Find the ISO week number for any date, or find which dates fall in any given week number. The UK follows the ISO 8601 standard where weeks start on Monday and week 1 is the week containing the first Thursday of the year.
How are UK week numbers calculated?
The UK uses the ISO 8601 standard for week numbering. Under this system weeks start on Monday and end on Sunday. Week 1 is defined as the week containing the first Thursday of the year. This means week 1 sometimes starts in late December of the previous year. A year can have either 52 or 53 weeks depending on which day of the week 1 January falls.
Common questions
Why isn’t week 1 always the first week of January?
Because ISO 8601 anchors week 1 to whichever week contains 4 January — if 1 January falls on a Friday, Saturday or Sunday, that partial week counts as the last week of the previous year instead.
Where are week numbers actually used?
Common in payroll, project planning, manufacturing schedules, and some European business contexts where “week 23” is used instead of a calendar date.
Does the week number reset differently in different countries?
Yes — the US commonly starts weeks on Sunday and counts differently, so a “week number” from a US source may not match this UK/ISO-based figure.
See also
Use our UK working days calculator to count working days in a date range, use our days between dates calculator for any date range, or see our how many working days in 2026 guide.