Week Number Calculator
Find the week number of any date, ISO 8601 or Sunday-start, with the first and last day of that week, or find the dates of a given week number.
Result
Week number: 39
ISO weeks can start in December or end in January, so the week-numbering year is shown. Sunday-start weeks are cut at 1 January and 31 December.
- First day of the week
- September 21, 2026
- Last day of the week
- September 27, 2026
- Week-numbering year
- 2026
- Weeks in that year
- 53
- ISO week date
- 2026-W39-5
https://calcgallery.com/date-time/week-number-calculator/
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How to use the Week Number Calculator
- Choose whether you have a date and want its week number, or have a week number and want its dates.
- [object Object]
- Read the week number, the first and last day of the week, the week-numbering year and how many weeks that year has.
Two numbering systems
ISO 8601 treats a year as a run of whole Monday-to-Sunday weeks. Week 1 is the week that contains the year’s first Thursday, which is the same as the week that contains 4 January. Every week has seven days, so the first or last few days of a calendar year can belong to the neighbouring week-year. The ISO week date writes a day as year, week and weekday, such as 2026-W39-5 for a Friday.
Sunday start, week 1 containing 1 January is the rule behind the default of the WEEKNUM spreadsheet function. Week 1 begins on 1 January whatever day that is and ends on the first Saturday, so it may be only a day long. The final week is cut short at 31 December in the same way. The page shows those cut weeks as they are rather than borrowing days from the next year.
ISO week 1 Monday = the Monday on or before 4 JanuaryISO week n Monday = week 1 Monday + 7 × (n − 1)Sunday-start week = floor((days since 1 January + weekday of 1 January) ÷ 7) + 1, counting weekdays from Sunday = 0Which one to use
Use whichever numbering the people you are working with use. A week 1 that starts in late December, or a week 53 in early January, can only be ISO. Otherwise, check a known date against the calendar in front of you if you are unsure, and pick the numbering that agrees. For a single date’s weekday and day of the year as well, the day of the week calculator shows those alongside the ISO week date.
With your numbers
- ISO weeks run Monday to Sunday; week 1 is the one containing the year’s first Thursday
- The week runs September 21, 2026 to September 27, 2026
- Week number = 39
Quick reference
ISO weeks can start in December or end in January, so the week-numbering year is shown. Sunday-start weeks are cut at 1 January and 31 December.
| Example | Result |
|---|---|
| 25 September 2026, ISO | Week number: 39 |
| 25 September 2026, Sunday start | Week number: 39 |
| 1 January 2021, ISO week 53 of 2020 | Week number: 53 |
| 30 December 2024, ISO week 1 of 2025 | Week number: 1 |
| ISO week 1 of 2026 | First day of the week: December 29, 2025 |
| ISO week 53 of 2026 | First day of the week: December 28, 2026 |
| Week 1 of 2021, Sunday start | First day of the week: January 1, 2021 |
| 31 December 2028, Sunday start | Week number: 54 |
Common questions about the Week Number Calculator
- What week number is it?
- Enter today’s date. The page never reads your device’s clock, so it gives the week of whatever date you type, which also means it works for any past or future date.
- Why do two calendars give different week numbers?
- Because there are two common rules. ISO 8601 starts weeks on Monday and counts week 1 from the week with the year’s first Thursday. The other rule starts weeks on Sunday and makes the week containing 1 January week 1, however short. Away from New Year the two numbers agree or differ by up to two: on 26 September 2021, a Sunday, the ISO week is 38 and the Sunday-start week is 40. Around New Year they can be far apart.
- How can 1 January be in week 53?
- In ISO numbering, 1 January belongs to the previous year’s last week whenever it falls on a Friday, Saturday or Sunday. 1 January 2021 was a Friday, so it is in week 53 of 2020. That is why the page shows the week-numbering year next to the week number.
- How many weeks are in a year?
- In ISO numbering, 52 or 53: a year has 53 weeks when it starts on a Thursday, or is a leap year starting on a Wednesday. In the Sunday-start numbering the first and last weeks are usually partial, which gives 53 weeks, or 54 in a leap year that starts on a Saturday.
- How do I find the dates of week 1?
- Choose the dates of a week number, enter the year and 1. In ISO numbering week 1 always contains 4 January, and it can start as early as 29 December of the year before.