Week Number Calculator - US vs ISO Work Week Standard
date ↔ week #Find the US and ISO week number for any date. Understand how American business weeks differ from the international ISO standard.
US Exclusive Use Cases
This week-number tool serves American business and financial reporting needs:
- Corporate weekly reporting: Assigning US work-week numbers for status reports and timesheets
- US stock-market weeks: Cross-checking NYSE/Nasdaq trading-week numbering
- Project sprint cycles: Mapping Agile two-week sprints onto US week numbers
- School instructional weeks: Counting teaching weeks in a US academic term
US vs Global Calculation Differences
US week numbering diverges from the international ISO 8601 standard in important ways:
- Week 1 start: US week 1 begins on January 1 regardless of weekday; ISO week 1 is the week containing the first Thursday
- Week start day: US convention treats Sunday as the first day; ISO mandates Monday
- Year-boundary weeks: late-December and early-January dates can carry different week numbers under US vs ISO rules
- Trading-week impact: US financial weeks follow the Sunday-start convention, which matters for weekly market reports
Common Calculation Mistakes
Common mistakes: applying ISO week numbers to US weekly reports (off by one near year boundaries), mislabeling NYSE trading weeks because of the Sunday-start convention, and assuming week 53 never occurs (it can under US rules in some years). Always confirm which standard your downstream report expects.
Tool Algorithm & Precision Explanation
The calculator computes both standards side by side so you can pick the right one:
- US week number: counts from January 1, Sunday-start, the American corporate default
- ISO week number: Monday-start, week 1 contains the first Thursday, the international standard
- Day-of-year and days-remaining: extra context for reporting accuracy
- No averaging: both numbers are exact integer results for the entered date
US Federal Holiday & Workday Reference
How US federal holidays typically fall across week numbers (varies by year):
| Holiday | Week Pattern | Reporting Note |
|---|---|---|
| New Year's Day | Week 1 | Annual reset week |
| MLK Jr. Day | Week 3-4 | 3rd Monday January |
| Labor Day | Week 36-37 | 1st Monday September |
| Thanksgiving | Week 47-48 | 4th Thursday November |
| Christmas Day | Week 52 | Year-end close |
Frequently Asked Questions
Which week-number standard do US companies use for weekly reports?
Most US corporations use the Sunday-start week 1 = January 1 convention. This calculator shows that US number alongside the ISO number so you can match your report.
How do US stock-market trading weeks differ from ISO weeks?
US market weeks follow the Sunday-start American convention, so a trading week can be numbered differently than the ISO week for the same date near year boundaries.
Why do my US and ISO week numbers differ by one?
Because US week 1 starts on January 1 (Sunday-start) while ISO week 1 is the week with the first Thursday. Near year boundaries this shifts the number by one.
Can a year have a week 53 under US rules?
Yes. Depending on the weekday of January 1, a US year can include a partial week 53, whereas ISO caps at 52 or 53 by its own rule.
Does the week start on Sunday or Monday in the US?
The US corporate convention starts the week on Sunday. ISO 8601 (used in Europe) starts on Monday. This tool shows both.
How do I map Agile sprints to US week numbers?
Use the US week number column to label each sprint. Two-week sprints will span two consecutive US week numbers, which keeps reporting consistent.