What is a week number?
A week number identifies which numbered week of the year a date belongs to.
Date & Time
Week Number Calculator
Find the week number for any date directly in your browser. Calculate ISO week numbers, calendar weeks, week year, and weekday information.
Tool
Week Number Calculator
Find the week number for any date directly in your browser. Calculate ISO week numbers, calendar weeks, week year, and weekday information.
This tool runs in your browser. Your input is processed locally and is not uploaded.
About this Tool
Use this free week number calculator to find which calendar week a date belongs to. It can show the ISO week number, week year, weekday, and useful calendar details.
Conversion Formula
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ISO week numbers follow ISO 8601 rules: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. This means the ISO week year can differ from the calendar year near New Year.
Examples
- 2026-01-01 belongs to ISO week 1 of 2026
- Find the calendar week for a project date or delivery date
- Check whether a date near New Year belongs to the previous or next ISO week year
Frequently Asked Questions
What is an ISO week number?
An ISO week number follows ISO 8601 rules, where weeks start on Monday and week 1 is based on the first Thursday of the year.
Can the week year differ from the calendar year?
Yes. Dates at the beginning or end of a year can belong to an ISO week year that is different from the calendar year.
Is this useful for business planning?
Yes. Week numbers are commonly used for production planning, delivery schedules, reporting, and project timelines.
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