Week Number Calculator — What Week Is It?
What week of the year is it? See today's week number in both the ISO 8601 system used in Europe and business, and the U.S. system, plus the start and end of every week of the year.
ISO week number
| Week | Starts | Ends |
|---|
How to find a week number
- Pick a date; today is filled in automatically.
- Read the ISO and U.S. week numbers and the date range of the week.
- Use the table to see every week of the year in the system you choose.
ISO vs U.S. week numbers
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4). A year has 52 or 53 ISO weeks, and early January dates can belong to the last week of the previous year.
The common U.S. system starts weeks on Sunday and counts the week containing January 1 as week 1, so it can have a partial first and last week. Spreadsheets reflect both: Excel's ISOWEEKNUM and WEEKNUM functions.
Week numbers are widely used in Europe for scheduling, in manufacturing and retail calendars, and in project planning, so it helps to agree on which system you mean.
Need a different tool? Suggest it here — we read every message.
Frequently asked questions
What week number is it today?
Open the page; today's ISO and U.S. week numbers appear automatically.
How many weeks are in a year?
52 or 53 ISO weeks; years starting on Thursday (or Wednesday in a leap year) have 53.
Why is January 1 in week 52 or 53?
In ISO, early January days belong to the previous year's last week if that week has the year's first Thursday after them.
Which system does Excel use?
ISOWEEKNUM gives ISO weeks; WEEKNUM defaults to the U.S. system.