Week Number Calculator

Find which ISO week number any date falls in.

Select Date

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Pick a date

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Week Start

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Week End

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Day of Year

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Weeks Left

How to use Week Number Calculator

Pick any date.
See its ISO week number, along with that week's start and end dates.

→ calculate the days between two dates

Frequently asked questions

What does "Weeks Left" account for?

It subtracts the selected date's ISO week from that year's actual final ISO week — determined via December 28, which always falls in the last ISO week — since some years have 53 ISO weeks instead of 52.

Why might the week number seem "off" compared to a regular calendar?

This uses the ISO 8601 standard, where weeks start on Monday and week 1 is the week containing the year's first Thursday — so late-December or early-January dates can belong to a week number from the adjacent year.

What does "Day of Year" show?

The 1-based day count from January 1st of that year up to the selected date.

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