Example
Week 36 (Tue, day 244)
Week Number Calculator
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.
Enter the values below, then press Calculate (or press Enter). The calculator updates your results instantly. Try different values to see how your inputs change the outcome.
- Date — enter a date.
- Week numbering system — enter a choice from the options.
- Review your results — the key output is highlighted, with supporting figures below.
Loading calculator…
Results
Week number
| Detail | Value |
|---|---|
| Day of year | |
| Day of week | |
| Week begins | |
| Year progress (%) | |
| Note |
What is this calculator for?
Determine the ISO-8601 (or US) week number of a given date, along with its day-of-year and week start.
How to Use This Calculator
Enter the values below, then press Calculate (or press Enter). The calculator updates your results instantly. Try different values to see how your inputs change the outcome.
- Date — enter a date.
- Week numbering system — enter a choice from the options.
- Review your results — the key output is highlighted, with supporting figures below.
What You'll Need
- Date
- a date · required
- Week numbering system
- a choice from the options (ISO 8601 (Mon start), US (Sun start)) · optional
About This Calculator
This week number calculator reports which week of the year a date falls into.
The default uses ISO 8601, where weeks start on Monday, week 1 contains the first Thursday of the year, and year boundaries can shift week numbering into the next/previous year. The US system starts weeks on Sunday and counts from January 1.
You also get the day-of-year and a quick year-progress percentage. Everything is computed locally in your browser.
Worked Examples
Frequently Asked Questions
Why can week 1 start in the previous year?
Under ISO 8601, if a year begins late in the week, the first days belong to week 52/53 of the prior year. The tool applies this correctly.
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 the counts can differ early in the year.