OneTaskBy Tim King · Updated
Week Number Calculator
Monday-to-Sunday Range, Week Date Format and Total Weeks in the Year
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How the Week Number Calculator works
Find the week number for today or any date using the ISO 8601 standard, the system used across Europe, in business planning and in most software. The result shows the week, the dates it runs from Monday to Sunday, its ISO week date and whether the year has 52 or 53 weeks.
How the ISO week number is worked out
ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday, which is the same as the week containing 4 January. The formula finds the Thursday of your date's week, notes which year that Thursday is in, and counts how many weeks it is from the start of that year.
Worked examples
For example, Monday 15 March 2027 is in week 11 of 2027. As a second example, Friday 1 January 2027 is not in week 1 at all: its Thursday is 31 December 2026, so it belongs to week 53 of 2026, written 2026-W53-5. The ISO year and the calendar year differ for a few days around New Year.
52 or 53 weeks
Most ISO years have 52 weeks. A year has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. The year 2026 is one of them, so the next 53-week year after that is 2032.
How many weeks are in a year?
The table lists when week 1 starts and how many weeks each ISO year has. In the US, some calendars count weeks from Sunday and call the week containing 1 January week 1, so a US wall calendar can be one week different from the ISO number.
ISO week 1 start date and number of weeks, 2020 to 2043
| ISO year | Week 1 starts | Weeks in the year |
|---|---|---|
| 2020 | Monday 30 December 2019 | 53 |
| 2021 | Monday 4 January 2021 | 52 |
| 2022 | Monday 3 January 2022 | 52 |
| 2023 | Monday 2 January 2023 | 52 |
| 2024 | Monday 1 January 2024 | 52 |
| 2025 | Monday 30 December 2024 | 52 |
| 2026 | Monday 29 December 2025 | 53 |
| 2027 | Monday 4 January 2027 | 52 |
| 2028 | Monday 3 January 2028 | 52 |
| 2029 | Monday 1 January 2029 | 52 |
| 2030 | Monday 31 December 2029 | 52 |
| 2031 | Monday 30 December 2030 | 52 |
| 2032 | Monday 29 December 2031 | 53 |
| 2033 | Monday 3 January 2033 | 52 |
| 2034 | Monday 2 January 2034 | 52 |
| 2035 | Monday 1 January 2035 | 52 |
| 2036 | Monday 31 December 2035 | 52 |
| 2037 | Monday 29 December 2036 | 53 |
| 2038 | Monday 4 January 2038 | 52 |
| 2039 | Monday 3 January 2039 | 52 |
| 2040 | Monday 2 January 2040 | 52 |
| 2041 | Monday 31 December 2040 | 52 |
| 2042 | Monday 30 December 2041 | 52 |
| 2043 | Monday 29 December 2042 | 53 |
FAQ
What week is it today?
Leave the date field empty and the calculator shows the ISO week number for today's date on your device. The number changes every Monday, so a week number shared in a meeting on Friday is still the same week until Sunday night.
Why does 1 January sometimes fall in week 52 or 53?
Because ISO week 1 must contain the year's first Thursday. When 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year, and week 1 begins on the following Monday.
What is an ISO week date?
It is a way of writing a date as year, week and weekday, such as 2027-W11-1 for Monday 15 March 2027. Weekdays are numbered 1 for Monday to 7 for Sunday. It is used in manufacturing, logistics and payroll where work is planned by week.
See also: all Calendars tools, Countdowns, Durations, Work and Contracts, Date Formats, Time Zones and Travel, Sun and Daylight.
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