📅 Week Number Calculator (ISO)
By ToolNimba Editorial Team · Updated 2026-06-19
Pick a date to see its ISO 8601 week number. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.
This week number calculator tells you which ISO 8601 week any date falls in. Enter a date, or just use today, and you will see the ISO week number, the ISO week-numbering year, and the weekday straight away. The ISO standard is the one used by business calendars, payroll systems, project planners, and most of Europe, so the answer here matches what your colleagues and software expect.
What is the Week Number Calculator?
A week number is simply a count of which week of the year a date sits in. The trouble is that there is more than one way to count, so the same date can land in different week numbers depending on the rule. ISO 8601 is the international standard that removes the ambiguity. Under ISO 8601, weeks always start on Monday and end on Sunday, and week 1 is defined as the week that contains the first Thursday of the year. An equivalent and easier way to remember it: week 1 is the week that contains January 4, because January 4 is always in the first ISO week.
Because of the Thursday rule, the ISO week-numbering year does not always match the calendar year at the edges. The first few days of January can belong to the last week (week 52 or 53) of the previous year, and the last few days of December can belong to week 1 of the next year. For example, January 1, 2021 is a Friday and falls in ISO week 53 of 2020, while December 30, 2024 is a Monday that belongs to ISO week 1 of 2025. That is why this tool reports both the week number and the separate ISO week year, so you always know which year the week belongs to.
Most ISO years have 52 weeks, but some have 53. A year has 53 ISO weeks when it either starts on a Thursday, or is a leap year that starts on a Wednesday. Those long years are the ones that pick up the extra week 53. Knowing whether you are in a 52 or 53 week year matters for anything that schedules or reports on a weekly cadence across a full year.
When to use it
- Quickly answering "what week is it?" using the same ISO numbering your calendar app and email client display.
- Filling in week numbers on timesheets, payroll runs, and production schedules that are organised by ISO week.
- Coordinating with teams in Europe or with software (such as spreadsheets and project tools) that default to ISO week numbers.
- Planning sprints, releases, or reporting periods where work is tracked by week of the year rather than by date.
How to use the Week Number Calculator
- Open the tool: it defaults to today, so you immediately see the current ISO week number.
- To check a different date, pick it in the date field (or press Today to jump back to the current date).
- Read off the ISO week number, the ISO week year it belongs to, and the weekday.
- Note the message below the boxes if the calendar year and ISO week year differ at the start or end of a year.
Formula & method
Worked examples
Find the ISO week for June 19, 2026.
- June 19, 2026 is a Friday, so the Monday-based weekday number is 5.
- Shift to the week’s Thursday: add (4 - 5) = -1 day, giving Thursday June 18, 2026.
- The Thursday is in 2026, so the ISO week year is 2026.
- Days from January 1, 2026 (a Thursday) to June 18, 2026 = 168 days.
- week = ceil((168 + 1) ÷ 7) = ceil(169 ÷ 7) = ceil(24.14) = 25.
Result: June 19, 2026 falls in ISO week 25 of 2026 (a Friday).
Find the ISO week for January 1, 2021 (a year-boundary case).
- January 1, 2021 is a Friday, so the Monday-based weekday number is 5.
- Shift to the week’s Thursday: add (4 - 5) = -1 day, giving Thursday December 31, 2020.
- The Thursday is in 2020, so the ISO week year is 2020, not 2021.
- Days from January 1, 2020 to December 31, 2020 = 365 days (2020 is a leap year).
- week = ceil((365 + 1) ÷ 7) = ceil(366 ÷ 7) = ceil(52.29) = 53.
Result: January 1, 2021 falls in ISO week 53 of 2020, even though the calendar year is 2021.
How week 1 lands depending on the weekday January 1 falls on
| January 1 is a | First Thursday of the year | Days at start in previous ISO year |
|---|---|---|
| Monday | January 4 | 0 (Jan 1 to 7 are in week 1) |
| Tuesday | January 3 | 0 |
| Wednesday | January 2 | 0 |
| Thursday | January 1 | 0 |
| Friday | January 7 | 3 (Jan 1 to 3 belong to last week of prior year) |
| Saturday | January 6 | 2 (Jan 1 to 2 belong to prior year) |
| Sunday | January 5 | 1 (Jan 1 belongs to prior year) |
Recent and upcoming 53-week ISO years
| ISO year | Number of ISO weeks | Reason |
|---|---|---|
| 2015 | 53 | Started on a Thursday |
| 2020 | 53 | Leap year starting on a Wednesday |
| 2026 | 53 | Started on a Thursday (January 1, 2026 is a Thursday) |
| 2032 | 53 | Leap year starting on a Thursday |
| 2037 | 53 | Started on a Thursday |
Common mistakes to avoid
- Assuming the week year always equals the calendar year. Early January dates can belong to week 52 or 53 of the previous year, and late December dates can belong to week 1 of the next year. Always pair the week number with its ISO week year, which this tool shows separately.
- Counting weeks from Sunday or from January 1. Some systems start the week on Sunday, and some simply number the partial week containing January 1 as week 1. ISO 8601 starts weeks on Monday and ties week 1 to the first Thursday, so its numbers can differ from those other schemes.
- Forgetting that some years have 53 weeks. Treating every year as having exactly 52 weeks breaks down in years that start on a Thursday, or leap years that start on a Wednesday. Those years carry an extra week 53.
- Mixing ISO numbers with the US week-of-year convention. The common US scheme makes January 1 the start of week 1 and starts weeks on Sunday, which usually gives a different number than ISO. Decide which standard your audience expects before sharing a week number.
Glossary
- ISO 8601
- The international standard for dates and times. Its week rule has weeks start on Monday with week 1 containing the first Thursday of the year.
- ISO week year
- The year a week belongs to under ISO rules. It can differ from the calendar year for the first or last few days of a year.
- Week 1
- The first ISO week of a year: the week that contains the first Thursday, equivalently the week that contains January 4.
- Week 53
- An extra week that exists only in ISO years that start on a Thursday, or leap years that start on a Wednesday.
- Weekday number
- Under ISO, Monday is 1 through Sunday is 7. This Monday-based numbering is used to find the Thursday of a given week.
Frequently asked questions
What week number is it right now?
Open the calculator and it defaults to today, showing the current ISO 8601 week number along with the ISO week year and the weekday. To check another date, just pick it in the date field.
How is the ISO week number calculated?
ISO 8601 starts each week on Monday and defines week 1 as the week that contains the first Thursday of the year. To find a date’s week, you shift to the Thursday of its week, take that Thursday’s year as the ISO week year, and count how many weeks have passed since the start of that year.
Why is January 1 sometimes in week 52 or 53 of the previous year?
Because week 1 must contain the first Thursday, any days in early January that fall before that Thursday belong to the last week of the previous ISO year. For example, January 1, 2021 is a Friday and sits in ISO week 53 of 2020.
Which years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Recent examples are 2015 and 2020, and 2032 is the next leap-year case. Most years have just 52 weeks.
Does ISO week numbering start on Sunday or Monday?
ISO 8601 weeks always start on Monday and end on Sunday. If your calendar shows weeks starting on Sunday, it is using a different convention, and the week number it displays may not match the ISO number.
Is the ISO week number the same as the US week number?
Not usually. The common US convention starts weeks on Sunday and treats the partial week containing January 1 as week 1, so its numbering often differs from ISO. This tool reports the ISO 8601 number, which is the international standard used by most business and software calendars.