What is an ISO week number?
ISO 8601 weeks run Monday–Sunday. Week 1 is the week with the year’s first Thursday.
Week Number Calculator
Look up the ISO week number for any date, including the ISO week-year and that week’s Monday–Sunday range.
Find a week number
Pick a date to see its ISO week, ISO year, and the week’s start and end dates.
About this calculator
How to use Week Number Calculator, plus the formulas and logic behind the results.
Tool description
The Week Number Calculator returns the ISO 8601 week of year for a chosen date. Teams use week numbers for planning, reporting, and international schedules.
How to use
- Select or enter a date.
- Select Calculate to view week number, ISO year, and week bounds.
Formulas & reference
ISO week = date formatted as ISO-8601 week (W)ISO week-year = year owning that week (o)week starts Monday; ends Sunday
ISO week rules
- Weeks start on Monday
- Week 1 contains the year’s first Thursday
- ISO week-year can differ near Jan 1 / Dec 31
Calculation logic
PHP date format tokens W and o provide ISO week and ISO week-year. The Monday of that week is found by shifting back (N − 1) days; Sunday is Monday + 6 days.
This tool uses ISO 8601 week numbering (common in Europe and many business systems), not US week systems that start on Sunday.
Week Number Calculator FAQ
Common questions about this calculator and how to use the results.
Why can January 1 be week 52 or 53?
Early January dates may belong to the previous ISO week-year.
Do weeks start on Sunday here?
No. ISO weeks start on Monday.
Is this the same as Excel WEEKNUM?
Not always. Excel has multiple systems; this tool matches ISO week numbering.
What is ISO week-year?
The year that “owns” the ISO week, which can differ from the calendar year near year boundaries.
Can I find today’s week number?
Leave the default date (today) and calculate.
Are there always 52 weeks?
Most years have 52 ISO weeks; some have 53.
Does locale change the week start?
No. This calculator always uses ISO Monday starts.