Week Number Calculator
See which ISO 8601 week any date falls in, and which week-year it belongs to.
Choose a date
The week number updates as you change the date.
ISO weeks start on Monday. Some countries number weeks differently; see below.
How to use this calculator
- 1
Pick a date. Any date from year 1 to 9999.
- 2
Read the week. The ISO week number, its week-year and the Monday–Sunday range are shown.
- 3
Check the year. Near January 1 the week-year can differ from the calendar year; the tool tells you when it does.
Example
October 20, 2026 is a Tuesday in week 43 of 2026, which runs from Monday, October 19 to Sunday, October 25.
January 1, 2027 is a Friday that belongs to week 53 of 2026.
How it is calculated
ISO 8601 weeks run from Monday to Sunday. Week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4). A year has 52 or 53 such weeks.
Calendar year versus week-year. Because weeks are never split, 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. Examples:
- January 1, 2021 → week 53 of 2020
- December 30, 2024 → week 1 of 2025
- January 1, 2027 → week 53 of 2026
The US and Canada often use other systems (weeks starting on Sunday, with week 1 containing January 1), so a week number from a US calendar can differ from the ISO number shown here.
Questions
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. 2020 and 2026 each have 53 ISO weeks; 2025 has 52.