What week is it? Week number calculator

The ISO standard, used across most of Europe and in most business software, starts weeks on Monday. Week 1 is the week that holds the year's first Thursday. So 1 January can belong to week 52 or 53 of the year before, and 29 December can belong to week 1 of the next.

The US convention starts weeks on Sunday and makes the week holding 1 January week 1, however few days of it fall in the new year. The two numbers often differ by one, and around New Year they can differ by more. If someone gives you a week number, ask which system. Usually it is ISO.

A year is 52 weeks and one day, or two in a leap year. Those extra days pile up, and every five or six years the ISO calendar needs a 53rd week to catch up. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. Payroll and planning that assume 52 weeks break in those years, which is a good reason to check.

The week number is worked out from your device's date, not a server's. Someone in Auckland is in a different day, and sometimes a different week, from someone in Hawaii. For a moment on first load the page is blank, while it reads your clock. That is the cost of getting Sunday night right.