Week Number Calculator
The ISO week number and day-of-year for any date.
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About this calculator
Manufacturing schedules, payroll, project plans and European business calendars run on ISO week numbers. This calculator gives the ISO 8601 week for any date, the Monday–Sunday span it covers, and the day-of-year — including the tricky year boundaries where 29 December can already be week 1.
The formula
ISO 8601: weeks start Monday; week 1 contains the year's first Thursday
Worked example
18 July 2026 falls in week 29, which runs Monday 13 July to Sunday 19 July.
Common mistakes
- Assuming week numbers reset on 1 January — the first days of January can be week 52/53 of the previous ISO year.
- Mixing ISO weeks with US-style weeks (Sunday start) in the same schedule.
- Using the calendar year with a week number — pair it with the ISO year shown to be unambiguous.
Frequently asked questions
Some diaries use US week numbering (weeks start Sunday, week 1 contains 1 January). ISO 8601 is the business standard in the UK and Europe.
Yes — years starting on a Thursday (or leap years starting Wednesday) have 53 ISO weeks.
The year the week belongs to, which can differ from the date's calendar year around New Year — that's why it's shown separately.
Assumptions & limitations
Strict ISO 8601 numbering.
Sources & verification
- ISO 8601 — date and time format — International standard defining week numbering (weeks start Monday; week 1 contains the first Thursday)
All sources last accessed 24 July 2026.
Formula last verified: 24 July 2026
Maintained by: Steve Davis, Founder & Editor — see our editorial standards
Spotted an error or an out-of-date figure? Report it — corrections are prioritised over all other work (see our corrections policy).
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