ISO Week Numbers Explained (With 2026 Examples)
How ISO 8601 week numbering works: why weeks start on Monday, why some years have 53 weeks, and how it differs from the US system.
Today is ISO week 44 of 2026. Week numbering sounds trivial until you notice that calendars disagree: a date can be week 40 in Europe and week 41 in the United States. Here is how the international standard works and why it sometimes produces a week 53.
The two rules of ISO weeks
- Weeks run Monday through Sunday — Monday is day 1, Sunday day 7
- Week 1 of a year is the week containing the year's first Thursday (equivalently, the week containing January 4)
The first-Thursday rule guarantees every ISO week belongs to exactly one year: if January 1 falls on a Friday, Saturday or Sunday, those days belong to the previous year's final week, and week 1 starts the following Monday.
When does a year get 53 weeks?
A standard year of 365 days spans 52 weeks plus one day; a leap year plus two. That extra day or two occasionally pushes a year to own 53 numbered weeks. Precisely: a year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year. 2026 is a 53-week year — week 53 of 2026 runs December 28 through January 3, 2027.
| Year | Weeks | Why |
|---|---|---|
| 2026 | 53 | Jan 1 is a Thursday |
| 2027 | 52 | Jan 1 is a Friday — Jan 1 itself belongs to week 53 of 2026 |
| 2028 | 52 | Jan 1 is a Saturday (leap year, but not the Wednesday case) |
| 2032 | 53 | Jan 1 is a Thursday |
ISO vs the US week system
The US convention starts weeks on Sunday and calls the week containing January 1 "week 1". Near year boundaries the two systems diverge by one: December 28, 2026 is in US week 1 of 2027, but ISO week 53 of 2026. Business reporting across Europe uses ISO; North American consumer calendars often use the Sunday system. When a date matters contractually, state the convention — or just link the week number page and let it speak for itself.
Every week of 2024–2027 has its own page here, e.g. week 40 of 2026, each listing all seven dates with weekdays.
Frequently asked questions
How many weeks are in a year?
Most years have 52 ISO weeks; a year gets a 53rd week when January 1 falls on a Thursday, or on a Wednesday in a leap year. 2026 has 53 weeks.
Do all countries start the week on Monday?
Most of the world follows the ISO standard with Monday first. The US, Canada, Japan and some others traditionally print calendars with Sunday first.
What week of the year is it now?
Today is ISO week 44 of 2026 — see the current week's page for all seven dates.