Hijri month length calculator

Enter a Hijri year from 1 AH onward and a month number from 1 through 12. The result identifies whether that month contains twenty-nine or thirty days and whether the supplied year is a tabular leap year. It uses fixed calendar arithmetic, so identical inputs always produce identical results without consulting an observation service, location, clock, or external calendar feed.

What this calculator tells you

A Hijri month can contain twenty-nine or thirty days, and the answer depends on which Islamic calendar convention is being used. This calculator answers the question specifically for the tabular Islamic calendar, a rule-based system that organizes years into a repeating thirty-year cycle. Supply the Hijri year as a positive integer and the month as a number from 1 through 12. The response repeats those inputs, reports the month length in the days field, and marks whether the year is a tabular leap year. That compact result is useful in validation rules, date pickers, schedules, teaching materials, and systems that already store tabular Hijri dates. It does not convert the date to the Gregorian calendar, identify a month by local naming conventions, or determine when a visible crescent was observed. By stating the calendar model directly, the result remains precise: it is an arithmetic calendar answer, not a claim about a religious authority's announced month boundary in a particular country.

How the tabular calculation works

The tabular Islamic calendar alternates month lengths in a regular pattern. Odd-numbered months normally have thirty days, while even-numbered months have twenty-nine. The twelfth month is the exception that connects the monthly pattern to the leap-year cycle: it has twenty-nine days in an ordinary year and thirty days in a leap year. Leap years are selected deterministically within a repeating cycle of thirty Hijri years. The calculation evaluates the supplied year against that cycle using integer arithmetic, then applies the appropriate rule for the supplied month. There is no dependence on the current date, a machine's local timezone, daylight-saving rules, geographic coordinates, network availability, or randomized data. Although the computation is suitable for execution in UTC environments, it never needs to construct a clock-based date at all. This avoids the common error of mixing local date methods with UTC serialization and guarantees that a browser and an API worker receive the same result for the same two integers.

Choose the calendar convention before relying on the result

Use this result when your specification explicitly calls for the tabular Islamic calendar or when deterministic arithmetic is more important than matching a locally proclaimed lunar month. It is especially appropriate for software tests, historical calculations that adopt a stated tabular convention, offline tools, and database constraints designed around the thirty-year cycle. Do not treat the answer as a substitute for an observational calendar, a national calendar authority, or a calendar such as Umm al-Qura. Those systems can assign month boundaries from observations, predictions, or published tables and may therefore differ from the tabular result. A sound integration records the convention alongside the stored Hijri date, validates that the year is at least 1 and the month is between 1 and 12, and uses the returned days value as the maximum permitted day. The endpoint costs $0.002 per request when automated through the API, while the same deterministic logic can run in the browser for an immediate individual check.