Week Number Calculator: ISO 8601 Week of the Year
See which week of the year a date falls in, using the ISO 8601 standard (weeks start on Monday).
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Week number
45
ISO 8601- Week starts (Monday)
- Mon, 2 Nov 2026
- Week ends (Sunday)
- Sun, 8 Nov 2026
- ISO week-year
- 2026
- Weeks in that year
- 53 weeks
This date is day 308 of the year. ISO weeks start on Monday.
Nearby weeks
| Week / year | Monday | Sunday |
|---|---|---|
| 43 / 2026 | Mon, 19 Oct 2026 | Sun, 25 Oct 2026 |
| 44 / 2026 | Mon, 26 Oct 2026 | Sun, 1 Nov 2026 |
| 45 / 2026 | Mon, 2 Nov 2026 | Sun, 8 Nov 2026 |
| 46 / 2026 | Mon, 9 Nov 2026 | Sun, 15 Nov 2026 |
| 47 / 2026 | Mon, 16 Nov 2026 | Sun, 22 Nov 2026 |
| 48 / 2026 | Mon, 23 Nov 2026 | Sun, 29 Nov 2026 |
How to use
- Enter a date (today is filled in).
- See the week number and the Monday to Sunday it covers.
- Open the table for the weeks before and after.
Good to know
ISO 8601 weeks run Monday to Sunday. Week 1 is the week that contains the year's first Thursday, so 1 January can belong to week 52 or 53 of the previous year.
Most years have 52 ISO weeks; a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. 2026 has 53 weeks.
Some calendars, Excel's WEEKNUM and US planners count weeks from Sunday or from 1 January, so their numbers can differ by one. Many Indian companies, SAP and ERP systems use ISO weeks.
Questions
What week number is it today?
Open the page: today's date is filled in and the big number is the ISO week.
When does week 1 start?
On the Monday of the week that contains the first Thursday of January.
Why does 1 January show week 53?
If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous ISO year.
Is this the same as Excel WEEKNUM?
Not always. Excel's ISOWEEKNUM matches this; WEEKNUM by default starts weeks on Sunday from 1 January.
Is my data stored?
No. Everything is calculated on your device and nothing is sent anywhere.