Two ways to number weeks
Week numbers are widely used in business planning, logistics, school calendars and manufacturing. There are two common systems:
- ISO 8601, the international standard used in Europe and by most companies: weeks start on Monday, and week 1 is the week that contains the year’s first Thursday (equivalently, January 4).
- US system: weeks start on Sunday, and week 1 is the week that contains January 1.
The calculator gives the week number in the system you choose, along with the first and last day of that week, the day of the year and the number of days left in the year.
The formulas
- ISO week = the week of the Thursday in the same Monday-to-Sunday week, counted from the first Thursday of its year.
- US week = floor((day of the year − 1 + weekday of January 1) ÷ 7) + 1, with Sunday = 0.
- First day of the week = the Monday (ISO) or the Sunday (US) on or before the date.
Worked example
November 3, 2026 is a Tuesday.
- ISO: the Thursday of that week is November 5, the 309th day of 2026. The first Thursday of 2026 is January 1, so November 5 is in week 45.
- The week runs from Monday, November 2 to Sunday, November 8.
- US: January 1, 2026 is a Thursday (weekday 4); November 3 is day 307, so floor((306 + 4) ÷ 7) + 1 = week 45 as well.
When the year boundary moves
In the ISO system, the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next year. January 1, 2027, a Friday, is in ISO week 53 of 2026. Week-based planning tools label such dates with the ISO year, for example 2026-W53.
Years with 53 weeks
An ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That happens about every 5 to 6 years.
| ISO year | Weeks |
|---|---|
| 2024 | 52 |
| 2025 | 52 |
| 2026 | 53 |
| 2027 | 52 |
| 2032 | 53 |
Which one should I use?
Use ISO weeks when working with European partners, ERP and supply-chain systems, or any tool that follows ISO 8601. Many US calendars and spreadsheets default to the Sunday-start system; in Excel, ISOWEEKNUM and WEEKNUM return the two numbers.