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Week Number Calculator

Find the ISO 8601 week number of any date, its Monday-to-Sunday range and the US week number, or turn a year and week number into dates. Includes a table of every week of the year.

Week of a date
Any date from the year 1 to 9999.

Dates of a week
From 1 to 52, or 53 in long years.

Read the guide How Week Numbers Work: ISO Weeks, US Weeks and Year Boundaries

How to use the calculator

  • Week of a date: the date starts on today. Pick another date and press Find the week to see its ISO week number, the week-year, the Monday and Sunday of that week, the number of ISO weeks in that year and the US-style week number. Today jumps back to the current date.
  • Dates of a week: enter a year and an ISO week number and press Find the dates to get the Monday and Sunday of that week.

Below the calculators, a table lists every ISO week of the selected year with its start and end dates, and the chosen week is highlighted.

How ISO 8601 week numbers work

The international standard ISO 8601 uses these rules:

  1. Weeks start on Monday and end on Sunday.
  2. Week 1 is the week that contains the first Thursday of the year. Put differently, it is the week that contains January 4.
  3. Each week belongs entirely to one year, the week-year: the year in which its Thursday falls.

Because of rule 3, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, Monday, December 29, 2025 is the first day of week 1 of 2026, written 2026-W01. In the same way, Sunday, January 3, 2021 still belongs to 2020-W53.

52 or 53 weeks?

Most years have 52 ISO weeks. A year has 53 when January 1 falls on a Thursday, or on a Wednesday in a leap year. Between 2000 and 2040 the long years are 2004, 2009, 2015, 2020, 2026, 2032 and 2037. December 28 is always in the last week of its year, which gives a quick way to check.

Worked example

Take Monday, October 19, 2026. Its Thursday is October 22, the 295th day of 2026. Days 1 to 7 form week 1, so day 295 is in week (295 − 1) ÷ 7 = 42, rounded down, plus 1 = week 43. The week runs from Monday, October 19 to Sunday, October 25, 2026, and 2026 has 53 ISO weeks because it starts on a Thursday.

The US week number

In the United States and some other countries, weeks are often counted from Sunday, and week 1 is simply the week that contains January 1, even if that week has only one day in the new year. In this system a year can have 53 or even 54 partial weeks. October 19, 2026 is in US week 43, which runs from Sunday, October 18 to Saturday, October 24.

Good to know

  • When a calendar, payroll system or project plan gives a week number, check which system it uses: ISO and US numbers often differ by one around the start of the year.
  • The ISO date notation 2026-W43-1 means week 43 of 2026, day 1 (Monday).
  • Dates from the year 1 to 9999 are supported, using the Gregorian calendar throughout.

Frequently asked questions

What week number is it today?

Open the calculator: the date field starts on today's date in your device's time zone and the result shows the current ISO week number, the week-year and the US week number.

Why does January 1 sometimes belong to week 52 or 53?

In ISO 8601 a week belongs to the year that contains its Thursday. If January 1 falls on a Friday, Saturday or Sunday, the Thursday of that week is in December, so those first days count as the last week of the previous year.

Which years have 53 weeks?

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Recent and upcoming examples are 2015, 2020, 2026, 2032 and 2037.

What is the difference between ISO and US week numbers?

ISO weeks start on Monday and week 1 contains the first Thursday of the year. US-style weeks start on Sunday and week 1 is the week that contains January 1. The two numbers are often equal but can differ by one, especially near the turn of the year.