Week Numbers: ISO 8601 vs US Week Numbering
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ISO 8601 weeks start on Monday and week 1 is the week with the year’s first Thursday. The common US style starts weeks on Sunday and calls the week containing January 1 week 1. Around New Year the two disagree.
The two rules
| ISO 8601 | US style | |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 is | The week with the first Thursday (the week containing January 4) | The week containing January 1 |
| Days at the turn of the year | Can belong to the last week of the previous year, or week 1 of the next | Always belong to their own calendar year |
| Weeks in a year | 52 or 53 | 53 or 54, with short first and last weeks |
Worked examples
| Date | ISO 8601 | US style |
|---|---|---|
| Thu Dec 31, 2026 | 2026, week 53 | Week 53 |
| Fri Jan 1, 2027 | 2026, week 53 | Week 1 |
| Sun Jan 3, 2027 | 2026, week 53 | Week 2 |
| Mon Jan 4, 2027 | 2027, week 1 | Week 2 |
| Fri Dec 31, 2027 | 2027, week 52 | Week 53 |
January 1, 2027 is a Friday. Its week has only three days in 2027, so ISO counts it as the last week of 2026. That makes 2026 a 53-week year in ISO numbering. In the US style the same Friday is in week 1, and a new week starts two days later on Sunday, January 3.
Make a calendar with week numbers
Which one to use
Use ISO week numbers when you share schedules across countries or with software that says “ISO week”. Use the US style when your calendar starts on Sunday and you want week 1 to begin with January 1. What matters is that everyone reading the calendar uses the same rule.
In Onesheet
Turn on “Week numbers” under “Change settings”. A Monday-start calendar is numbered by ISO 8601 and a Sunday-start calendar by the US style, so the numbers always match the rows you see.
Found a mistake, or a rule that has changed? Email contact@lumenlab.page. We check it, fix it and put the new date on this guide.