📅 Date & Time

Week Number Calculator

Find the ISO-8601 week number for any date.

Results

ISO week number
42
Day of the year
287
Days left in year
78

📊 Week Number Calculator chart

Proportional breakdown of the result.

What is the Week Number Calculator?

Find the ISO-8601 week number for any date. ISO-8601 week numbers are used in business planning, logistics and international teams, and they do not always match simple counting.

Week Number Calculator – free online date & time tool illustration
Week Number Calculator — free, exact and private.

How to use the Week Number Calculator

  1. Enter your date.
  2. Results update automatically as you type; you can also press Calculate.
  3. Read the output: iso week number, day of the year, days left in year.
  4. Use the 📊 chart to see the result visually, then copy or share it.

Formula and method

ISO weeks start Monday; week 1 contains the year’s first Thursday

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.

Where the Week Number Calculator helps in real life

Your company plans two-week sprints by week number. The calculator shows the ISO week for any date, so everyone refers to the same week.

You coordinate shipments with a European supplier who says goods leave in week 47. Converting tells you the exact dates to expect delivery.

Common mistakes to avoid

  • Assuming week 1 always starts on January 1.
  • Mixing ISO and US week-numbering systems.
  • Ignoring dates near year end.
  • Starting weeks on Sunday.

💡 Practical tip: Some early January dates belong to the last ISO week of the previous year.

Illustration: a workspace with a wall calendar, a clock and a laptop
Week Number Calculator in everyday work — illustration.

Tips for better results

  • Use ISO weeks for international work.
  • Check dates near year end.
  • Record both date and week number.

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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