ISO 8601 week numbering is the international standard used across European business, manufacturing planning and project scheduling. Weeks run Monday to Sunday, and week 1 is defined as the week containing the first Thursday of January — equivalently, the week containing 4 January.
That definition has a consequence that surprises people: 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. So a week number is only meaningful together with its week-year, which is not always the calendar year.
Most years have 52 weeks, but a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. 2026 is such a year, with 53 weeks. The 53-week years arrive on an irregular pattern, roughly every five to six years.
The formula
D- The date in question
Thu- The Thursday of that date’s Monday-to-Sunday week
WY- The week-year — the calendar year that Thursday falls in
W- Week number, counted from the first Thursday of the week-year
How it works, step by step
- Enter any date.
- The gauge shows the ISO week number, 1 to 53.
- The week-year is shown alongside — it can differ from the calendar year.
- The Monday and Sunday bounding the week are given in the breakdown.
Worked examples
9 August 2026
This falls in ISO week 32 of week-year 2026, written 2026-W32. The week runs Monday 3 August 2026 to Sunday 9 August 2026.
When the week-year and calendar year disagree
1 January 2027 falls in ISO week 53 of 2026, not week 1 of 2027, because it is a Friday and its week’s Thursday is still in December. Conversely 30 December 2024 already belongs to week 1 of 2025.
How to read your score
Frequently asked questions
Why does 1 January sometimes fall in week 52 or 53?
Because ISO weeks never split: a week belongs entirely to the year containing its Thursday. If 1 January is a Friday, Saturday or Sunday, its week has more days in the old year, so it keeps the old year’s numbering.
Which years have 53 weeks?
Years starting on a Thursday, and leap years starting on a Wednesday. Recent and upcoming examples are 2004, 2009, 2015, 2020, 2026, 2032, 2037.
Is this the same as the American week numbering?
No. US convention commonly starts weeks on Sunday and treats the week containing 1 January as week 1, which can give a different number for the same date. Always state which system you mean.
How do I find the Monday of a given ISO week?
Take 4 January of the week-year, step back to its Monday, then add seven days for each week beyond the first. The calculator does this in reverse to bound the week you asked about.
ISO week reference
| Year | 1 Jan weekday | ISO week of 1 Jan | Weeks in the year |
|---|---|---|---|
| 2022 | Saturday | 2021-W52 | 52 |
| 2023 | Sunday | 2022-W52 | 52 |
| 2024 | Monday | 2024-W01 | 52 |
| 2025 | Wednesday | 2025-W01 | 52 |
| 2026 | Thursday | 2026-W01 | 53 |
| 2027 | Friday | 2026-W53 | 52 |
| 2028 | Saturday | 2027-W52 | 52 |
| 2029 | Monday | 2029-W01 | 52 |
| 2030 | Tuesday | 2030-W01 | 52 |
| 2031 | Wednesday | 2031-W01 | 52 |
| 2032 | Thursday | 2032-W01 | 53 |
A year has 53 ISO weeks when it begins on a Thursday, or on a Wednesday in a leap year.