Week Number Calculator (ISO 8601)
Pick a date to see its ISO week number. Weeks start on Monday, and the date starts as today.
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Formula
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday.
- Find the Thursday of the date's week:
thursday = date + (4 − weekday)days, where Monday is 1 and Sunday is 7 - The ISO year is the calendar year of that Thursday
week = floor((days from 1 January of the ISO year to that Thursday) ÷ 7) + 1
Worked example
Sunday 3 January 2021:
- weekday is 7, so its Thursday is 3 − 3 = 31 December 2020
- the ISO year is 2020
- 31 December 2020 is 365 days after 1 January 2020 (a leap year), and floor(365 ÷ 7) + 1 = week 53
Frequently asked questions
How is the ISO week number defined?
In ISO 8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday. Monday 1 January 2024 therefore starts week 1 of 2024.
Why can early January be in week 52 or 53?
If a week has its Thursday in the old year, the whole week belongs to that year. Sunday 3 January 2021 is in week 53 of 2020.
Why can late December be in week 1?
If a week's Thursday falls in the new year, the week is week 1 of that year. Monday 30 December 2024 is in week 1 of 2025.
Which years have 53 weeks?
Years that start on a Thursday, or leap years that start on a Wednesday. 2026 starts on a Thursday, so 31 December 2026 is in week 53.