You check a calendar app in late December and it shows "Week 1" — even though it's still December. Or you see a week number in a business report that doesn't match your phone's calendar. That's ISO 8601 at work.
Here's a plain-English explanation of exactly how it works.
What Is ISO 8601?
ISO 8601 is a standard published by the International Organization for Standardization. It defines how dates, times, durations, and — importantly — week numbers are written and calculated.
The goal is consistency across countries, software systems, and industries. Before ISO 8601, different countries counted weeks differently. Some started on Sunday, some on Monday. Week 1 could mean different things in different places.
ISO 8601 fixed that.
The Two Core Rules
Rule 1: Weeks start on Monday
Every ISO week runs from Monday to Sunday. Saturday and Sunday are always the last two days of the week — never the first.
This is the biggest difference from the US system, where weeks start on Sunday.
Rule 2: Week 1 contains the first Thursday of the year
This is the rule that surprises people. Week 1 of a year is defined as the week containing the first Thursday of that year.
Why Thursday? Because Thursday is the middle of the ISO week (Mon = 1, Thu = 4, Sun = 7). Anchoring on Thursday means at least four days of Week 1 always fall in January.
What This Means in Practice
Take 2026. January 1 is a Thursday. So the week containing that Thursday — December 29, 2025 to January 4, 2026 — is Week 1 of 2026.
That means December 29, 30, and 31 of 2025 officially belong to ISO year 2026, not 2025.
And the last three days of 2025 (Dec 29–31) are in Week 1 of 2026. Their ISO year label is 2026.
ISO Year vs Calendar Year
This is where people get confused. ISO 8601 uses two different "year" values:
- Calendar year (Y): The Gregorian year — Jan 1 to Dec 31
- ISO year (o in PHP / G in most systems): The year the ISO week belongs to
At year boundaries, these can differ. December 28–31 might show an ISO year of the following calendar year. January 1–3 might show an ISO year of the previous calendar year.
For almost every day of the year, they match. Only a few days at the very start and end of January differ.
How the Week Number Is Calculated
The standard algorithm:
- Find what day of the week January 4 of the year falls on
- That day is always in Week 1
- Go back to the Monday of that week — that's the start of Week 1
- Count forward from there
In code (PHP):
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52 vs 53 Weeks
Most years have 52 ISO weeks. A year gets a week 53 when:
- January 1 falls on a Thursday, or
- It is a leap year and January 1 falls on a Wednesday or Thursday
2026 is a 53-week year. Week 53 runs from December 28, 2026 to January 3, 2027.
Who Uses ISO Week Numbers?
Practically everywhere outside North America:
- Europe: Business planning, payroll, logistics, manufacturing
- International trade: Shipping schedules, freight documentation
- Software: Databases, spreadsheets, API date fields
- Retail: Planograms, inventory cycles, weekly sales reports
The US, Canada, and a few other countries still commonly use the Sunday-start system. Both are valid — they just produce different numbers for the same date near week boundaries.
US Week Numbers vs ISO Week Numbers
| ISO 8601 | US / North American | |
|---|---|---|
| Week starts | Monday | Sunday |
| Week 1 rule | Contains first Thursday | Starts January 1 |
| Max weeks | 52 or 53 | Always 52 or 53 |
| Used in | Europe, international | US, Canada |
The difference only shows up near year boundaries. Most of the year, both systems return the same or adjacent week numbers.
Quick Reference
- What week is it today? → Check the homepage
- Convert a date to ISO week → Date to Week Calculator
- See all weeks of 2026 → 2026 Week Calendar
ISO 8601 is not complicated once you know the two rules: weeks start Monday, and Week 1 contains the first Thursday. Everything else follows from there.