Dates and time
How to find the ISO week number
You need the ISO week of a civil date. You will read the week label and the weekday in this tab. Monday is 1 and Sunday is 7. A date in late December can belong to next year's week 1.
Open week number when you have a civil date. This is not a week that starts on Sunday. Nothing is uploaded.
Open Week numberContents
Use the week that starts on Monday
Monday is weekday 1. Sunday is weekday 7. A week number that starts on Sunday is a different system. This page will not produce that number.
Monday is 1. Sunday is 7. Week 1 holds the first Thursday. Open week number on cv.cm
The week stays in this tab.
Open Week numberPick the civil date
The date is YYYY-MM-DD. Clock time does not move the week. 1 January 2026 is Thursday, weekday 4, and it is 2026-W01.
Read the ISO year separately from the calendar year
Week 1 is the week that contains the year's first Thursday. 1 January 2021 is a Friday, so it stays in 2020-W53. 30 December 2024 is a Monday in 2025-W01.
1 Jan 2021 is 2020-W53. 30 Dec 2024 is 2025-W01. Copy the label and the weekday
The label looks like 2020-W53. The weekday is a number from 1 to 7. A spreadsheet that wants a Sunday-start week cannot use this label as-is.
Check one Thursday you already know
The first Thursday of the year is inside week 1. If 1 January is a Thursday, that date is weekday 4 of week 1 of the same year, as 2026-01-01 is 2026-W01.
FAQ
Can I find an ISO week number free online?
Yes. cv.cm shows the ISO year, week, and weekday in this tab. Nothing is uploaded.
Which day starts the week?
Monday. Monday is 1 and Sunday is 7. A Sunday-start week is a different number.
Can January belong to the previous year?
Yes. 1 January 2021 is a Friday in 2020-W53. Week 1 is the week with the first Thursday.
Can late December belong to the next year?
Yes. 30 December 2024 is a Monday in 2025-W01.
Is the date uploaded?
No. Closing the tab discards it.