Week number calculator

Find a date’s ISO week number, or turn an ISO week into its Monday-to-Sunday range.

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Updated 2026-09-20

Using Week number calculator

ISO weeks start on Monday. Week 1 contains January 4, so the first days of January can belong to the previous week-year. Reverse mode checks whether week 53 actually exists in the chosen year.

A worked example

January 1, 2021 belongs to 2020-W53, which runs from December 28, 2020 through January 3, 2021.

Before you use the result

This uses ISO week numbering, not a Sunday-start US week scheme. The complete week range must fit within calendar years 0001–9999.

Next: Calendar generator

Next: Day of the year calculator

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ToolOctopus. “Week number calculator.” Updated 2026-09-20. https://tooloctopus.com/week-number-calculator.

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