Why one date can have two week numbers
Countries do not agree on how to number weeks, so this page shows two systems. ISO 8601, common in Europe and in business planning worldwide, starts weeks on Monday and treats week 1 as the week with the first Thursday of the year. The US-style count starts weeks on Sunday and makes week 1 the week that contains 1 January.
Enter any date and both numbers appear, along with the start and end dates of that week in the chosen system and the number of weeks left in the year. The top of the page shows today's week using your device date.
The difference matters near New Year. Friday 1 January 2027 belongs to ISO week 53, which is the last week of the previous ISO year, yet it is already week 1 in the US count. Monday 4 January 2027 starts ISO week 1.
Use it to check a project calendar, match a supplier's week-based delivery date or read a spreadsheet that numbers its columns by week. If a colleague quotes a week number and your own calendar disagrees, the system is usually the reason.
Worked examples
ISO week years 2024 to 2035
Week 1 is the week with the first Thursday of the year, which is the same as the week containing 4 January. Most years have 52 weeks. A year has 53 when 1 January is a Thursday, or a Wednesday in a leap year.
| Year | Week 1 starts | Weeks | Last week ends |
|---|---|---|---|
| 2024 | Mon 1 Jan 2024 | 52 | Sun 29 Dec 2024 |
| 2025 | Mon 30 Dec 2024 | 52 | Sun 28 Dec 2025 |
| 2026 | Mon 29 Dec 2025 | 53 | Sun 3 Jan 2027 |
| 2027 | Mon 4 Jan 2027 | 52 | Sun 2 Jan 2028 |
| 2028 | Mon 3 Jan 2028 | 52 | Sun 31 Dec 2028 |
| 2029 | Mon 1 Jan 2029 | 52 | Sun 30 Dec 2029 |
| 2030 | Mon 31 Dec 2029 | 52 | Sun 29 Dec 2030 |
| 2031 | Mon 30 Dec 2030 | 52 | Sun 28 Dec 2031 |
| 2032 | Mon 29 Dec 2031 | 53 | Sun 2 Jan 2033 |
| 2033 | Mon 3 Jan 2033 | 52 | Sun 1 Jan 2034 |
| 2034 | Mon 2 Jan 2034 | 52 | Sun 31 Dec 2034 |
| 2035 | Mon 1 Jan 2035 | 52 | Sun 30 Dec 2035 |
Dates are worked out by this site from the rule above and checked against the calculator's own engine on every build.
Questions about this tool
Which system should I use?
Use ISO if your team, supplier or software says ISO 8601 or starts weeks on Monday. Use the US count for calendars that start on Sunday.
Can a year have 53 weeks?
Yes. Under ISO, some years have 53 weeks. For example, 2026 has 53, while 2027 has 52.
Why does 1 January sometimes show the previous year's week?
In the ISO system a week belongs to the year that holds its Thursday, so early January days can sit in the last week of the old year.
How do I get the ISO week number in Excel or Google Sheets?
Use =ISOWEEKNUM(A1) with the date in A1. The plain WEEKNUM(A1) function counts the US way by default, with week 1 holding 1 January, so near New Year the two can differ by one, which is the difference this page shows.
Related tools
Where this comes from: ISO 8601-1:2019, the date and time standard ยท ISO week date on Wikipedia