Week number calculator

The ISO 8601 week number is the international standard most UK businesses and payroll systems use. Weeks start on Monday and week 1 is the week that contains the year's first Thursday, which means it always contains 4 January. Because of this rule, a few days in early January can belong to the previous ISO year, and a year has either 52 or 53 ISO weeks. Enter a date to read off its week number, or enter a year and week number to see the Monday-to-Sunday dates of that week.

This works both ways: type a date to get its ISO week number, or switch mode to turn an ISO year and week number back into the Monday and Sunday that bracket it.

Week number lookup

What do you want to do? Find the ISO week number for a date Find the dates of an ISO week number Date ISO year Week number

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How this is worked out

We use the ISO 8601 week-date system, the international standard also used by most UK businesses and payroll.

weeks start on Monday
week 1 is the week containing the year's first Thursday
so week 1 always contains 4 January
a year has 52 or 53 ISO weeks; dates in early January can belong to the previous ISO year

This differs from a simple "count the Sundays" approach: under ISO 8601 the week number can roll into the next or previous year for a few days around New Year. Source: ISO 8601 date and time standard.

Need the gap between two dates instead? Use the date difference calculator, or see the 2026 UK bank holidays.

Source: the ISO 8601 date and time standard.