Time & Date
Week Number Calculator
Find the ISO 8601 week number for any date, or look up the date range for a given week of the year.
Find the week number
ISO week
39 / 2026
Find the date range
Monday – Sunday
Sep 21, 2026 – Sep 27, 2026
How the Week Number Calculator works
Enter a date to find its ISO 8601 week number, or enter a week number and year to find that week's exact date range.
Frequently asked questions
What is an ISO week?
ISO 8601 defines week 1 of the year as the week containing the year's first Thursday, with weeks running Monday to Sunday — the standard used across most of Europe.
Can I find the date range for a specific week?
Yes — enter a week number and year to see that week's Monday-to-Sunday date range.
Guides that cover this
Mastering Dates, Deadlines, and Time ZonesThe everyday date math — ages, countdowns, business days, time zones — that's easy to get wrong by hand.What an ISO Week Number Actually Means (and Why Week 1 Isn't Always January 1)How the ISO 8601 week numbering standard defines week 1, and why it doesn't simply start on January 1st every year.Why Your Calculated Week Number Didn't Match Your Calendar AppWhy different apps and countries can show a different week number for the same date, based on which numbering convention they use.ISO Week Numbering vs. the US Convention: What's DifferentHow the ISO 8601 standard and the common US week-numbering approach differ in week-start day and where week 1 begins.How to Use the Week Number CalculatorA quick walkthrough of finding the ISO week number for any date.Week Number Calculator: How It Works and How to Use ItA quick guide to the week number calculator — what it calculates, how the math works, and tips for getting an accurate result.