Week Numbers Explained: How ISO and US Week Numbers Work

Week numbers are a quick way to talk about time. "Let's ship it in week 42" is shorter than "the week of October 12". They are used a lot in business, factories, shipping and schools, especially in Europe.

The catch is that there is more than one way to count them. The same day can be week 1 in one system and week 2 in another. This guide explains the two systems you will meet most, and how to tell which one someone means.

Quick Answer

• A year has 52 or 53 weeks in the ISO system, which is the international standard.
• ISO weeks start on Monday, and week 1 is the week with the year's first Thursday in it.
• The common US style starts weeks on Sunday, and week 1 is simply the week that contains January 1.
• The two systems often give different numbers for the same date, especially in early January.

The ISO Week Number System

ISO 8601 is the international standard for dates and times. Its week rules are:

1. Every week starts on Monday and ends on Sunday.
2. Week 1 is the week that contains the first Thursday of the year. Another way to say this: week 1 always contains January 4.
3. Every week is a full 7 days. A week never gets split between two years.

Because of rule 3, the first few days of January can belong to the last week of the year before, and the last few days of December can belong to week 1 of the next year.

An example you can check

January 1, 2027 is a Friday. The first Thursday of 2027 is January 7, so ISO week 1 of 2027 runs from Monday, January 4 to Sunday, January 10. That means January 1, 2 and 3, 2027 are still in week 53 of 2026.

Our monthly calendar pages use the ISO system. For example, the January 2027 calendar shows which ISO week each date falls in.

The US-Style Week Number System

Many American calendars and the default setting in Excel count weeks a simpler way:

1. Weeks start on Sunday.
2. Week 1 is the week that contains January 1, even if it only has one or two days.
3. A new week number starts every Sunday after that.

This is easy to understand, but the first and last weeks of the year are often short. A year usually shows 53 week numbers this way, and now and then 54. That happens in a leap year that starts on a Saturday, like 2028, where December 31 falls on a Sunday and starts a week of its own.

Same Date, Two Different Week Numbers

Here is what that looks like in practice in early 2026. January 1, 2026 is a Thursday.

The Sunday is where they split. In ISO, Sunday is the last day of a week. In the US style, Sunday is the first day of a new one. If you share week numbers with people in other countries, agree on ISO to avoid mix-ups.

How Many Weeks Are in a Year?

A normal year has 365 days, which is 52 weeks and 1 day. A leap year has 52 weeks and 2 days. Those extra days are why some years get a 53rd week.

In the ISO system, a year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Otherwise it has 52.

So 2026 is a 53-week year. Its last ISO week, week 53, runs from Monday, December 28, 2026 to Sunday, January 3, 2027. The next 53-week ISO year after that is 2032.

Who Uses Week Numbers?

• Businesses in Europe, where "week 12" is normal in everyday work talk.
• Factories, warehouses and shipping, for production plans and delivery dates.
• Project teams, for sprints and release plans.
• Schools, for term plans and timetables that repeat every week.
• Payroll, for weekly or every-other-week pay periods.

How to Find Week Numbers in Excel and Google Sheets

Both programs have two formulas, one for each system. Say your date is in cell A1:

If week numbers in your spreadsheet look one off from someone else's, check which formula you are both using.

How to Show Week Numbers in Your Calendar App

• Google Calendar (web): Settings, then View options, then turn on "Show week numbers".
• Google Calendar (Android): Settings, then General, then turn on "Show week number".
• iPhone Calendar: open the Settings app, tap Calendar, then turn on Week Numbers.
• Outlook (desktop): File, then Options, then Calendar, then tick "Show week numbers in the month view".

Menu names can change a little between app versions, but the setting is usually under Calendar or View options.

Tips for Using Week Numbers Without Confusion

• Say which system you mean the first time, for example "ISO week 14".
• Add the year near the start and end of the year, for example "week 1 of 2027", because early January can belong to the year before.
• For important dates, write the actual date next to the week number.
• Print a calendar that shows week numbers. In our custom calendar maker, switch on "Show Week Numbers" to add ISO weeks to every row.