ISO vs US Week Numbers

Two systems, two rules

The ISO system and the common US system both give each week of the year a number, but they follow different rules. The ISO system starts the week on Monday and makes week 1 the week with at least four days in the new year, which is the week with the first Thursday. The US system, as used by default in Excel and many calendar apps, starts the week on Sunday and makes week 1 the week that contains 1 January, however few days of it are in the new year.

Both are legitimate. They solve the same problem, how to number weeks across a year boundary, with different priorities. ISO prefers weeks that belong clearly to one year. The US rule prefers a simple rule: the week of 1 January is the first.

When they agree

For most dates in the middle of a year, the two numbers match. Thursday 22 October 2026 is in week 43 in both systems, and so is every day from Monday 19 October to Saturday 24 October. The only day in that week where the numbers differ is Sunday 25 October, which the US system counts as the first day of week 44 and ISO counts as the last day of week 43.

That Sunday effect is the most common difference. Because the US week starts on Sunday, every Sunday is the first day of a new US week and the last day of an ISO week. Whenever you look up a Sunday, the US number is likely to be one higher than the ISO number. The calculator shows both numbers in the rows under the main result.

Where they differ most: New Year

Around 1 January the difference grows. For 2026, 1 January is a Thursday. In the ISO system week 1 starts on Monday 29 December 2025, so 29 to 31 December 2025 are already in week 1 of 2026. In the US count, which numbers days by the year they are in, those three December days are still in week 53 of 2025, and week 1 of 2026 begins on 1 January. The same Monday, 29 December 2025, is week 1 in one system and week 53 in the other.

To see a clean example, take Friday 1 January 2027. In the ISO system it is in week 53 of 2026. In the US system it is in week 1 of 2027, because it is the week containing 1 January. The same date has number 53 in one system and number 1 in the other.

In the US count the week that contains 1 January is split between two years: its December days belong to the last week of the old year and its January days to week 1. The system can also produce a week 54. In 2028, a leap year that starts on a Saturday, Sunday 31 December 2028 is in US week 54, while in ISO it is in week 52 of 2028. A report that expects a maximum of 53 will break on that day.

A comparison of dates

Here are some dates that show the difference. Sunday 5 April 2026 is in ISO week 14 and US week 15. Friday 1 January 2027 is in ISO week 53 of 2026 and US week 1 of 2027. Sunday 3 January 2027 is in ISO week 53 of 2026 and US week 2 of 2027. Monday 4 January 2027 is in ISO week 1 of 2027 and US week 2 of 2027.

For Tuesday 15 June 2027 the ISO number is 24 and the US number is 25. For 22 October 2026 both are 43. Over a whole year, the numbers agree on most weekdays and differ mainly on Sundays and around New Year, and the pattern depends on the weekday of 1 January.

Which one should you use?

Use ISO weeks whenever people from different countries or systems have to agree. It is the international standard, it is the default in most European calendars, and it gives every week one clear year. It is also the choice when week numbers appear in contracts, supply chains or software that exchanges data.

Use the US rule if your audience expects it, for example for some retail reporting in the United States, or when a spreadsheet function already uses it and the results have to match old reports. In that case, write down that the weeks start on Sunday and that week 1 contains 1 January.

Never mix the two inside one document. If some rows use ISO weeks and others use US weeks, the totals for the first and last week of a year will be wrong in ways that are hard to see.

How to get each number from the calculator

The first tab shows the ISO week as the large result and the US week in a row under it, labelled with its rule. The row below that shows a third variant, the Monday-start version of the same idea, where weeks start on Monday and week 1 contains 1 January. Together the three cover the most common systems.

If you copy the result, the ISO form is copied, for example "Week 43 of 2026". For the US number, read it off the row and add the label yourself, so that nobody mistakes it for the ISO number.