Math, Science & Calculators · Free Tool

ISO Week Number

Find the ISO-8601 week number for any date — plus the week’s Monday-to-Sunday range and the day of the year. Runs entirely in your browser.

Date

ISO week

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ISO week-year

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Day of year

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Weeks in year

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How it works

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year’s first Thursday. The number is found by taking the Thursday of your date’s week and counting weeks from the first Thursday of that ISO year — which is why early-January or late-December dates can belong to the previous or next year’s week count. All computed locally.