Week Number Calculator — ISO and US Week Values
Find ISO and US week numbers for a date, including the Monday-to-Sunday span of its ISO week.
Loading the tool… JavaScript is required for processing on your device. The instructions and examples below are available without JavaScript.
How to use it
Pick a date (today is filled in) to see its ISO week number and the week’s dates.
Good to know
ISO weeks start on Monday and week 1 contains the year’s first Thursday. US weeks start on Sunday with week 1 containing 1 January.
Identify a reporting week
Enter a date to see two common numbering systems. For 2027-01-01, the tool reports Week 53 of 2026 (ISO); that ISO week begins on 2026-12-28 and ends on 2027-01-03, while the US week number is 1.
Agree on the convention first
ISO weeks start on Monday and week 1 is the week containing the year’s first Thursday. The US count shown starts weeks on Sunday and puts 1 January in week 1. Businesses and countries can define fiscal or operational weeks differently, so confirm the required convention before labelling a report or deadline.
Frequently asked questions
What week number is it today?
Today’s date is filled in automatically. Run the tool to see the current ISO and US week numbers.
Why do ISO and US week numbers differ?
ISO weeks start on Monday and the first week must contain a Thursday. The US system starts weeks on Sunday and week 1 always contains 1 January.
Can a year have 53 weeks?
Yes. ISO years have 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year.