How ISO week numbers work
ISO 8601 is the international standard used across Australia, Europe and most business calendars.
ISO week = week (Monday to Sunday) counted from the week holding the year's first Thursday
- Weeks run Monday to Sunday.
- Week 1 is the week with the year's first Thursday in it. That is also the week containing 4 January.
- Find the Thursday of your date's week. Its year is the ISO week-year, and counting weeks from week 1 gives the number.
- Most years have 52 weeks. A year has 53 when it starts on a Thursday, or on a Wednesday in a leap year.
Worked example — 29 December 2025 is a Monday. That week's Thursday is 1 January 2026, so it is week 1 of 2026, running 29 Dec 2025 to 4 Jan 2026.
Week number examples
| Date | ISO week number | Week runs |
|---|---|---|
| 29 Dec 2025 | Week 1 of 2026 | 29 Dec 2025 to 4 Jan 2026 |
| 1 Jan 2026 | Week 1 | 29 Dec 2025 to 4 Jan 2026 |
| 31 Dec 2026 | Week 53 | 28 Dec 2026 to 3 Jan 2027 |
| 3 Jan 2021 | Week 53 of 2020 | 28 Dec 2020 to 3 Jan 2021 |
| 1 Jan 2027 | Week 53 of 2026 | 28 Dec 2026 to 3 Jan 2027 |
Swipe the table sideways to see the rest.
ISO 8601 weeks. Dates near New Year can belong to the neighbouring year.
Things to consider
- Planners and payroll calendars in Australia usually use ISO weeks.
- Check the ISO week-year in late December and early January.
- US calendars often start weeks on Sunday, so numbers can differ by one.
- Week 53 only happens in some years, such as 2020 and 2026.
Frequently asked questions
What week number is it today?
The calculator opens on today's date in your time zone and shows its ISO week number.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday, and week 1 holds the first Thursday of the year. The common US system starts weeks on Sunday, and week 1 is the week containing 1 January. The two can differ, especially near New Year.
Why is 29 December in week 1 of next year?
Under ISO rules a week belongs to the year its Thursday falls in. If that Thursday is 1 January, the whole week counts as week 1 of the new year.
Does every year have 52 weeks?
No. ISO years have 52 or 53 weeks. 2026 has 53 because it starts on a Thursday.
Which system does Australia use?
Most Australian business, government and planner calendars follow ISO 8601, with Monday as the first day.
Results are estimates for general information only, not financial, medical or professional advice.