Week Numbers USA

What is a week number?

A week number tells you which week of the year a date falls in. Instead of writing out a date range, you can simply refer to the number: the current week is week 42 in the US system.

In the United States, week numbers are less common in everyday life than in Europe, but they're widely used in business: in payroll and scheduling, retail and manufacturing, logistics and project management. If you work with international partners, you'll come across them even more often.

How do US week numbers work?

The most common system in the United States has two simple rules. A week starts on Sunday and ends on Saturday. And week 1 is the week that contains January 1, no matter how many days of that week fall in the new year. If January 1 is a Saturday, week 1 is just one day long, and week 2 starts the next day.

Because of that, the first and last weeks of the year are usually partial weeks. A year normally has 53 numbered weeks, and a leap year that starts on a Saturday even has a week 54, consisting of only December 31. Every week belongs to the calendar year it falls in, so there's never any confusion about the year.

ISO 8601 week numbers

Most of Europe, and many international companies, use the ISO 8601 standard instead. Under ISO 8601 a week starts on Monday, every week has seven full days, and week 1 is the week containing the first Thursday of January. As a result, the first days of January can belong to the last week of the previous year, or the last days of December to week 1 of the next year.

An ISO year has 52 or 53 weeks: 2026 has 53. The current ISO week is week 42, while the US week number is 42. The two numbers can be the same or one week apart, depending on the time of year, so always check which system a colleague, customer or supplier means.

Week numbers in Excel, Outlook and other software

Software often uses the US system by default. In Excel, the WEEKNUM function counts weeks starting on Sunday, with week 1 containing January 1. For ISO week numbers, use ISOWEEKNUM, or WEEKNUM with return type 21. Outlook shows week numbers starting from January 1 by default, but you can change the first week of the year in the calendar options.

If week numbers in two programs don't match, the difference in system is almost always the reason. This site shows both, so you can compare them side by side.

Retail and fiscal weeks

Many companies use their own fiscal calendar, with weeks numbered from the start of the fiscal year rather than January 1. Retailers often follow a 4-5-4 calendar, which divides the year into quarters of 13 weeks, so that sales can be compared week by week from year to year. In those cases the week number in your company's systems can differ from both the US and the ISO week numbers shown here.