Week Number Calculator
Use this free Week Number Calculator to quickly get accurate results in seconds, free and instantly offline with no signup required ever, trusted by professionals in Austin, Chicago, Miami, Denver, Seattle, and New York for reliable daily use without accounts, subscriptions, limits, or delays on any device for everyday personal and business workflows daily.
Find the ISO week number and calendar week of any date instantly in your browser. Free online week number calculator with day-of-year info — no uploads.
Calculator Inputs
About Week Number
Use this free Week Number Calculator to quickly get accurate results in seconds, free and instantly offline with no signup required ever, trusted by professionals in Austin, Chicago, Miami, Denver, Seattle, and New York for reliable daily use without accounts, subscriptions, limits, or delays on any device for everyday personal and business workflows daily.
Week Number Calculator: This free tool tells you which week of the year any given date falls into, supporting both the ISO-8601 international standard and the US Sunday-start system. It also shows the day of year, day of week, and a year progress percentage — all computed instantly offline with no signup required ever.
ISO 8601 Week Numbering: The default ISO system starts weeks on Monday. Week 1 contains the first Thursday of the year. This means if January 1 falls on a Friday, Saturday, or Sunday, those early days belong to week 52 or 53 of the previous year. This is the standard used by most of the world for business and logistics.
US Week Numbering: The US system starts weeks on Sunday and week 1 is simply the first 7 days of January. This is simpler but can produce different week numbers than ISO during the first and last weeks of the year. Choose the system that matches your organization's convention.
Day of Year and Progress: Along with the week number, you get the day of year (1-365 or 1-366 for leap years) and a percentage showing how far through the year the date is. This is useful for tracking project timelines and annual goal progress.
Real-World Applications: ISO week numbers appear on shipping documents, airline tickets, and European business calendars. US week numbers are used in retail scheduling, payroll processing, and academic planning. Converting between the two systems is a common need for companies operating across borders.
Browser-Based Privacy: Everything runs locally in your browser. No date data is transmitted anywhere, and the tool is completely free with no signup required. Works offline once loaded.
Where it is used
A project manager in Austin tracks sprint deadlines using week 36 for a 09/01/2026 release. A Chicago logistics coordinator fills in shipping manifests requiring ISO week numbers. A Miami payroll clerk aligns timesheets to US week boundaries for overtime calculations. A Denver HR director schedules performance reviews by week number across a 500-employee company. A Seattle data analyst exports week-numbered reports for quarterly audits. A New York event planner coordinates vendor deliveries using week-based milestones for a $500,000 wedding.
How to use the Week Number
- Date — enter the value.
- Week numbering system — enter the value.
- Calculate — press the Calculate button to see the result instantly above.
Examples
ISO week
US week
New Year edge
Frequently Asked Questions
- Why can week 1 start in the previous year?
- Under ISO 8601, if January 1 falls on Friday, Saturday, or Sunday, those days belong to week 52 or 53 of the prior year because week 1 must contain the first Thursday.
- How is this different from a US week number?
- US weeks start Sunday and week 1 is simply the first 7 days of the year. ISO weeks start Monday and follow the first Thursday rule, so counts can differ in early January.
- Is this tool free to use?
- Yes, this week number calculator is completely free. Determine week numbers instantly offline with no signup required ever.
- Which system should I use?
- Use ISO 8601 for international business, shipping, and logistics. Use the US system if your organization starts weeks on Sunday for payroll or retail scheduling.
- What is the maximum week number?
- ISO weeks go up to 52 or 53 depending on the year. Years with 53 weeks start on Thursday in common years or Wednesday in leap years.
- Does this handle leap years?
- Yes. Leap years have 366 days and may have 53 ISO weeks. The tool correctly accounts for February 29 when calculating week boundaries.