About the Week Number Calculator
This week number calculator shows the current week of the year as soon as you open the page. You can also check which week any date falls in, or turn a week number such as “week 40” into its exact dates – handy for payroll periods, sprints, school schedules, shipping and production planning.
Two numbering systems are supported: the US system (weeks start on Sunday, week 1 contains January 1) and the international ISO 8601 standard (weeks start on Monday) used in Europe and by many companies.
How to use
- Open the page: the top card shows today's week number and the dates of the current week.
- Choose US (Sunday) or ISO (Monday) numbering with the buttons at the top.
- Under Week number of a date, pick any date to see its week and the week's date range.
- Under Dates of a week number, enter a year and week to get its first and last day, or scroll the full-year calendar at the bottom.
US vs ISO week numbers
| US system | ISO 8601 | |
|---|---|---|
| First day of week | Sunday | Monday |
| Week 1 | Week containing January 1 | Week containing the first Thursday (January 4) |
| Weeks per year | 53 (54 in rare leap years) | 52 or 53 |
| Excel | =WEEKNUM(date) | =ISOWEEKNUM(date) |
The two systems often differ by one week. For example, Sunday, October 4, 2026 is in week 41 in the US system but in ISO week 40.
How ISO weeks work
- Every ISO week runs Monday to Sunday and belongs entirely to one “week-year”.
- Week 1 is the week with the year's first Thursday, so January 1–3 can belong to week 52 or 53 of the previous year.
- Years that start on a Thursday (or leap years starting on a Wednesday) have 53 weeks: 2015, 2020, 2026, 2032.
- ISO weeks are written like
2026-W40, or2026-W40-4for the Thursday of that week.
FAQ
What week number is it today?
The top of this page shows the current week number automatically, based on your device's date. For example, October 1, 2026 is week 40 in both the US and ISO systems.
Why does January 1 show as week 53?
In the ISO system, week 1 is the week containing the first Thursday. If January 1 falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous year – January 1, 2021 was in 2020-W53. In the US system, January 1 is always in week 1.
How many weeks are in 2026?
2026 has 53 ISO weeks because it starts on a Thursday; ISO week 53 runs from December 28, 2026 to January 3, 2027. In the US system 2026 also has 53 weeks, the last one being just December 27–31.
How do I get the week number in Excel or Google Sheets?
Use =WEEKNUM(A1) for the US system (Sunday start) and =ISOWEEKNUM(A1) for ISO weeks. Both functions work the same way in Google Sheets.