Week Number Calculator
Find the ISO 8601 and US/Canada week number for any date.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this week number calculator
- Pick a date in the date field.
- Press Calculate to see the ISO 8601 week number, the US/Canada week number, and the day of the year.
- Use the result to fill in a form, schedule a sprint, or check fiscal-week numbering.
- Use Copy to grab the result, or Reset to start over.
About this week number calculator
The week number calculator turns any calendar date into a week-of-year number, using two different conventions: ISO 8601 (the international standard, used across Europe, Asia, and most enterprise software) and the US/Canada convention (Sunday-starts, Week 1 contains January 1). The two systems disagree by one in the first and last weeks of the year, which is why "Week 1" can mean different things in different places.
Behind the scenes the ISO 8601 calculation uses the standard algorithm: weeks start on Monday, and Week 1 of a year is the week containing the year's first Thursday. This means a date in early January can belong to Week 52 or 53 of the previous year, and a date in late December can belong to Week 1 of the next year — both are correct under ISO 8601. The US/Canada calculation is simpler: weeks start on Sunday, and Week 1 always contains January 1. We use the browser's native Date object, so leap years are handled correctly.
For example, 2024-12-30 is ISO Week 1 of 2025 (because the week of Mon Dec 30 contains Thu Jan 2, 2025) but US/Canada Week 53 of 2024. The tool shows both so you do not have to guess which convention the form you are filling in uses.
FAQ
- What is the difference between ISO and US week numbering?
- ISO 8601 weeks start on Monday and Week 1 is the week containing the year's first Thursday. US/Canada weeks start on Sunday and Week 1 always contains January 1. The two systems often disagree by one in early January and late December.
- Can a date be in Week 53?
- Yes. Long years (leap years that start on Thursday, or any year that starts on Thursday) have 53 ISO weeks. 2020 and 2026 are examples.
- Why is January 1 sometimes in last year's ISO week?
- Because under ISO 8601 the week is anchored to its Thursday. If January 1 falls on a Friday, Saturday, or Sunday, that week's Thursday is in December, so the week belongs to the previous year. The tool shows the correct ISO year alongside the week number.
- Which convention should I use?
- If you are in Europe or working with enterprise software, use ISO. If you are in the US or Canada and the form does not specify, use the US/Canada number. When in doubt, check what other dates in the system look like — Week 1 starting on a Sunday is the giveaway for US/Canada.
- Does the tool work for any year?
- Yes. It uses native JavaScript Date arithmetic, so it works for any year the browser supports — which in practice means anything from year 100 to year 275760.