“What week is it?” sounds like a trivial question until you hit the turn of the year. Our new Week Number Calculator uses the real ISO 8601 rule — not an approximation — and works in both directions.
- Date → week number. Pick any date and get its ISO week number instantly, computed with the standard Thursday-anchoring algorithm (week 1 is whichever week contains the year’s first Thursday).
- Week number → date range. Enter a week and a year and get the exact Monday–Sunday dates back, with a live count of how many ISO weeks that year actually has.
- The ISO year, shown honestly. Near New Year’s, a date’s ISO week-numbering year can differ from its calendar year — 31 December 2024 falls in ISO week 1 of 2025, and 1 January 2027 falls in ISO week 53 of 2026. Most tools quietly pick one year and hope you don’t notice the other; this one labels both so nothing gets silently mixed up.
- 52 or 53 weeks — actually checked, not assumed. Most years have 52 ISO weeks, but roughly one in five or six — including 2026 — have 53. The tool computes the real count for whichever year you enter.
- Private by default. Every calculation runs in your own browser; nothing you enter is sent to a server.
Whether you’re filling in a timesheet that wants “week 33” instead of a date, or working backward from “week 1” to figure out which Monday that actually is, this tool gets you there in one step — including the edge cases most calculators quietly get wrong.