▣ CALCULATORS Updated July 25, 2026

Week Number Calculator

ISO week number, day of year, and quarter.

Date
Result
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How it works

Week numbers follow ISO 8601, where week 1 is the week containing the first Thursday of the year and weeks start on Monday.

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Frequently asked questions

Why does 1 January sometimes fall in week 52?

Under ISO 8601 the first days of January can belong to the last week of the previous year.

Do weeks start on Sunday?

Not in ISO 8601 — they start on Monday, which is the standard used across Europe and in most business software.

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