Week Number Calculator - ISO Week of Any Date

About the Week Number Calculator

Many businesses, schools and project teams organise work by week number rather than by date: "the delivery is due in week 40", "the sprint runs weeks 12 to 14", "the factory closes for weeks 31 and 32". Week numbers are especially common in Europe and in manufacturing, logistics, retail and payroll. But working out which week a date falls in — or which dates a week covers — is not obvious, because the first and last weeks of a year can spill into the neighbouring years.

This week number calculator gives the ISO 8601 week number of any date, the Monday-to-Sunday range of that week, the ISO week code, the US-style week number, and how many weeks the year has. It also works in reverse, giving the dates of any week in any year.

How to Use the Week Number Calculator

Choose Week of a date or Dates of a week.

For a date, enter it and the week number appears.

For a week, enter the year and week number to see its dates.

The ISO 8601 Rules

  weeks start on Monday and end on Sunday
  week 1 is the week containing the year's first Thursday
      (equivalently, the week containing 4 January)
  a year has 52 or 53 weeks
  the last days of December can be in week 1 of the next year,
  and the first days of January in week 52 or 53 of the previous year

A simple way to find the ISO week of a date: find the Thursday of the same week; the week belongs to that Thursday's year, and its number is the count of whole weeks from 1 January of that year, plus one.

Step-by-Step Example

Tuesday 29 September 2026.

  Thursday of that week:  1 October 2026
  Days from 1 January:    273
  273 ÷ 7 = 39 remainder 0 → week 39 + 1 = week 40
  Week runs Monday 28 September to Sunday 4 October 2026
  ISO code: 2026-W40-2 (Tuesday is day 2)

Friday 1 January 2027. Its Thursday is 31 December 2026, so it is in week 53 of 2026. Week 1 of 2027 starts on Monday 4 January 2027.

Week 1 of 2026 runs from Monday 29 December 2025 to Sunday 4 January 2026, because it contains Thursday 1 January 2026.

52 or 53 Weeks?

Most years have 52 ISO weeks, but about 71 years in every 400 have 53. A year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks; the next such years are 2032 and 2037. Businesses that plan in weeks need to know this, because a 53-week year has an extra week of sales, wages and production.

ISO and US Week Numbers

The ISO system is the international standard and is used across Europe and much of the world, and by many software tools. In the United States and some other places, calendars and spreadsheets often use a different system: weeks start on Sunday, and week 1 is simply the week containing 1 January. The two systems usually agree for most of the year but can differ by a week near the start and end. For 29 September 2026 both give week 40, but for early January or late December they often do not.

The ISO Week Date Code

ISO 8601 also defines a compact week date: year, "W", week number, and day number, where Monday is 1 and Sunday is 7. So 2026-W40-2 means the Tuesday of week 40 of 2026. The year in this code is the ISO week-numbering year, which can differ from the calendar year for dates in late December and early January — 1 January 2027 is 2026-W53-5.

Week Numbers in Spreadsheets

Spreadsheets offer both systems, which is a common source of mismatches. In Excel, ISOWEEKNUM(date) returns the ISO week, while WEEKNUM(date) uses the Sunday-start, 1-January system by default and gives different answers near the new year; WEEKNUM with a return type of 21 matches ISO. Google Sheets has the same functions. If a report and a calendar disagree by a week, check which function produced each number.

Uses for Week Numbers

Manufacturing and logistics: production schedules, delivery windows and shipping plans. Retail: comparing sales for the same week in different years. Projects and agile teams: sprint planning by week. Education: term calendars and timetables in some countries. Payroll: weekly pay periods. Personal planning: diaries and planners that print week numbers.

Understanding Your Result

The headline gives the ISO week number, or the week's dates in reverse mode.

The ISO week line gives the week-numbering year and the ISO week code.

The dates line gives the Monday-to-Sunday range.

The US week line gives the Sunday-start, 1-January-based week number.

The weeks in year line says whether the year has 52 or 53 weeks.

When Should You Use This Calculator?

Use it to find the week number of a delivery, deadline or event.

Use it to find the dates of a week given as a number.

Use it to check whether a year has 53 weeks.

Use it to reconcile ISO and US week numbers in reports and calendars.

Common Mistakes

Assuming week 1 starts on 1 January. In ISO it starts on the Monday of the week containing 4 January.

Using the calendar year for early-January dates. They may belong to the previous week year.

Mixing ISO and US week numbers. Check which system is being used, especially in spreadsheets.

Forgetting 53-week years. They add an extra week to annual plans, budgets and year-on-year comparisons.