Week Number Calculator
Find the ISO 8601 week a date falls in, or the dates a week number covers.
Which ISO week a date falls in, and which seven days a week number covers. The ISO year is shown next to the week number, because around New Year the two do not agree.
How it works
ISO-8601 defines week 1 as the week containing the first Thursday of the year, and every week as running Monday to Sunday. The consequence is that the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next.
Because of that, the year reported here is the ISO year, which is the year the week belongs to rather than the year the date falls in. 1 January 2027 is a Friday: it is in week 53 of 2026.
An ISO year has 52 weeks, or 53 when it either starts on a Thursday or is a leap year starting on a Wednesday. Asking for week 53 of a year that has 52 returns an error rather than a silent wrap into January.
Examples
| Case | Input | Result |
|---|---|---|
| The first ISO week can start in the old year | 2026-01-01 | 2026-W01, which began on 2025-12-29 |
| A year with fifty-three weeks | Week 53 of 2026 | 2026-12-28 to 2027-01-03 |
Frequently asked questions
Why does my spreadsheet give a different week number?
Most spreadsheets default to a rule where week 1 is simply the week containing 1 January, and weeks start on Sunday. That is not ISO-8601 and it disagrees for a few days each year. This tool implements ISO-8601, the standard used across Europe and in most business calendars.
Why is the year sometimes not the one I typed?
Because it is the ISO year. A date in early January can belong to the final week of the year before, and the week number on its own would then be meaningless. The pair year and week is what identifies a week.
Can a year have 53 weeks?
Yes. Roughly every five or six years an ISO year is long. 2020 and 2026 both have 53 weeks; 2024 and 2025 have 52.
Good to know
- Weeks run Monday to Sunday, and week 1 is the week holding the first Thursday of the year. Other week numbering conventions exist and are not implemented here.