What is the Week Number Calculator?
Find the ISO-8601 week number for any date. Dates are deceptively tricky — months have different lengths and leap years shift everything. These date and time calculators handle calendar math exactly, using UTC to avoid daylight-saving surprises.
How to use the Week Number Calculator
- Enter your date.
- Results update automatically as you type; you can also press Calculate.
- Read the output: iso week number, day of the year, days left in year.
- Use the 📊 chart to see the result visually, then copy or share it.
Formula and method
Worked example
With date = 2026-10-14, the Week Number Calculator returns iso week number: 42; day of the year: 287; days left in year: 78.
Tips for better results
- Decide whether you need to include the end date when counting days.
- Business-day counts exclude weekends but not public holidays unless you subtract them.
- Timestamps are shown in UTC; convert to your local time zone as needed.
Frequently asked questions
What does the Week Number Calculator do?
Find the ISO-8601 week number for any date.
How is it calculated?
The Week Number Calculator uses this method: ISO weeks start Monday; week 1 contains the year’s first Thursday.
Can you show an example?
With date = 2026-10-14, the Week Number Calculator returns iso week number: 42; day of the year: 287; days left in year: 78.
Is the Week Number Calculator accurate?
Yes. It uses standard formulas and exact constants, and the results are verified by our automated test suite. Values are displayed to up to 12 significant digits.
Is this tool free and private?
Yes — the Week Number Calculator is completely free with no sign-up, and it runs entirely in your browser, so the values you enter are never uploaded to our servers.
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