What Is an ISO Week Number?
The ISO 8601 standard defines a universal numbering system for calendar weeks. Used across Europe, finance, manufacturing, and international trade, it provides a language-neutral way to reference any week of the year without ambiguity.
ISO Week Number Rules
- Weeks run Monday through Sunday (not Sunday through Saturday like the US system)
- ISO Week 1 is the week containing the first Thursday of January
- A year always has 52 or 53 weeks (53 weeks in years where January 1 or December 31 is Thursday)
Practical consequence
Because of rule 2, late December dates can belong to Week 1 of the following year, and early January dates can belong to Week 52 or 53 of the previous year. This trips up many spreadsheets that use simpler week formulas.
Week Numbers in 2026
| Week | Monday (Start) | Sunday (End) |
|---|---|---|
| Week 1 | Dec 29, 2025 | Jan 4, 2026 |
| Week 2 | Jan 5, 2026 | Jan 11, 2026 |
| Week 10 | Mar 2, 2026 | Mar 8, 2026 |
| Week 26 | Jun 22, 2026 | Jun 28, 2026 |
| Week 52 | Dec 21, 2026 | Dec 27, 2026 |
Why Week Numbers Matter in Business
Week numbers simplify scheduling across time zones, languages, and cultures. Common uses include:
- Manufacturing: Production runs are scheduled by week (e.g., "Ship by W42")
- Finance: Payroll and reporting cycles often align to ISO weeks
- Retail: Sales reports compare Week 23 this year vs. Week 23 last year
- Project management: Sprints and milestones are tracked by week number
- Academia: Semester schedules in Europe use ISO week references
Frequently Asked Questions
What week number is today?
The calculator at the top of the page shows today's ISO week number and the date range (Monday to Sunday) of the current week, updating automatically each day.
How many weeks are in 2026?
2026 has exactly 52 ISO weeks. The year 2026 starts on Thursday and is not a long year. Years that have 53 weeks include 2015, 2020, and 2026 does not qualify. Scroll up and check the full year calendar to see all 52 weeks.
What is the difference between ISO weeks and US weeks?
ISO weeks (ISO 8601) start on Monday and use a specific rule for Week 1. US weeks typically start on Sunday and treat January 1 as always being in Week 1. In practice, this means the week numbers can differ by 1 for dates in early January and late December. Excel, Google Sheets, and SAP all have separate functions for each system (WEEKNUM vs ISOWEEKNUM).
How do I calculate the ISO week number in Excel?
Use the ISOWEEKNUM() function: =ISOWEEKNUM(A1) where A1 is a date cell. For US-style week numbers, use =WEEKNUM(A1, 2). The second argument specifies Monday as week start.
Can a year have 53 weeks?
Yes. If January 1 is a Thursday (since Thursday is mid-week by the ISO definition), or if December 31 is a Thursday, the year will have 53 ISO weeks. About 71 out of every 400 years have 53 weeks. Recent 53-week years include 2015, 2020, and the next will be 2032.