How ISO week numbers work
ISO 8601 numbers the weeks of a year from 1. Each week runs from Monday to Sunday, and week 1 is the week that contains the first Thursday of the year. That is the same as saying 4 January always falls in week 1. A year has either 52 or 53 full weeks.
Because weeks always start on Monday, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, 1 January 2027 is a Friday and sits in week 53 of 2026. The calculator shows the ISO year as well, so you can see when this happens.
When week 1 starts, 2026 to 2031
| ISO year | Week 1 starts | 1 Jan is a | Weeks |
|---|---|---|---|
| 2026 | Mon 29 Dec 2025 | Thu | 53 |
| 2027 | Mon 4 Jan 2027 | Fri | 52 |
| 2028 | Mon 3 Jan 2028 | Sat | 52 |
| 2029 | Mon 1 Jan 2029 | Mon | 52 |
| 2030 | Mon 31 Dec 2029 | Tue | 52 |
| 2031 | Mon 30 Dec 2030 | Wed | 52 |
Week numbers on the first of each month, 2026
| Date | ISO week | US style week |
|---|---|---|
| 1 January | 1 | 1 |
| 1 February | 5 | 6 |
| 1 March | 9 | 10 |
| 1 April | 14 | 14 |
| 1 May | 18 | 18 |
| 1 June | 23 | 23 |
| 1 July | 27 | 27 |
| 1 August | 31 | 31 |
| 1 September | 36 | 36 |
| 1 October | 40 | 40 |
| 1 November | 44 | 45 |
| 1 December | 49 | 49 |
Common questions
What week number is it today?
Open this page and the calculator fills in today's date and shows its ISO week number straight away. As a worked example, 1 October 2026 is in week 40 of 2026, and week 40 runs from Monday 28 September to Sunday 4 October.
How is an ISO week number defined?
ISO 8601 says weeks start on Monday and week 1 is the week that contains the first Thursday of the year. The simple test is that 4 January is always in week 1. Every ISO year has either 52 or 53 whole weeks.
Why does 1 January sometimes fall in week 52 or 53?
Weeks always run Monday to Sunday, so the first days of January can belong to the last week of the year before. In 2027, 1 January is a Friday and falls in week 53 of 2026. Week 1 of 2027 starts on Monday 4 January.
Which years have 53 weeks?
A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. The recent and upcoming 53-week years are 2015, 2020, 2026, 2032 and 2037. Every other year has 52.
What is the difference between ISO and US week numbers?
The US style used by many calendars starts weeks on Sunday and makes week 1 the week that contains 1 January. ISO starts on Monday and puts the first Thursday in week 1. The two systems can differ by one, and the table above shows both for 2026.
Which countries use ISO week numbers?
ISO weeks are widely used across Europe and in payroll, shipping, manufacturing and project planning, where a Monday start is standard. Calendars in the United States, Canada and Australia more often show the Sunday-start style, or no week numbers at all.
Does the Excel WEEKNUM function match the ISO week?
Not by default. WEEKNUM uses the Sunday-start US style unless you choose return type 21, which gives the ISO week. The ISO.WEEKNUM function also returns the ISO number. If a spreadsheet and this page disagree, that setting is the usual cause.
How do I find the dates of a given week number?
Use the Find the dates of a week panel above: enter the ISO year and the week number, and it shows the Monday and Sunday. It also warns you when a week does not exist, such as week 53 in a 52-week year. To move by whole weeks from a date, use the add or subtract days calculator.