🗓️ Week Number Calculator
Any date → its ISO 8601 week number, with the week's exact Monday–Sunday range. Today's week number shown by default.
Week 29
of ISO year 2026 · Mon, Jul 13, 2026 → Sun, Jul 19, 2026
ISO 8601: weeks start Monday; week 1 contains the year's first Thursday.
How the week number calculator works
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January). This means the days around New Year can belong to the other year's week numbering: 29–31 December can fall in week 1 of the next year, and 1–3 January in week 52 or 53 of the previous one. Years have 52 or 53 ISO weeks.
ISO is not the only convention: the US system starts weeks on Sunday and calls the week containing 1 January "week 1", so a US planner and a European one can disagree by one on the same date. This tool computes the ISO standard — the one meant when a supplier says "delivery in week 34".
Frequently asked questions
How is week 1 of a year defined?
Under ISO 8601, week 1 is the week (Monday–Sunday) containing the year's first Thursday — equivalently the week containing 4 January. 1 January can therefore sit in week 52/53 of the previous year.
Can a year have 53 weeks?
Yes — years starting on a Thursday, and leap years starting on a Wednesday, have 53 ISO weeks. It happens roughly every 5–6 years; 2026 has 53.
Why does my US calendar show a different week number?
The US convention starts weeks on Sunday and numbers the week containing 1 January as week 1. The two systems disagree for part of most years. European business and manufacturing use ISO.
What day does an ISO week start?
Monday, always. The tool shows each week's full Monday-to-Sunday date range so there is no ambiguity.
Is the date I enter transmitted?
No — the week number is computed in your browser with a few lines of calendar arithmetic.