ISO Week Number
Find the ISO 8601 week number and week-year for any date, with the Monday-start and first-Thursday rules explained and a 2026 worked example.
Results
ISO week
- ISO week-year
- 2026
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Formula / how it works
ISO 8601: weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January). A year has 52 or 53 ISO weeks.
Read the guide: Date and Time Calculations: Leap Years, Business Days and Time Zones
Detailed guide
What it is, how to use it, how to read the numbers, common mistakes, a worked example, formulas, and FAQs.
What this calculator is
The ISO week number is the standard way to number weeks in project plans, payroll, manufacturing and many European calendars. Under ISO 8601 a week runs Monday to Sunday and week 1 is the week that contains the first Thursday of the year. That rule means the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. This calculator gives the ISO week and the ISO week-year for a date. The week-year can differ from the calendar year near the year boundary, which is a common source of reporting errors. US calendars often use a different system in which weeks start on Sunday and week 1 contains 1 January, so the same date can have a different week number.
When to use / who it helps
- Your company reports by week number.
- You are reading a delivery date or production schedule given as week 37.
- You are building a spreadsheet or report that groups by ISO week.
- You are checking which year a date near 1 January belongs to for weekly reporting.
- You are converting between a date and a week reference.
How to use
Steps
- Pick or enter the date.
- Read the ISO week number.
- Read the ISO week-year, especially for dates in late December or early January.
- Remember that weeks run Monday to Sunday.
- Do not mix this with US-style week numbering.
- For the dates in a given week, count back to the Monday.
How to interpret results
- The ISO week number runs from 1 to 52 or 53.
- A year has 53 ISO weeks if 1 January is a Thursday, or a Wednesday in a leap year.
- The week-year can be one year ahead of or behind the calendar year.
- Week 1 always includes 4 January.
- Week numbers differ between ISO, US and some other systems.
Common mistakes & gotchas
- Using the calendar year instead of the ISO week-year.
- Assuming week 1 starts on 1 January.
- Counting weeks starting from Sunday.
- Assuming all years have 52 weeks.
- Mixing ISO weeks with another system in the same report.
Worked example
Worked example — 9 September 2026 and the 2026–27 year end
- 1 January 2026 is a Thursday, so week 1 of 2026 is Monday 29 December 2025 to Sunday 4 January 2026.
- 9 September 2026 is a Wednesday, 254 days after 29 December 2025, which is 36 full weeks plus 2 days, so it falls in ISO week 37.
- 2026 starts on a Thursday, so it has 53 ISO weeks.
- 31 December 2026 and 1 January 2027 both fall in ISO week 53 of 2026, so the week-year for 1 January 2027 is 2026.
Result: 9 September 2026 is ISO week 37 of 2026.
To load the form's own demo values, tap Try example above the Calculate button.
How to calculate / formula
ISO 8601: weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January). A year has 52 or 53 ISO weeks.
Frequently asked questions
What is ISO week numbering?
A standard in which weeks start on Monday and week 1 contains the year's first Thursday.
How many weeks are in a year?
52 or 53 ISO weeks.
Why does 1 January sometimes fall in week 52 or 53?
Because week 1 is defined as the week with the first Thursday.
Is the US system the same?
No. US-style numbering commonly starts weeks on Sunday and puts 1 January in week 1.
What is the ISO week-year?
The year the ISO week belongs to, which can differ from the calendar year near 1 January.
What day does an ISO week start on?
Monday.