Date & Time calculator
Week Number Calculator
Find the ISO week number for any date.
About the week number calculator
The week number calculator finds the ISO 8601 week number for any date.
How to use this calculator
- 1
Enter date
Select any date.
- 2
View results
See the ISO week number and year.
How the calculation works
ISO 8601 week numbering: Week 1 is the week with the year's first Thursday. Weeks start on Monday.
Example: Jan 1, 2024 is in week 1. Dec 31, 2024 is in week 1 of 2025.
How date calculations work
Calendar math is trickier than it seems. Months have 28–31 days, leap years add an extra day every four years (except century years not divisible by 400), and time zones complicate exact hour counts. Our calculators use UTC-based date arithmetic to ensure consistent results regardless of your local time zone.
Important limitations
This tool provides a mathematical estimate based only on the values entered. It does not account for regulations, taxes, lender fees, market conditions, or individual circumstances. Results are for planning purposes only — consult a qualified professional for regulated, medical, financial, or construction decisions.
Frequently asked questions about week number calculator
How accurate is the week number calculator?+
The calculation follows the displayed formula and retains full numeric precision before formatting the result.
Can I use the result professionally?+
Use it as a planning estimate. Confirm regulated, medical, financial, or construction decisions with a qualified professional.