Week Number Calculator
Finds the ISO 8601 week number for a date.
ISO week number
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Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
ISO week and year —
Monday of that week —
Sunday of that week —
Weeks in that ISO year — weeks
Reading this —
The formula
week = 1 + whole weeks from 1 January to this week’s Thursday
ISO 8601 weeks start on Monday, and week 1 is the one holding the year’s first Thursday.
What to watch out for
In the United States a common scheme starts weeks on Sunday and puts 1 January in week 1. If you are matching a calendar or a payroll system, check which it uses.
Frequently Asked Questions
Which week does 1 January fall in?
It depends on the year. 1 January 2026 is in week 1 of 2026, but 1 January 2021 is in week 53 of 2020.
How many weeks are in a year?
52 or 53. ISO year 2026 has 53 and 2025 has 52. A year has 53 when it holds 53 Thursdays.