Week number calculator
Pick a date, get its week number.
What week number is it?
Pick today in the calculator. Weeks are numbered by the ISO 8601 standard: they start on Monday, and week 1 is the week with the year’s first Thursday. September 27, 2026 falls in week 39.
How many weeks are in a year?
Most years have 52 ISO weeks; some have 53. 2026 has 53 weeks because it starts on a Thursday.
Why can January 1 be in week 52 or 53?
If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year. January 1, 2027 is a Friday, so it is in week 53 of 2026.
How do I find the dates of a week number?
Choose "Dates of a week" and enter the year and week: week 1 of 2026 runs from Monday, December 29, 2025 to Sunday, January 4, 2026.
Do US calendars number weeks differently?
Some US calendars start weeks on Sunday and count the week with January 1 as week 1, so their numbers can be one off from ISO weeks. This calculator uses ISO 8601, the international standard used in business and software.
Which years have 53 weeks?
A year has 53 ISO weeks if January 1 is a Thursday, or if it is a leap year and January 1 is a Wednesday. Between 2015 and 2032 that gives 2015, 2020, 2026, 2032; every other year in that span has 52. The reason is that 52 weeks are 364 days, and years of 365 or 366 days accumulate extra days that eventually add a full week.
How is the ISO week written in full?
ISO 8601 writes a week date as year, W and week number, optionally followed by the day of the week from 1 (Monday) to 7 (Sunday). Sunday, September 27, 2026 is 2026-W39-7. The year in this notation is the ISO year, which differs from the calendar year around New Year.
The rules of ISO 8601 weeks
Weeks run from Monday to Sunday. Week 1 of a year is the week that contains that year's first Thursday, which is the same as the week containing January 4. It follows that the days from the end of December and the start of January can belong to the week-numbering year next to them. The calculator finds the Thursday of the week of your date, takes the year of that Thursday as the ISO year, and counts how many weeks have passed since the first of January of that year. In the opposite direction, for a year and week number, it starts from the Monday of week 1 and adds the required number of weeks.
Worked example: dates around New Year
Sunday, January 1, 2023 is in week 52 of 2022, not in 2023, because its Thursday is December 29, 2022. Tuesday, December 31, 2024 is in week 1 of 2025, since its Thursday is January 2, 2025. Monday, December 29, 2025 starts week 1 of 2026, and because 2026 begins on a Thursday, 2026 has 53 weeks and its last week ends on Sunday, January 3, 2027. Week 1 of 2027 starts on Monday, January 4, 2027.
Where week numbers are used
Week numbers appear in business planning, project schedules, logistics, manufacturing date codes, school calendars in many countries and in software that follows ISO 8601. In the United States they are more rarely used in daily life, and several systems number weeks differently. For example, spreadsheet functions that count a week starting on Sunday with the week that contains January 1 give a different number from the ISO week for many dates, and Excel has a separate ISOWEEKNUM function for the ISO value. Fiscal calendars may also number weeks from the start of a fiscal year, such as a retail 4-5-4 calendar.
Common mistakes
Pairing the week number with the wrong year is the main one: for dates in late December or early January the ISO year can differ from the calendar year, so a report titled "week 1, 2025" may start in December 2024. Assuming a week starts on Sunday gives results one day off at the week boundary. Mixing up the US week count with the ISO count can shift a number by one for the whole year. And a year does not always have 52 weeks: a schedule built on 52 weeks per year slips by a week every five or six years.
Sources: ISO 8601 (date and time format; the ISO week date, week 1 as the week with the first Thursday); Microsoft documentation for WEEKNUM and ISOWEEKNUM. Related: date calculator, days from today calculator and time zone converter.