Week number calculator
ISO week 1 contains 4 January. Weeks start on Monday. Try today, 1 January or 31 December — 1 January can sit in the previous ISO year.
Read more in how ISO week numbers work.
Week 37 of 2026
12 September 2026 falls in ISO week 37. The week runs Monday to Sunday.
How this works
Week 1 is the week containing 4 January. Weeks start on Monday, following ISO 8601. 1 January can belong to the previous ISO year.
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Frequently asked questions
No. ISO week 1 contains 4 January. Use the chips for 1 January and 4 January to see the difference.