How do I find the week number of a date?
- Find the Thursday of the week. Weeks run Monday to Sunday, so go back to Monday and forward 3 days.
- Note that Thursday's year. That is the week-year, which can differ from the date's calendar year near New Year.
- Count the weeks from the first Thursday of that year: the Thursday's day of the year, minus 1, divided by 7 (rounded down), plus 1.
The calculator above does this for any date and also shows the US-style week number. Open All ISO weeks to see the Monday and Sunday dates of every week in that year.
Why does ISO 8601 use the first Thursday?
Thursday is the middle of a Monday-to-Sunday week, so the week containing the first Thursday always has at least four days in the new year. That rule gives every week exactly one number and stops a week from being split between two years. The trade-off is that up to three days at the start of January can belong to the previous year's last week, and up to three days at the end of December can belong to the next year's week 1.
Worked examples
Example 1: a date in the middle of the year
Tuesday, November 3, 2026 is in ISO week 45 (2026-W45-2), which runs from Monday, November 2, 2026 to Sunday, November 8, 2026.
Example 2: New Year's Day in the previous year's week
Friday, January 1, 2027 falls in week 53 of 2026. The week's Thursday is December 31, 2026, so the whole week belongs to 2026. Its US-style week number is 1.
Example 3: late December in next year's week 1
Monday, December 30, 2024 is in week 1 of 2025, because that week's Thursday is January 2, 2025.
How many weeks does each year have?
| ISO year | Weeks | Week 1 starts | Last week ends |
|---|---|---|---|
| 2025 | 52 | Monday, December 30, 2024 | Sunday, December 28, 2025 |
| 2026 | 53 | Monday, December 29, 2025 | Sunday, January 3, 2027 |
| 2027 | 52 | Monday, January 4, 2027 | Sunday, January 2, 2028 |
| 2028 | 52 | Monday, January 3, 2028 | Sunday, December 31, 2028 |
| 2029 | 52 | Monday, January 1, 2029 | Sunday, December 30, 2029 |
| 2030 | 52 | Monday, December 31, 2029 | Sunday, December 29, 2030 |
Weekly timesheets usually cover one Monday-to-Sunday or Sunday-to-Saturday week; total one with the time card calculator. For a two-week pay period, use the biweekly time card.