Week Number Calculator
Find the ISO 8601 week number for any date — plus the day of the year, the calendar quarter, and how many days are left in the year. Defaults to today.
How ISO week numbers work
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year’s first Thursday (equivalently, the week containing 4 January). This means early-January or late-December dates can belong to a neighbouring year’s week — the calculator handles that correctly.
Alongside the week number you get the ordinal day of the year, the quarter, and the number of days remaining until 31 December.
Frequently asked questions
What is an ISO week number?
It is the week of the year under ISO 8601, where weeks run Monday to Sunday and week 1 contains the first Thursday of the year. Values run from 1 to 52 or 53.
Why might week 1 include days from December?
Because week 1 is anchored to the first Thursday, the last days of December can fall in week 1 of the next year, and early January can fall in the last week of the previous year.
How many weeks are in a year?
Most years have 52 ISO weeks; some have 53. The calculator returns the correct number for the date you enter.