Week Number Calculator
Find the ISO 8601 week number for any date, with the ISO week-year and day of the week. Handles the week 53 and year-boundary cases correctly.
Independently verified for accuracy
Calculator by Toolsloft ↗- ISO week number
- 29
- ISO week-year
- 2026
- Day of week
- Sunday (ISO 7)
Find the ISO 8601 week number for any date, along with the ISO week-year and the day of the week. This is the week numbering used across business, payroll, and manufacturing calendars in much of the world. Enter a date and read the week without counting on a wall calendar.
How this is calculated
Week numbers follow ISO 8601: weeks start on Monday, and week 1 is the week containing the year first Thursday. Because the Thursday of a week decides which year it belongs to, a date in late December or early January can fall in week 52 or 53 of a neighbouring year, which is reported as the ISO week-year.
How to use
- Enter a date in YYYY-MM-DD format.
- Read the ISO week number and ISO week-year.
- Check the day of the week shown alongside.
Examples
- Mid-year:
2026-07-19 → week 29 - Year boundary:
2021-01-01 → week 53 of 2020
FAQ
- Why can January 1 be in week 53 of the previous year?
- Under ISO 8601, a week belongs to the year that holds its Thursday. If January 1 falls on a Friday, Saturday, or Sunday, its Thursday is in December, so that week is week 52 or 53 of the previous ISO year.
- Which day does the week start on?
- ISO weeks start on Monday and end on Sunday. The ISO weekday runs from 1 for Monday to 7 for Sunday, which is what the calculator reports.
- How many weeks are in a year?
- An ISO year has either 52 or 53 weeks. Most years have 52; a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year.