62 · Calculators

Week Number Calculator

Pick a date to see its ISO week number, the Monday-to-Sunday week it falls in, and the day of the year.

ISO week number

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How to use the week number calculator

  1. Pick a date; today is filled in for you.
  2. Read the ISO week number.
  3. See the week’s dates and the day of the year.

ISO week numbers

ISO 8601 weeks start on Monday, and week 1 is the week containing the year’s first Thursday. So the first days of January can belong to the last week of the previous year, and some years have 53 weeks.

Examples

October 29, 2026

Week 44 · day 302 of 365

January 1, 2027

Week 53 of 2026

Questions

Why does early January show week 52 or 53?

Those days belong to the last ISO week of the previous year until the week with the first Thursday begins.

Does the US use ISO weeks?

Many US calendars start weeks on Sunday and number them differently. ISO weeks are standard in Europe and in business.