📆 Week Number Calculator

How the Week Number Calculator Works

Week numbers are counted differently around the world. Most of Europe and Asia — and most business and project-planning software — use the ISO 8601 standard: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. That means the first days of January can belong to week 52 or 53 of the previous year.

The US system starts weeks on Sunday and counts the week containing January 1 as week 1. This calculator shows both, together with the day of the year and how many days are left in the year. You can also go the other way: enter a year and an ISO week number to get the Monday-to-Sunday dates of that week.

Formula

ISO week: find the Thursday of the date's week (Monday-based);
          ISO year = year of that Thursday;
          ISO week = 1 + floor((day-of-year of that Thursday − 1) / 7)
US week  = floor((day-of-year − 1 + weekday of Jan 1 [Sun=0]) / 7) + 1

How to Use It

1. Pick a date to see its ISO and US week numbers.
2. Or enter a year and an ISO week number in the second section.
3. Read the Monday–Sunday dates that belong to that week.

Frequently Asked Questions

What week number is it according to ISO 8601?

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4). Pick any date in this calculator to see its ISO week number instantly.

How many weeks are in a year?

An ISO year has either 52 or 53 weeks. It has 53 weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year.

Why do the ISO and US week numbers sometimes differ?

The two systems start weeks on different days (Monday vs. Sunday) and define week 1 differently, so the same date can be in a different week — and around New Year even in a different year — depending on the system.

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Formula and content last reviewed: September 2026. All calculations run in your browser — nothing you enter is sent to or stored on our servers.