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.
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.
Common questions
How is the ISO week number worked out?
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year’s first Thursday, which means week 1 always contains 4 January. The calculator applies this rule to turn a date into its week number and to turn a week number back into its dates.
Why can a date in early January fall in the previous year’s week?
Because ISO week 1 is fixed to the first Thursday, a few days at the very start of January can belong to the last ISO week of the previous year. The calculator handles this so the week number and the ISO week-year stay consistent.
Can a year have 53 weeks?
Yes. Most years have 52 ISO weeks, but some have 53, depending on which weekday the year starts on and whether it is a leap year. The calculator returns the correct count for the year you enter.
Is the ISO week the same as the week number my payroll uses?
Usually, yes. ISO 8601 is the international standard that most UK businesses and payroll systems follow. Some organisations number their weeks a different way, so check with your employer if the exact number matters.
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Editor, Dates & Times
Oliver Mackman is the named editor of Dates & Times and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every result to cite the underlying date maths and the official UK source behind it, such as the gov.uk Bank Holidays API.
Last reviewed: 12 June 2026