If you've ever needed to know a birthday, an anniversary, or a historical event in "the other" calendar — or wondered why Ramadan lands about 11 days earlier every year — this guide walks through how Hijri-Gregorian conversion actually works, and how to use it correctly.
How is a Gregorian date converted to Hijri?
Quick answerThe Gregorian date is converted into a continuous Julian day number first, then mapped onto the Hijri calendar's 30-year leap cycle, where 11 of every 30 years get an extra day (355 instead of 354). This arithmetic method — often called the Kuwaiti or Islamic Civil calendar — is what most free online converters use, because it can be computed instantly without a lookup table.
The Hijri calendar is purely lunar: each of its 12 months alternates between 30 and 29 days, following the approximate length of a lunar cycle. A converter doesn't track the actual moon — instead it applies a fixed rule that has been checked against the real lunar cycle over centuries, so month lengths are known in advance rather than announced month by month.
A tool built this way typically also lets you go the other direction — entering a Hijri day, month, and Hijri year (AH, for Anno Hegirae) to get the matching Gregorian date — using the same Julian day number as the bridge between the two systems.
Julian day number: a simple running count of days that has nothing to do with the Julian calendar by name — it's just a calendar-agnostic day counter, which makes it a convenient common format to convert between the Gregorian and Hijri systems.
Why does the Hijri-Gregorian gap keep changing?
Quick answerA Hijri year is about 11 days shorter than a Gregorian year (354-355 days versus 365-366). That shortfall means Hijri dates drift earlier through the Gregorian calendar year after year, and the numeric year gap between the two calendars slowly narrows — closing by roughly one year about every 33.6 years.
This is the reason Ramadan, Eid al-Fitr, and Eid al-Adha don't fall on the same Gregorian dates each year — they cycle slowly backward through the seasons, completing a full loop through all four seasons roughly once every 33 years.
Example (illustrative only): Suppose someone was born on a Gregorian date that converts to 1 Rajab in a given Hijri year. Because the Hijri year is shorter, their Hijri-calendar age in years will typically be a little higher than their Gregorian age by the time they're older — since more Hijri years fit into the same span of real time. This is just an illustration of the math, not a specific person's actual data.
Example conversions using the arithmetic (Kuwaiti) method
| Gregorian date | Day of week | Hijri equivalent |
|---|
| January 1, 2026 | Thursday | 12 Rajab 1447 AH |
| July 20, 1969 | Sunday | 5 Jumada al-awwal 1389 AH |
| July 4, 1776 | Thursday | 17 Jumada al-awwal 1190 AH |
What are the Hijri months, and which are sacred?
Quick answerThe 12 Hijri months, in order, are Muharram, Safar, Rabi' al-awwal, Rabi' al-thani, Jumada al-awwal, Jumada al-thani, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Qadah, and Dhu al-Hijjah. Of these, Muharram, Rajab, Dhu al-Qadah, and Dhu al-Hijjah are the four sacred months.
Ramadan, the ninth month, is the month of fasting. The first day of the following month, Shawwal, is Eid al-Fitr; the tenth day of Dhu al-Hijjah — the final month, and the month of Hajj — is Eid al-Adha. A good converter will flag these landmark dates and note when a converted date falls inside one of the four sacred months.
Good to know: a purely arithmetic converter can differ by a day (occasionally two) from an official local moon-sighting announcement, because many mosques and national Islamic authorities confirm the start of a month by observation rather than a fixed formula. For prayer times, Ramadan, or Eid, always defer to your local mosque or Islamic authority.
Convert any date between Hijri and Gregorian instantly
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Frequently asked questions
How is a Hijri date calculated from a Gregorian date?
The Gregorian date is first converted into a continuous Julian day number, a day count that doesn't depend on any particular calendar. That number is then run through the Hijri calendar's 30-year leap cycle (years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26, and 29 have 355 days instead of 354) to produce the Hijri year, month, and day. This is the arithmetic (Kuwaiti / Islamic Civil) method, and it runs entirely in your browser with no data sent to a server.
Why does the gap between Hijri and Gregorian years keep changing?
A Hijri year is lunar and runs about 354-355 days, roughly 11 days shorter than a 365-day Gregorian year. That 11-day drift means Hijri months move earlier through the Gregorian seasons every year, and it also means the Hijri year slowly catches up on the Gregorian year, closing the numeric gap by about one year roughly every 33.6 years.
Will an online Hijri converter match my local mosque's Ramadan or Eid announcement?
Usually, but not always. An arithmetic converter uses a fixed mathematical rule to set month lengths in advance. Many local Islamic authorities instead confirm the start of a month by sighting the crescent moon, or by using observation-based criteria. The two approaches typically agree but can differ by a day. For religious observance, always follow the announcement from your local mosque or Islamic authority rather than an online calculator.
What are the four sacred months in the Hijri calendar?
Muharram, Rajab, Dhu al-Qadah, and Dhu al-Hijjah are traditionally set apart as sacred months. Three of them fall consecutively around the Hajj season (Dhu al-Qadah, Dhu al-Hijjah, and Muharram), and Rajab stands alone in the middle of the year.
What's the difference between the Kuwaiti/Tabular method and Umm al-Qura?
Kuwaiti / Tabular (Islamic Civil) sets Hijri month lengths using a fixed 30-year mathematical leap cycle. Umm al-Qura is Saudi Arabia's official calendar, normally published as a lookup table rather than a formula; an online tool typically applies a documented day-offset approximation of it rather than the exact official table. The two usually agree but can differ by a day, occasionally two, in a given month.
A note on methodology: the figures above (year lengths, drift rate, example conversions) reflect the standard arithmetic Islamic Civil calendar algorithm and are provided for general, informational purposes. They are not a religious ruling — for prayer times, fasting, or holiday observance, always follow guidance from your local mosque or Islamic authority.