Week Number
Week Number — runs in your browser. Inputs are not stored.
Week number calculator
Enter values, then press Calculate.
ISO 8601 (week starts Monday; week 1 is the week that contains the year’s first Thursday).
How to use
- Pick a Date on the calendar. When you open the page, today is often the default.
- Press Calculate to see ISO week and ISO year.
- Use Week start (Mon) and Week end (Sun) to see that week’s bounds.
- Copy the sentence in the Summary field into a report title.
- Press Today to return to today.
Key concepts
The week-number calculator follows ISO 8601. Weeks start on Monday, and week 1 is the week that contains the year’s first Thursday. The number can differ from a US Sunday-start calendar. Around New Year, ISO year and calendar year can disagree, so the week can be 53 or week 1 of the next year.
When putting a week number in a weekly report or sprint name, first agree whether the team starts weeks on Monday. Calculation runs only in the browser. Holidays and working-day counts are not computed. The summary sentence is a convenient shape to copy into a Slack title.
Example
If 1 January is a Wednesday, the ISO week may still be attached to the previous year’s 53. Press Today, then change only the other date to compare. Confirm week start is Monday and end is Sunday, then align it with the weekly document period. Around New Year, ISO year and calendar year can differ, so the week can be 53 or week 1 of the next year. When putting a week number in a weekly report title, check that Monday start matches the team rule. Press Today to set the baseline, then change only the other date to compare. When trying the same input several times, reset and paste again so it does not mix with the previous output. On a shared computer, clear the fields when you finish so the original is not left on the monitor. Closing the browser tab also discards what you just pasted; you do not need to send a delete request to a server. Button names and field titles match the tool at the top of the screen, so you can follow along as you read. If the result is empty, first check that required fields are filled and that an error message is not hidden. This article is usage guidance and does not mean input is kept in external storage.
Related: Unix timestamp converter · Online clock
Frequently asked questions
What is an ISO week?
It starts on Monday, and week 1 is the week that contains the year’s first Thursday. This screen follows that rule.
Is it different from a Sunday-start calendar?
It can be. US week numbers and ISO often disagree.
Is week-number calculation done on a server?
No. It is calculated only in the browser.
When do ISO year and calendar year differ?
Often around New Year. January can still be the previous year’s week, or December can be week 1 of the next year.
What weekdays are week start and end?
Start is Monday and end is Sunday.
Does it subtract public holidays?
No. It only shows the date’s ISO week and the week span.
Last reviewed: 2026-09-04