Week Number Calculator
Weeks run Monday through Sunday, and week 1 is whichever week contains the year's first Thursday — the ISO 8601 standard. Everything below is computed instantly in your browser.
How the ISO week number is worked out
Every ISO 8601 week runs Monday to Sunday, and week 1 of a
year is fixed as whichever Mon–Sun week contains that year's
first Thursday. The standard trick for computing this is
Thursday-anchoring: shift any date to the
Thursday of its own Mon–Sun week, then count whole weeks from
that Thursday back to 4 January's Thursday of the same
stretch. It's the same algorithm behind spreadsheet functions
like ISOWEEKNUM and most date libraries — this
tool doesn't invent its own rule.
Worked example
Take 31 December 2024 — a Tuesday. Its Mon–Sun week runs 30 December 2024 to 5 January 2025. That week contains 2 January 2025, a Thursday, so it belongs to ISO week 1 of 2025 — even though the date itself is still in calendar year 2024. Flip it around: take 1 January 2027, a Friday. Its Mon–Sun week runs 28 December 2026 to 3 January 2027, whose Thursday (31 December 2026) sits in ISO year 2026 — so 1 January 2027 is ISO week 53 of 2026, the last week of the previous ISO year. Both are correct outputs of the same rule, not edge-case bugs.
Frequently asked questions
How is the ISO week number actually calculated?
ISO 8601 weeks run Monday to Sunday, and week 1 of a year is defined as whichever week contains that year's first Thursday — which is exactly equivalent to "the week containing 4 January." To find a date's week number, you find its Thursday (shifting forward or back within the same Mon–Sun week), then count how many Thursdays away that is from the first Thursday of its ISO year. It sounds fiddly, but it's the same standard algorithm used by spreadsheet ISOWEEKNUM functions and most programming-language date libraries.
Why did this tool show me a different year than the one I typed?
Because the ISO week-numbering year isn't always the same as the calendar year — it happens whenever the Monday–Sunday week containing 1 January or 31 December crosses into the neighboring year. Two real examples: 31 December 2024 is a Wednesday, and its Mon–Sun week (30 Dec 2024 – 5 Jan 2025) is ISO week 1 of 2025, not week 53 of 2024. Going the other way, 1 January 2027 is a Friday, and its week belongs to ISO week 53 of 2026, not week 1 of 2027. This tool always labels which is which so the two years never get silently mixed up.
Does every year have 53 weeks?
No — most years have 52 ISO weeks, and roughly one year in five or six has 53. A year gets a 53rd week if 1 January falls on a Thursday, or if the year is a leap year and 1 January falls on a Wednesday. 2026 is a 53-week year (1 January 2026 is a Thursday); 2020 was too, as a leap year starting on a Wednesday. This tool computes the real count for whichever year you enter rather than assuming 52.
Why do ISO weeks start on Monday instead of Sunday?
That's simply what the ISO 8601 standard specifies, and it's the convention most business, payroll, and project-planning calendars follow internationally — distinct from the Sunday-start week some US calendar apps default to. If you're cross-checking against a Sunday-start calendar app, expect its "week number" to sometimes disagree with the ISO number by one, especially near year boundaries.