Week Numbers Explained: ISO Weeks, Excel Formulas and Calendar Settings
Make sense of "W42" on a work schedule and why week numbers sometimes disagree.
Updated:
Why the same day has different week numbers
There are three common systems, so the same date can differ by a week depending on your company, school or country.
| System | Week runs | Week 1 is | Used by |
|---|---|---|---|
| Sunday start | Sun–Sat | The week with Jan 1 | US calendars, Excel default |
| Monday start | Mon–Sun | The week with Jan 1 | Some companies and schools |
| ISO 8601 | Mon–Sun | The week with the year's first Thursday | Europe, global companies, production and logistics |
Compare all three instantly in the Week Number Calculator.
Reading ISO week notation
Global schedules write 2026-W42: ISO week 42 of 2026. Add a weekday and it becomes 2026-W42-3 (Wednesday; Monday = 1 … Sunday = 7).
Watch the turn of the year. Because ISO counts from the first Thursday, January 1–3 can belong to week 52 or 53 of the previous year, and December 29–31 to week 1 of the next. That's how "2027-W01" can start in December 2026.
Years with 53 weeks
A year is 52 weeks plus one or two days, so every few years there's a week 53. In ISO terms, years starting on a Thursday (or a Wednesday in leap years) have 53 weeks. 2026 is one of them — a 52-column yearly plan will come up a week short.
Excel and Google Sheets formulas
- Sunday start:
=WEEKNUM(A1) - Monday start:
=WEEKNUM(A1,2) - ISO:
=ISOWEEKNUM(A1) - Monday of an ISO week (year in B1, week in C1):
=DATE(B1,1,4)-WEEKDAY(DATE(B1,1,4),3)+(C1-1)*7
No formulas needed: enter a year and week in the calculator's Week → Dates tab.
Showing week numbers in your calendar
- Google Calendar (web): Settings → View options → "Show week numbers"
- Outlook: File → Options → Calendar → "Show week numbers in the month view and date navigator"
- iPhone Calendar: Settings → Calendar → "Week Numbers"
Names can vary by version. To count days to a deadline, use the Day Counter.
Tools used in this guide
FAQ
My company's week number is one off from the calculator.
Check which system your company uses. Global firms and production plans mostly use ISO; school and payroll calendars often use Sunday or Monday start. Switch the system at the top of the calculator.
What week of the month is it?
Usually the row containing the 1st is week 1 of the month, so if the 1st is a Saturday, that single day is week 1. Organisations may differ.
Can I see today's week number right away?
Yes. The Week Number Calculator shows this year's and this month's week for today as soon as it opens.