Week number of any date
Pick a date and the tool shows its week number using the US convention (weeks start Sunday, week 1 holds January 1) and the ISO 8601 standard (weeks start Monday, week 1 holds the first Thursday). It also gives the week start and end dates and the day of the year. Enter a year and week to get its dates.
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How week numbers work in the US
The common U.S. convention (used by many calendars and payroll systems) starts weeks on Sunday. Week 1 is the week that contains January 1, so it can begin in late December. Weeks run Sunday to Saturday.
\[ \text{US week} = \left\lfloor \dfrac{\text{days since the Sunday on or before Jan 1}}{7} \right\rfloor + 1 \]
Many businesses and software use ISO 8601 instead, so always check which one a spreadsheet or schedule uses.
The ISO 8601 standard
ISO 8601 starts the week on Monday, and week 1 is the week with the year's first Thursday, which is the week that contains January 4. A year has 52 or 53 weeks.
| Year | ISO weeks | Monday of week 1 |
|---|---|---|
| 2024 | 52 | 01/01/2024 |
| 2025 | 52 | 12/30/2024 |
| 2026 | 53 | 12/29/2025 |
| 2027 | 52 | 01/04/2027 |
| 2032 | 53 | 12/29/2031 |
A year has 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year.
Why the two numbers can differ
Around New Year, the same date can sit in week 53 of the old year under ISO and in week 1 of the new year under the US rule. In the middle of the year the two usually match or differ by one.
| Date | US week | ISO week |
|---|---|---|
| 12/29/2025 | 1 (2026) | 1 (2026) |
| 01/01/2027 | 1 (2027) | 53 (2026) |
| 01/04/2027 | 2 (2027) | 1 (2027) |
| 10/30/2026 | 44 (2026) | 44 (2026) |
3 real-life examples
Today, October 30, 2026
Friday, October 30, 2026 is in week 44 under both systems. The ISO week runs Monday, October 26 to Sunday, November 1, and the US week runs Sunday, October 25 to Saturday, October 31. It is the 303rd day of the year.
January 1, 2027
This Friday is in US week 1 of 2027 but in ISO week 53 of 2026. ISO week 1 of 2027 starts Monday, January 4. This mismatch trips up many year-end schedules.
December 29, 2025
This Monday already belongs to week 1 of 2026 in both systems, because its week contains Thursday, January 1, 2026.