Week number calculator
Find the ISO 8601 week number for any date and see the Monday-to-Sunday range for that reporting week.
Today is Saturday, September 26, 2026
→ ISO Week 39 of 2026
ISO week number
Week 39 of 53 — 2026
Week Starts (Monday)
September 21, 2026
Week Ends (Sunday)
September 27, 2026
Day 269
Day of Year
Saturday
Weekday
Q3
Quarter
53
Weeks in Year
When to Use a Week Number Instead of a Date Difference
Week numbers are useful when teams plan by reporting cycles, sprint cadence, factory schedules, and international operations. This calculator uses the ISO 8601 standard, where weeks start on Monday and Week 1 is the week containing the first Thursday of the year.
📊 Business Reporting
Financial quarters and weekly reports often reference ISO week numbers for global consistency.
🏗️ Project Management
Agile sprints and project schedules often use week numbers for clear communication across time zones.
📅 ISO 8601 Standard
The international standard ensures week 1 always contains January 4th, with Monday as the first day.
Why this tool is separate
This page is not for counting days. It is specifically for turning a date into an ISO week reference so you can match business reports, sprint boards, manufacturing calendars, and Europe-first planning systems that use week numbers.
ISO week 1 is the week that contains the first Thursday of the year, so early January can belong to week 52 or 53 of the previous ISO year. US retail calendars that start weeks on Sunday will not match this output.
Do not use week numbers as a substitute for a public-holiday list. A week can contain a federal holiday and still be week 27. Open the month calendar or holiday page when the question is “is this a day off.”