Week Number Calculator

Find the week number (ISO) for any date. Also shows the Monday–Sunday range for that week.

Week number

Result

How it’s calculated

ISO: Week 1 is the week with the first Thursday of the year. Weeks start on Monday. In US mode, weeks start on Sunday.

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Frequently Asked Questions

What is an ISO week number?

ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the year.

What’s the difference between ISO and US week numbers?

ISO weeks are Monday–Sunday. US weeks commonly run Sunday–Saturday, which can change the week number near year boundaries.

Why does the week number change near New Year’s?

The last few days of a year can belong to Week 1 of the next year (or Week 52/53 of the previous year) depending on the week system.

Does the calculator show the week’s date range?

Yes. It shows the start and end dates for the selected week.

How do I use this tool?

Pick a date and select ISO or US mode to see the week number, year, and date range.