Two week numbers for the same day
A week number labels each week of the year, from week 1 to week 52 or 53. Schedules, factory plans, school calendars and delivery slips use them because “week 12” is shorter than “March 16 to 22”.
The catch: two numbering systems are in common use, and they often give different numbers for the same day. The United States mostly uses a system where weeks start on Sunday and week 1 is the week that contains January 1. Europe and most of the world use ISO 8601, where weeks start on Monday and week 1 is the week with the first Thursday of the year. This page shows the US number by default and tells you, under the result, when the ISO number is different. If you work with people in Europe, switch to ISO 8601 and the tool remembers your choice.
The ISO 8601 rules
- Weeks run from Monday to Sunday.
- Week 1 is the week with the year’s first Thursday. Put another way, it is the week that contains January 4, or the one whose Monday is closest to January 1.
- Every week belongs to exactly one year, so a year has 52 or 53 whole weeks.
- The edges can spill over. January 1 to 3 may still sit in the last week of the previous year, and December 29 to 31 may already be week 1 of the next one.
Example: January 1, 2027 is a Friday. ISO week 1 of 2027 starts on Monday, January 4, so January 1, 2 and 3 are in week 53 of 2026. The same days are week 1 in the US system.
The US system
Weeks run from Sunday to Saturday, and week 1 is simply the week that contains January 1, even if that week holds a single day of the new year. The last week of December is counted too, even when it is cut short. That is why a US year can have 53 week numbers, and occasionally 54: in 2028, January 1 is a Saturday and December 31 is a Sunday, so both are alone in their week and the year runs from week 1 to week 54. This is the system Excel uses when you type =WEEKNUM(date) without a second argument.
Week 1 in both systems
| Year | US week 1 | ISO week 1 | Weeks (US / ISO) |
|---|---|---|---|
| 2025 | Dec 29, 2024 to Jan 4, 2025 | Dec 30, 2024 to Jan 5, 2025 | 53 / 52 |
| 2026 | Dec 28, 2025 to Jan 3, 2026 | Dec 29, 2025 to Jan 4, 2026 | 53 / 53 |
| 2027 | Dec 27, 2026 to Jan 2, 2027 | Jan 4, 2027 to Jan 10, 2027 | 53 / 52 |
| 2028 | Dec 26, 2027 to Jan 1, 2028 | Jan 3, 2028 to Jan 9, 2028 | 54 / 52 |
| 2029 | Dec 31, 2028 to Jan 6, 2029 | Jan 1, 2029 to Jan 7, 2029 | 53 / 52 |
| 2030 | Dec 30, 2029 to Jan 5, 2030 | Dec 31, 2029 to Jan 6, 2030 | 53 / 52 |
Why some years have 53 weeks
Fifty-two weeks make 364 days, so every year has one day left over, two in a leap year. Those spare days add up, and every five or six years the ISO calendar needs a 53rd week to catch up. Under ISO 8601, a year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. That happens 71 times in every 400 years. Recent and coming examples: 2015, 2020, 2026, 2032, 2037 and 2043. 2026 is one of them: its week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
Week numbers in Excel and Google Sheets
- ISO week: =ISOWEEKNUM(A1), or =WEEKNUM(A1, 21).
- US week: =WEEKNUM(A1), which counts from Sunday with week 1 holding January 1.
- Weeks starting on Monday, US style: =WEEKNUM(A1, 2).
Google Sheets accepts the same formulas. Most calendar apps can also print week numbers in the margin: look for a “week numbers” option in their settings, and check which system it follows.
The ISO year is not always the calendar year
Because ISO weeks are never split, the first days of January can belong to the previous year’s last week, and the last days of December to the next year’s week 1. So an ISO week has its own year. December 29, 2025, for example, is in week 1 of 2026. In a spreadsheet, =YEAR(A1-WEEKDAY(A1,2)+4) returns the ISO year of the date in A1: it finds the Thursday of the same week, and that Thursday always sits in the right year. Use it next to ISOWEEKNUM whenever you build week labels such as 2026-W01, or your reports will mix up the two ends of the year.
How to work out an ISO week number by hand
Take the day of the year (January 1 is day 1) and the weekday as a number from 1 (Monday) to 7 (Sunday). Then: week = (day of year − weekday + 10) ÷ 7, rounded down. Example: November 2, 2026 is day 306 and a Monday (1). (306 − 1 + 10) ÷ 7 = 45, so it is in week 45. If you get 0, the date belongs to the last week of the previous year. If you get 53 in a year that only has 52 weeks, it is week 1 of the next year.
Where week numbers show up
In Germany, people say “KW 45” (short for Kalenderwoche, calendar week) in emails and meetings. Week numbers are just as common in the Nordic countries and the Netherlands, on printed calendars, in shipping and in manufacturing. In the US they appear mostly in payroll, retail and project plans. When a supplier abroad promises delivery “in week 12”, it almost always means the ISO week.
Full tables with every week and its dates: week numbers 2026, 2027 and 2028. To find the weekday of a date, use the day of the week calculator; to count the days between two dates, the date calculator; for working days only, the business days calculator.