Week Number Calculator
Find the ISO week number for any date. See week start, end, and remaining weeks in the year.
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Week 36
Year 2026
Saturday
Aug 31
Sep 6
248
of 365
Q3
Smart Insights
How It Works
The ISO week number is calculated using the ISO-8601 standard. Week 1 is the week containing the first Thursday of the year. Weeks run Monday to Sunday.
Frequently Asked Questions
How is the ISO week number calculated?
ISO-8601 defines week 1 as the week containing the first Thursday of the year. Weeks run Monday through Sunday.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday. US weeks start on Sunday. The calculator uses the ISO standard by default.
Can a year have 53 weeks?
Yes. A year has 53 weeks if January 1 is a Thursday, or if it's a leap year and January 1 is a Wednesday.
How are remaining weeks calculated?
Remaining weeks = days remaining ÷ 7, rounded up to account for partial weeks.
What is 'Day of Year'?
The sequential day number from January 1 (day 1) to December 31.
Can I check a past date?
Yes. The calculator works for any date — past, present, or future.
Last reviewed: July 20, 2026