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Week Number Calculator – What Week Number Is It?

Find the week number of any date using the international ISO 8601 standard, and the US-style week number too. Or go the other way: enter a year and week number to see the Monday-to-Sunday dates of that week.

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How to use the week number calculator

Week number of a date: choose Week number of a date and pick any date (it starts on today). You’ll see the ISO week and its Monday–Sunday dates, the US week number, and how many ISO weeks the year has.

Dates of a week: choose Dates of a week, enter the year and the ISO week. You’ll see the Monday and Sunday of that week.

How ISO week numbers work

The ISO 8601 rules are:

  1. Weeks start on Monday and end on Sunday.
  2. Week 1 is the week with the year’s first Thursday in it.
  3. A year has 52 or 53 weeks.

A quick way to find the ISO week of a date: move to the Thursday of the same week, then count which week of that Thursday’s year it is.

ISO week = ⌈(day of year of that Thursday) ÷ 7⌉

Worked examples

  • 8 November 2026 (a Sunday) → the Thursday of its week is 5 November, day 309 of the year → 309 ÷ 7 rounded up = week 45.
  • 1 January 2026 (a Thursday) → week 1 of 2026.
  • 1 January 2027 (a Friday) → its Thursday is 31 December 2026 → week 53 of 2026.

ISO and US week numbers side by side

DateISO weekUS week
Sat 3 January 202611
Sun 4 January 202612
Mon 5 January 202622

US weeks start on Sunday, so they roll over a day earlier.

Where week numbers are used

  • Business planning in Europe — “delivery in week 12”.
  • Payroll and shift rotas.
  • Manufacturing and logistics schedules and production codes.
  • School terms and academic calendars.
  • Retail reporting, where sales are compared week by week.
  • Software: many date libraries and spreadsheets have ISO week functions (ISOWEEKNUM in Excel and Google Sheets).

Week numbers in spreadsheets

In Excel and Google Sheets, =ISOWEEKNUM(A1) returns the ISO week of the date in cell A1, while =WEEKNUM(A1) uses the US system by default (weeks starting on Sunday). If your totals don’t match a colleague’s, check which function each of you used — it is one of the most common causes of week-number mix-ups between European and American teams.

Tips

  • When working with international teams, say which system you mean — “ISO week 45”.
  • Watch the year: the ISO week-year can differ from the calendar year for the first and last few days.
  • To count working days in a period, use the business days calculator.

Other date tools: day of the week calculator and date difference calculator.

Frequently asked questions

What week number is it?

The calculator opens on today’s date and shows the current ISO week number. For example, 8 November 2026 is in ISO week 45.

What is an ISO week?

ISO 8601 weeks start on Monday, and week 1 is the week that contains the first Thursday of the year — equivalently, the week containing 4 January. Most of Europe and much of the world use this system.

Why is 1 January sometimes in week 52 or 53?

If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year. 1 January 2027 is a Friday, so it is in ISO week 53 of 2026.

How many weeks are in a year?

Most years have 52 ISO weeks; some have 53. 2026 has 53 weeks because it begins on a Thursday.

How do US week numbers differ?

In the common US convention, weeks start on Sunday and week 1 is the week containing 1 January, however short. So US and ISO week numbers can differ by one.

How do I find the dates of week 1?

Choose “Dates of a week”, enter the year and week 1. ISO week 1 of 2026 runs from Monday 29 December 2025 to Sunday 4 January 2026.

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