Week Number Calculator – Find ISO Week Number for Any Date
Look up the week number for any date instantly. Our ISO week number calculator also shows the start and end dates for any given week of the year.
How to Find Week Number
Select a Date
Choose any date using the date picker or enter manually.
Click Calculate
The calculator determines the ISO week number for your selected date.
View Week Details
See the week number, year, and the full date range for that week.
Key Features of This Week Calculator
**ISO 8601 Standard**
Uses the international ISO week date standard for consistent week numbering.
**Week Range Display**
Shows the Monday-to-Sunday date range for any week number.
**Any Date Support**
Find week numbers for past, present, or future dates.
**Free & Instant**
Get week numbers immediately without any registration.
Frequently Asked Questions
What is ISO week number?
ISO week numbers follow the ISO 8601 standard. Week 1 is the week containing the first Thursday of the year. Weeks start on Monday and are numbered 01-53.
How many weeks are in a year?
Most years have 52 weeks. Some years have 53 weeks when January 1st falls on a Thursday (or Wednesday in leap years).
Why does week 1 sometimes start in the previous year?
ISO week 1 must contain January 4th. If Jan 1-3 fall in the previous week, they belong to week 52 or 53 of the prior year.
What day does the ISO week start?
ISO weeks always start on Monday and end on Sunday. This differs from some calendars that start weeks on Sunday.
How is week number used in business?
Week numbers are used for project planning, manufacturing schedules, financial reporting, and international coordination where consistent week references are needed.
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