About this week number calculator
Many businesses plan in week numbers, but there are two common ways to count them. ISO 8601, used across Europe and in most software, starts weeks on Monday and makes week 1 the week that contains the year’s first Thursday, so a year has 52 or 53 weeks and early January can belong to the previous year. The North American system starts weeks on Sunday and makes week 1 the week that contains 1 January. This calculator shows both numbers for any date, the full week as a strip of seven days, and the dates of any week number you enter.
How to find a week number
- Choose what to look up. Pick The week of a date, or The dates of a week number. Press an example to try both.
- Enter the date or week. Choose a Date, or type a Year and a Week number.
- Choose the numbering. Select ISO 8601 for European and international use, or North American for calendars that start on Sunday.
- Press Find week. Read the week number, its date range, and the seven days of the week.
Frequently asked questions
Why does 1 January sometimes belong to week 52 or 53?
In ISO 8601, week 1 is the week containing the first Thursday of the year. If 1 January falls on a Friday, Saturday, or Sunday, it is part of the last week of the previous year.
Which years have 53 weeks?
In ISO 8601, a year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 and 2032 are such years.
What is the difference between the two systems?
ISO weeks start on Monday and can begin in the previous year. North American weeks start on Sunday, and week 1 always contains 1 January, so the last days of December can be week 53 while 1 January is week 1.
How is the week of the month counted?
Week 1 of a month is the week that contains the first day of that month, and each new week starts on Monday or Sunday depending on the system you choose.
How should I write an ISO week?
The standard form is the year, a capital W, and a two-digit week, such as 2026-W40. Add the day number to name one day, such as 2026-W40-6 for Saturday.