ISO Week Number Calculator (week-of-year)
Pick any date and see its ISO 8601 week number (1-53), the ISO weekday (1=Monday, 7=Sunday), the Monday-Sunday range of that week, and useful related counts (day of year, weeks in year).
- Week number
- 19
- Weekday
- Thursday
- Week starts (Monday)
- Mon, May 4, 2026
- Week ends (Sunday)
- Sun, May 10, 2026
- Day of year
- 127
- Weeks in year
- 53
How it works
What's an ISO week
ISO 8601 defines a calendar week starting Monday and ending Sunday. The first week of any ISO year is the one containing that year's first Thursday — or equivalently, the week containing January 4. This means a year can have 52 or 53 ISO weeks, and the ISO year doesn't always match the calendar year for late-December and early-January dates.
Format: 'YYYY-Www-D'. So '2026-W19-3' means Wednesday of week 19 of 2026. Weekday number: 1=Monday, 7=Sunday. Week number: 1-52 or 1-53.
Why ISO weeks differ from calendar weeks
US calendars typically count Sunday as the first day. ISO is Monday-first. This means: a Sunday in your local calendar that's at end of week 30 is, ISO-wise, end of week 30. But your typed Sunday might fall on a different ISO week if it's the last Sunday of one year and the first Monday is another.
Also: ISO years can be off by one from calendar years. Dec 31 2024 is in ISO week 1 of 2025 (week containing the first Thursday of 2025). Jan 1 2024 was Monday — week 1 of 2024 starts Jan 1. Jan 1 2025 is Wednesday — week 1 of 2025 starts Dec 30 2024.
Where ISO weeks are used
Business: many European companies invoice and ship by ISO week. 'Order will arrive in W23' is unambiguous and language-independent.
Software: many programming languages have ISO week functions. JavaScript Date doesn't directly, so libraries like date-fns and dayjs add it. Used in scheduling, log analysis, and reporting.
Healthcare epidemiology: weekly disease incidence reports use ISO weeks. WHO and CDC use them in their flu and COVID dashboards.
Sports / gaming: many leagues schedule by 'week 1, week 2, …' which often align with ISO weeks (especially European leagues).
Frequently asked questions
›Why doesn't this match Excel's WEEKNUM function?
Excel's WEEKNUM uses different conventions. The default is US-style (Sunday-first). For ISO weeks in Excel, use ISOWEEKNUM or specify return_type=21 in WEEKNUM.
›Can a year have 53 weeks?
Yes. A year has 53 ISO weeks if Jan 1 is a Thursday OR Dec 31 is a Thursday. About 71 in every 400 years.
›What's the ISO year vs calendar year?
Most days they're the same. Late-December and early-January edge cases differ. Dec 29 2008 is ISO week 1 of 2009 (calendar year 2008, ISO year 2009).
›Is week 1 always Jan 1's week?
Not always. ISO week 1 contains the year's first Thursday. So if Jan 1 is a Friday-Sunday, that day is part of the previous year's last ISO week.
›What's the difference between W1 and Week 1?
Same thing in ISO context. The W prefix is the ISO format convention. '2026-W01' means ISO week 1 of 2026.
›Why Monday as week start?
ISO 8601 standardized this. Most of the world uses Monday-first; only Saudi Arabia, US, Canada, and a few other countries customarily use Sunday-first.
›Can I export to a calendar?
Not directly from this tool. Use the week start/end dates and create your own calendar event.
›Does the data leave my browser?
No. Calculation runs locally; nothing is sent to a server.
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