Week Number Calculator
What week is it? Find the ISO week number of any date, the week’s first and last day, and go the other way from a week number to its dates.
About this tool
Uses ISO 8601 weeks, the standard in South Africa, Europe and most business software: weeks start on Monday, and week 1 is the week with the year’s first Thursday. Change the date to get its week, or change the year and week to jump to that week’s Monday.
Find out what week number a date falls in, or which dates belong to a week number. Planners, payroll, school timetables and many diaries use ISO week numbers, where weeks start on Monday. Type a date to see its week, or type a year and week number to jump to that week.
How to use the Week Number Calculator
- Pick a date. The tool shows its ISO week number and that week’s Monday and Sunday.
- To go the other way, type a year and a week number. The date changes to the Monday of that week.
- Check the number of weeks in the year: most years have 52, some have 53.
Sunday 18 October 2026
18 October 2026 is in week 42. That week starts on Monday 12 October and ends on Sunday 18 October 2026. Going the other way, week 1 of 2026 starts on Monday 29 December 2025, because 1 January 2026 is a Thursday and week 1 is the week that contains the year’s first Thursday.
When week 1 starts
Worked out with this calculator.
| Year | Week 1 starts on | Weeks in the year |
|---|---|---|
| 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 |
How ISO week numbers work
The ISO 8601 standard numbers the weeks of a year from 1 to 52 or 53. Every week runs from Monday to Sunday, and week 1 is the week that contains the first Thursday of January (which is the same as the week that contains 4 January). This means the first few 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. A year has 53 weeks when it starts on a Thursday, or is a leap year that starts on a Wednesday, so 2026 has a week 53.
Common mistakes
- Mixing ISO weeks with US-style weeks, which start on Sunday and count 1 January as week 1. The two systems can differ by a week.
- Using the calendar year instead of the ISO year near New Year. 1 January 2027 is in week 53 of 2026.
- Assuming every year has 52 weeks. 2026 has 53, so a week-53 deadline is real.
Frequently asked questions
What week is it today?
Choose today’s date and the tool shows its ISO week number.
Does South Africa use ISO week numbers?
ISO weeks starting on Monday are the common standard, and Excel’s ISOWEEKNUM uses them.
Why is 1 January in week 52 or 53?
Because ISO week 1 is the week with the first Thursday. If 1 January is a Friday, Saturday or Sunday, it still belongs to the last week of the old year.
How many weeks are in 2026?
53. 2026 starts on a Thursday, so it has an extra ISO week, from Monday 28 December 2026 to Sunday 3 January 2027.