Free & Instant
Week Number Calculator
Find the ISO week number for any date, plus how many weeks remain in the year.
Week Number Calculator
Select a date to find its ISO 8601 week number.
Week Number
Week —
—Total Weeks in Year
—Weeks Remaining
About ISO Week Numbers
The ISO 8601 standard defines week numbers where each week starts on Monday, and Week 1 is the week containing the year's first Thursday. This system is used widely in Europe for scheduling, invoicing, and project planning.
ISO Week Rules
| Rule | Detail |
|---|---|
| Week Start | Monday |
| Week 1 Definition | Contains the year's first Thursday |
| Weeks per Year | 52 or 53 (varies by year) |
Frequently Asked Questions
Why do some years have 53 weeks?+
A year has 53 ISO weeks when January 1st falls on a Thursday, or in leap years when it falls on a Wednesday — this happens roughly once every 5-6 years.
Can early January dates belong to the previous year's last week?+
Yes. If January 1st-3rd falls before the year's first Thursday, those days are counted as part of the final week of the previous year under ISO 8601 rules.