Week Number Calculator
Find the ISO 8601 and US week number for any date. Shows both standards side by side with explanation of differences.
How to use
Select a date
Select or type a date (defaults to today).
View week numbers
View the ISO 8601 and US week numbers.
See additional info
See additional info like day of year and days remaining.
Use cases
Business planning
"Reference ISO week numbers for sprint planning or quarterly reports."
International scheduling
"Know which calendar week you're in for cross-border coordination."
Academic planning
"Identify week numbers for semester scheduling."
Frequently Asked Questions
?What's the difference between ISO and US week numbering?
ISO weeks start on Monday and week 1 contains January 4. US weeks start on Sunday and week 1 contains January 1.
?Which standard should I use?
ISO 8601 is the international standard used in most countries. The US system is mainly used in North America.
?Is my data private?
Yes. Everything runs locally in your browser. No data is sent to any server.
?Can I check past or future dates?
Yes, select any date to see its ISO and US week numbers.
?Is this tool free?
Yes. Completely free with no limits, no sign-up required.
?Why do ISO and US week numbers differ?
Because they use different rules for when week 1 starts and which day begins the week.
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