Week Number Calculator
Find the ISO week number of any date, and the dates of any week of the year.
Enter your values to see the result.
Continue your work
- Current Time Now — See today’s week number.
- Date Calculator (Add & Subtract) — Add weeks to a date.
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Clear instructions
Find steps, examples and limitations below.
Use online
Open the tool in a supported web browser.
Free to use
No sign-up required. Tool-specific limits may apply.
How to use the Week Number Calculator
Find the ISO week number of any date, and the dates of any week of the year.
- 1 Choose Week of a date or Dates of a week.
- 2 Enter a date, or a year and week number.
- 3 Read the week number or the dates of that week.
Example and practical tips
30 September 2026 is in ISO week 40. Week 1 of 2027 starts on Monday 4 January.
Frequently asked questions
How are ISO weeks counted?
Weeks start on Monday, and week 1 is the week containing the year’s first Thursday.
Can a year have 53 weeks?
Yes. Years that start on a Thursday, or leap years that start on a Wednesday, have 53 ISO weeks.
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