BYTETOOLS

Week Number Calculator

Find the ISO 8601 week number for any date, plus the Monday-to-Sunday date range of that week and the day of the week. Free and 100% in-browser.

Week 35
ISO week number
2026
ISO week-year
Aug 24, 2026 – Aug 30, 2026
Week range (Mon–Sun)
Monday
Day of week

What is the Week Number Calculator?

The ByteTools Week Number Calculator tells you which ISO 8601 week of the year any date falls in. Enter a date and it returns the week number (1–53), the Monday-to-Sunday range that defines that week, the year the ISO week belongs to, and the day of the week your date lands on.

  • Official ISO 8601 week numbering (1–53)
  • Shows the Monday-to-Sunday range of the week
  • Reports the correct ISO week-year at year boundaries
  • Displays the day of the week for your date
  • One-click Today button
  • Runs entirely in your browser — instant and private

How to use the Week Number Calculator

  1. 1

    Pick a date (or click Today).

  2. 2

    Read the ISO 8601 week number for that date.

  3. 3

    Check the Monday-to-Sunday date range of that week.

  4. 4

    See the day of the week and the ISO week-year.

About the Week Number Calculator

The ByteTools Week Number Calculator tells you which ISO 8601 week of the year any date falls in. Enter a date and it returns the week number (1–53), the Monday-to-Sunday range that defines that week, the year the ISO week belongs to, and the day of the week your date lands on.

ISO week numbers are used across business, manufacturing, logistics and payroll — especially in Europe — where schedules refer to "week 32" rather than a specific date. This tool removes the guesswork by applying the official ISO rule: weeks start on Monday and week 1 is the week containing the first Thursday of the year.

Everything is computed locally in your browser using JavaScript. No dates are sent anywhere, so it is a safe, instant reference for planning and reporting.

Frequently asked questions

How is the ISO 8601 week number calculated?

Under ISO 8601, weeks run Monday to Sunday and week 1 is the week that contains the year's first Thursday — equivalently, the week containing January 4. The tool applies this rule so week numbers match calendars used across Europe and in business software.

Why can a date in late December be week 1 of the next year?

If December 29, 30 or 31 falls in a week whose Thursday is already in January, ISO assigns it to week 1 of the following year. That is why the tool also shows the ISO week-year, which can differ from the calendar year.

Can a year have 53 weeks?

Yes. Years that start on a Thursday, or that are leap years starting on a Wednesday, contain 53 ISO weeks. All other years have 52. The calculator handles both cases automatically.

What day does an ISO week start on?

ISO 8601 weeks always start on Monday and end on Sunday. This differs from the US convention where weeks often start on Sunday, so week numbers can vary between systems that use different rules.

What is week number used for?

Week numbers give a compact way to refer to periods in planning, manufacturing schedules, payroll, retail calendars and project timelines — saying "ship in week 40" is clearer than quoting a specific date range.

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