arithmetical (tabular) Islamic calendar, the JDN of a date (year y, month m, day d) is:
JDN = d + ⌈29.5 × (m − 1)⌉ + 354 × (y − 1) + ⌊(3 + 11y) ÷ 30⌋ + 1,948,439
The first part counts the days in the earlier months (alternately 30 and 29), the second the days in earlier common years, and the third adds the 11 leap days of each 30-year cycle. The Gregorian date is then recovered from the JDN with the standard Gregorian formulas.
Calculated tables
For 1356–1500 AH (1937–2077), HijriTime does not use the formula but the published Umm al-Qura tables, which store the actual start of each month. These can differ from the tabular calendar by a day or two.
Why your result might be one day off
A converter gives the date in one particular calendar. If your country starts each month with a local moon sighting, the Hijri date you see on the street may be one day behind. For historical documents, also remember that the writer's community may have used its own sighting. Try the Hijri date converter to convert any date, including your date of birth.
Sources and Verification
This article was written by Fatah Nasrat and checked against the official, scientific and academic sources below. HijriTime does not copy source texts; it explains their methods and findings in its own words.
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