About Week Number Calculator
Pick any date and this page returns its ISO 8601 week number, the system used across Europe and in international business, where weeks run Monday to Sunday. Thursday 8 October 2026, for example, is in week 41, is day 281 of 365, sits in Q4 and has 84 days left in the year. The result also names the week-numbering year, which can differ from the calendar year for a few days around New Year: 1 January 2027 belongs to week 53 of 2026. A copyable line gives the formal notation, such as 2026-W41-4.
ISO weeks per year and where 1 January falls, 2024 to 2033
Results from this page for 1 January, 28 December (always in the last ISO week) and 31 December of each year.
| Year | 1 January is in | ISO weeks in the year | 31 December is in |
|---|---|---|---|
| 2024 | Week 1 of 2024 (Monday) | 52 | Week 1 of 2025 |
| 2025 | Week 1 of 2025 (Wednesday) | 52 | Week 1 of 2026 |
| 2026 | Week 1 of 2026 (Thursday) | 53 | Week 53 of 2026 |
| 2027 | Week 53 of 2026 (Friday) | 52 | Week 52 of 2027 |
| 2028 | Week 52 of 2027 (Saturday) | 52 | Week 52 of 2028 |
| 2029 | Week 1 of 2029 (Monday) | 52 | Week 1 of 2030 |
| 2030 | Week 1 of 2030 (Tuesday) | 52 | Week 1 of 2031 |
| 2031 | Week 1 of 2031 (Wednesday) | 52 | Week 1 of 2032 |
| 2032 | Week 1 of 2032 (Thursday) | 53 | Week 53 of 2032 |
| 2033 | Week 53 of 2032 (Saturday) | 52 | Week 52 of 2033 |
A year has 53 ISO weeks when 1 January is a Thursday, or a Wednesday in a leap year. The last 53-week year before 2026 was 2020.
Week number examples for specific dates
Each result is the output of the calculator for that date.
An ordinary mid-year date
Date 1 July 2026
ISO week 27, week-numbering year 2026. Day 182 of 365, Q3, 183 days remaining, Wednesday, year progress 50%. ISO notation 2026-W27-3.
New Year's Day in the previous year's last week
Date 1 January 2027
ISO week 53, week-numbering year 2026. Day 1 of 365, Q1, 364 days remaining, Friday. ISO notation 2026-W53-5.
Week 1 of 2027 does not begin until Monday 4 January 2027.
A late December date already in next year's week 1
Date 30 December 2024
ISO week 1, week-numbering year 2025. Day 365 of 366, Q4, 1 day remaining, Monday. ISO notation 2025-W01-1.
The calendar year is 2024 but the week belongs to 2025, which is why the year must be stored with the week.
A leap day
Date 29 February 2024
ISO week 9, week-numbering year 2024. Day 60 of 366, Q1, 306 days remaining, Thursday. ISO notation 2024-W09-4.
Christmas Day 2026
Date 25 December 2026
ISO week 52, week-numbering year 2026. Day 359 of 365, Q4, 6 days remaining, Friday, year progress 98%. ISO notation 2026-W52-5.
How ISO 8601 week numbering works
ISO 8601 sets two rules. A week starts on Monday and ends on Sunday, and week 1 of a year is the week that contains that year's first Thursday. The same rule can be stated in other ways that always give the same answer: week 1 is the week containing 4 January, and it is the first week with at least four of its days in the new year.
The point of the Thursday rule is that a week is never split between two years. Each week is assigned whole to the year that holds most of its days. The cost is that up to three days at the start of January can belong to the last week of the old year, and up to three days at the end of December can belong to week 1 of the new one. In 2026 the start is clean, because 1 January is a Thursday and falls in week 1, but the last week runs on into the next year: 1 to 3 January 2027 are still in week 53 of 2026.
That is why the result shows a week-numbering year as well as the week. For most dates it equals the calendar year. For the few dates where it does not, quoting the week with the wrong year points to a date about twelve months away.
ISO week numbers compared with other week systems
| System | Week starts on | Week 1 is | Where it is used |
|---|---|---|---|
| ISO 8601 | Monday | The week containing the first Thursday of the year | Europe, much of Asia and international business |
| US and Canada convention | Sunday | The week containing 1 January | North American calendars, Excel WEEKNUM default |
| Retail 4-4-5 calendar | Set by the retailer | Set by the fiscal year start | Retail and accounting periods |
This page calculates ISO 8601 weeks only. Under the US convention, 1 January is always in week 1 and the first and last weeks of the year can be shorter than seven days.
How to get the ISO week number in Excel and code
| Where | Expression | Notes |
|---|---|---|
| Excel and Google Sheets | =ISOWEEKNUM(A1) | =WEEKNUM(A1) uses the US system unless you pass 21 as the second argument |
| Python | datetime.date.today().isocalendar()[1] | isocalendar() also returns the week-numbering year and the weekday |
| PHP | date("W") | Use date("o") for the matching week-numbering year |
| Linux and macOS shell | date +%V | Use %G for the week-numbering year |
| PostgreSQL | EXTRACT(WEEK FROM date_column) | Use EXTRACT(ISOYEAR FROM ...) for the year |
| MySQL | WEEKOFYEAR(date_column) | Same as WEEK(date_column, 3) |
| Java | date.get(IsoFields.WEEK_OF_WEEK_BASED_YEAR) | For a java.time.LocalDate |
JavaScript's Date object has no method for the ISO week. It has to be computed from the Thursday of the date's week, or taken from a date library.
What the other results mean
- Day of year: the date's position counted from 1 on 1 January, out of 365 or 366. This is sometimes called the ordinal date.
- Quarter: calendar quarters, Q1 January to March, Q2 April to June, Q3 July to September, Q4 October to December. Fiscal quarters that start in another month are not covered.
- Days remaining: the number of days after the chosen date up to and including 31 December. On 31 December it is 0.
- Day of week: the weekday name of the chosen date.
- Year progress: day of year divided by the days in the year, rounded to a whole percent. It follows the calendar year, not the week-numbering year.
- ISO notation: week-numbering year, W plus the two-digit week, then the weekday number from 1 (Monday) to 7 (Sunday).
Step by step
- 1
Choose a Date with the date picker or type it in. It starts on today's date, so the page opens on the current week number.
- 2
Read ISO week in the Calendar position panel. The small text under it gives the week-numbering year that week belongs to.
- 3
Check Day of year, Quarter, Days remaining, Day of week and Year progress for the date's place in the calendar year.
- 4
Copy the ISO notation line when you need the date in year-week-day form, for example 2026-W41-4.