Week Number Calculator
Get the ISO week number for any date and see the exact Monday-to-Sunday range that week covers.
How it works
ISO week = the Monday–Sunday week containing the date, numbered by where its Thursday falls
ISO-8601 weeks run Monday through Sunday, and week 1 is the week that contains January 4 — equivalently, the week of the year’s first Thursday. That means up to three days of early January can belong to the last week of the previous ISO year, and late-December days can already be week 1 of the next. An ISO year has 52 weeks, or 53 when January 1 falls on a Thursday (or on a Wednesday in a leap year).
Worked example
September 11, 2026 falls in ISO week 37 of 2026, which runs Monday, September 7 through Sunday, September 13. January 1, 2026 is a Thursday, so it opens week 1 — and gives 2026 a rare 53rd week.
Frequently asked questions
Why can January 1 be week 52 or 53 of the previous year?
Because week 1 must contain the first Thursday. January 1, 2016 was a Friday, so it belonged to week 53 of 2015; the ISO year label follows the week, not the calendar year.
Who uses ISO week numbers?
They are standard across most of Europe for business planning, manufacturing schedules and invoicing (“delivery in week 37”), and in ISO-compliant software everywhere.
How often does a 53-week year happen?
About every 5–6 years — 71 times in each 400-year Gregorian cycle. 2026 has 53 ISO weeks; the next after that is 2032.
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This calculator is for educational purposes. Double-check important results before acting on them.