Guides ·
How to use Week Number Calculator
Week Number Calculator. ISO 8601 and US week numbers. ISO 8601, used across Europe and in most business software.
The short answer
To find a week number, pick a date: ClickZM shows the ISO 8601 week (weeks start Monday; week 1 contains the first Thursday) with its date range, and the US week number (weeks start Sunday). 4 November 2026 is ISO week 45.
It runs in your browser. What you enter is not uploaded.
How to use it
Enter your values in the fields; defaults show a worked example.
Pick the units or options that apply to you.
Read the result, which updates instantly as you type.
Check the formula note under the result to see exactly how it was worked out.
Limits worth knowing
Which week numbering does Europe use? ISO 8601, used across Europe and in most business software.
Can a year have 53 weeks? Yes. Under ISO rules, years starting on a Thursday (or leap years starting on Wednesday) have 53 weeks.