Week Number Calculator
Find the ISO week number for any date, or discover which dates fall within a specific week. Includes day of year, quarter information, and more.
Week Number Calculator
Find the ISO week number for any date or get dates for a specific week
💡 ISO 8601: Week 1 is the week containing the first Thursday of the year. Weeks start on Monday.
Week Numbers Reference
| Month | Typical Weeks | Quarter |
|---|---|---|
| January | 1-5 | Q1 |
| February | 5-9 | Q1 |
| March | 9-13 | Q1 |
| April | 13-18 | Q2 |
| May | 18-22 | Q2 |
| June | 22-26 | Q2 |
| July | 26-31 | Q3 |
| August | 31-35 | Q3 |
| September | 35-40 | Q3 |
| October | 40-44 | Q4 |
| November | 44-48 | Q4 |
| December | 48-52/53 | Q4 |
* Week numbers vary slightly each year based on how days fall
Years with 53 Weeks
These years have 53 ISO weeks instead of the usual 52:
How to Use
- 1Choose mode — Select whether to find a week number from a date or find dates from a week number.
- 2Enter your input — Either select a date or enter a year and week number.
- 3View results — See the ISO week number, day of year, quarter, and all dates in that week.
Frequently Asked Questions
What is an ISO week number?
ISO 8601 defines a standard way to number weeks. Week 1 is the week containing the first Thursday of the year. All weeks start on Monday and end on Sunday.
Why do some years have 53 weeks?
Most years have 52 weeks, but years that start on Thursday (or Wednesday in leap years) have 53 weeks. This happens roughly every 5-6 years.
Why might the ISO week year differ from the calendar year?
The ISO week year can differ at the start and end of the calendar year. For example, December 31 might be in Week 1 of the next year, or January 1 might be in Week 52/53 of the previous year.
How is the day of year calculated?
The day of year counts from January 1 (day 1) to December 31 (day 365 or 366 in leap years). It's useful for tracking progress through the year.
What is a quarter?
A quarter divides the year into four 3-month periods: Q1 (Jan-Mar), Q2 (Apr-Jun), Q3 (Jul-Sep), and Q4 (Oct-Dec). Quarters are commonly used in business and finance.