Week Number Calculator
Find the ISO-8601 week number for any date.
How to use
- 1 Pick a date with the date field.
- 2 Read the ISO week number and its label, e.g. 2026-W26.
- 3 Check the ISO week-numbering year, which can differ from the calendar year.
- 4 See whether the year has 52 or 53 weeks.
- 5 Copy the week label if you need it.
About Week Number Calculator
The Week Number Calculator tells you which ISO-8601 week any date falls in.
Pick a date and it shows the week number, the matching ISO label such as "2026-W26", the weekday and how many weeks that year contains.
ISO weeks are the standard used across business, manufacturing, finance and much of Europe, and they have two rules that surprise people.
Weeks start on Monday, not Sunday, and week 1 is the week that contains the year's first Thursday — equivalently, the week holding 4 January.
Because of that, the first days of January can belong to the last week of the previous year, and the final days of December can belong to week 1 of the next year.
This tool surfaces that distinction by reporting the ISO week-numbering year separately from the plain calendar year, so you are never caught out at the boundaries.
It also tells you whether a year is a "long" 53-week year or an ordinary 52-week year, which matters for payroll cycles, sprint planning and reporting calendars.
Dates are interpreted in UTC so the answer never shifts with your timezone, and the calculation is deterministic.
Everything runs locally in your browser with no uploads, so it is private and works offline.
FAQ
Which week-numbering standard does this use?
ISO-8601. Weeks start on Monday and week 1 is the week containing the year's first Thursday (the week with 4 January).
Why does the ISO year sometimes differ from the calendar year?
Early-January or late-December dates can belong to a neighbouring year's week, so the ISO week-numbering year is shown separately.
How can a year have 53 weeks?
Years whose first Thursday placement leaves room for an extra Monday-start week are 53-week "long" years; the tool reports this for each year.