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Week Calculator

Find the week number of any date (ISO 8601 and US), count the weeks between two dates, or add weeks to a date. Free week number calculator.

Week numbers and counting weeks

Week numbers are used for planning, payroll and production schedules. In the ISO 8601 standard, weeks start on Monday and week 1 is the week with the year's first Thursday, so a year has 52 or 53 weeks: 2026 has 53. 30 October 2026 is in ISO week 44. In the US, weeks often start on Sunday and week 1 is the one containing 1 January, so numbers can differ near the new year. You can also count the weeks between dates or add weeks to a date.

How to use it

  1. 1

    Choose the calculation

    Week number, weeks between dates or add weeks.

  2. 2

    Enter the date or dates

    Or use today.

  3. 3

    Read the result

    ISO and US week numbers, or the week count.

Frequently asked questions

What week is it?

The calculator opens on today's date and shows the current ISO week number.

Why do some years have 53 weeks?

A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year.

How do I count weeks between dates?

Divide the days between them by 7. The calculator shows whole weeks and leftover days.

Is anything stored?

No. The calculation runs in your browser.

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