Week Number Calculator
Find the ISO-8601 week number for any date, along with its ISO week-numbering year.
ISO week number
- ISO week-numbering year
- 2,024
Updates live as you type
Frequently asked questions
Under ISO-8601, weeks run Monday to Sunday and week 1 is the week containing the year’s first Thursday. Week numbers therefore range from 1 to 52 or 53.
Early-January days can belong to the last ISO week of the previous year, and late-December days to week 1 of the next. The ISO week-numbering year reflects this.
It depends on the weekday. For 2024, 1 January is a Monday, so it falls in ISO week 1. In other years it can land in week 52 or 53 of the prior year.
They are common in business planning, manufacturing, payroll and logistics across Europe, where schedules are often expressed as “week 14” rather than a date.
Some years, "1 January" belongs to last year's final week
1 January 2024 returns ISO week 1 of 2024 — but only because that date happens to fall on a Monday, so the Monday-to-Sunday week containing it also contains 4 January (a Thursday), satisfying the ISO rule directly. That's not true every year: when 1 January falls on a Friday, Saturday or Sunday instead, that date actually belongs to the last ISO week of the previous year, since the week containing the year's first Thursday starts later than 1 January.
The rule behind the numbering
Many industries plan by week rather than by date — "ship in week 32," "results in week 14" — and the ISO-8601 standard defines exactly how that numbering works: weeks run Monday to Sunday, and week 1 is whichever week contains the year's first Thursday (equivalently, the week containing 4 January). That rule produces a clean 1–52 or 1–53 numbering with no partial weeks, which is why it underlies business and manufacturing calendars, especially across Europe.
Why the ISO year can differ from the calendar year
A subtle but important detail is that the ISO week-numbering year can differ from the calendar year at the boundaries. A date in early January may belong to the final week (52 or 53) of the previous ISO year, and a late-December date may already belong to week 1 of the next ISO year — both are correct under the standard, just counter-intuitive if you assume week numbers reset cleanly on 1 January every time. This calculator reports the correct ISO year alongside the week number so there is never ambiguity about which year a given week number belongs to.
Use it to align with European business schedules, plan sprints and production runs that are tracked by week, or simply to answer "what week is it?" for any date, past or future.