ISO week 27 ISO week-year 2026 Weekday Monday Day of year 180 US week 27 (Sunday-start)
About ISO 8601 Week Number Calculator — What Week Is It
"What week is it?" sounds simple until you hit a year boundary. The ISO 8601 week number isn't the calendar week you might count by hand — weeks start on Monday, and week 1 is the week containing the year's first Thursday. That rule means early-January and late-December dates can belong to a different ISO week-year than the calendar year, which trips up payroll, sprint, and reporting calculations.
This free week number calculator finds the ISO week number for any date, along with the ISO week-year, the weekday, and the day-of-year. It also shows the US-style week (Sunday-start) so you can compare the two conventions side by side. Everything runs entirely in your browser — no date ever leaves your device.
Features
- ISO 8601 week number and ISO week-year for any date
- Correctly handles year-boundary weeks (1, 52, and 53)
- Weekday name and day-of-year alongside the week number
- US (Sunday-start) week number for comparison
How to use
- Enter a date (or press Today to use the current date).
- Read the ISO week number and ISO week-year in the output.
- Compare the US (Sunday-start) week and check the day-of-year.
- Copy the breakdown with the Copy button.
Frequently asked questions
Why does the ISO week-year sometimes differ from the calendar year?
ISO 8601 assigns whole weeks to a year, and a week belongs to the year that contains its Thursday. So January 1, 2021 (a Friday) falls in ISO week 53 of 2020, because that week's Thursday was December 31, 2020.
Can a year have 53 weeks?
Yes. A year has 53 ISO weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year. Most years have 52. For example, 2020 has 53 ISO weeks.
What is the difference between the ISO week and the US week?
ISO weeks start on Monday and week 1 contains the first Thursday of the year. The common US convention starts weeks on Sunday and counts week 1 as the week containing January 1. The two numbers can differ near year boundaries.
Is my date sent anywhere?
No. The calculation happens entirely in your browser using the built-in date functions. Nothing is uploaded, logged, or stored.
Related tools
Everything runs locally in your browser — your input is never uploaded.