Week Number Calculator
Find the ISO week number and day of the year for any date.
Results update as you type.
About this calculator
A week number calculator returns the ISO 8601 week number for a date — the standard used across business, logistics and Europe, where weeks run Monday to Sunday and week 1 is the week containing the first Thursday of the year. It also gives the ISO week-numbering year and the day-of-year number.
Frequently asked questions
How is the ISO week number defined?
Under ISO 8601 weeks start on Monday and week 1 is the week that contains the first Thursday of the year — equivalently, the week containing 4 January. This is why early-January and late-December dates can belong to a neighbouring year.
Why does the ISO year sometimes differ from the calendar year?
Dates in the first days of January can fall in the last ISO week of the previous year, and late-December dates can fall in week 1 of the next year. The ISO week-numbering year reflects which year the week belongs to.
Which quarter is the date in?
The quarter is the calendar quarter of the ordinary date: Q1 is January–March, Q2 April–June, Q3 July–September and Q4 October–December.
Results are estimates for general guidance only, not financial, medical or tax advice.