Week Number Calculator

What week is it?

Enter a date - or use today, which is filled in for you - and this gives the week number, the days the week runs between, and how many weeks are left in the year.

ISO 8601, and why it sometimes surprises people

The international standard defines a week as running Monday to Sunday, and week 1 as the week containing the year's first Thursday. Equivalently: week 1 is the week containing 4 January. Every week belongs entirely to one year.

Two consequences catch people out, and neither is a bug:

• Early January can belong to the previous year. If 1 January is a Friday, Saturday or Sunday, those days are in the last week of the year before - week 52 or 53.
• Late December can belong to the next year. If 31 December is a Monday, Tuesday or Wednesday, it is already in week 1 of the coming year.

This is deliberate. It means a week is never split across two years, which is precisely what makes weekly reporting comparable.

53-week years

Most years have 52 weeks. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That works out at roughly 71 years in every 400. If your planning assumes 52, it will be wrong about once every seven years.

ISO and US numbering are not the same

The United States commonly numbers weeks from Sunday, with week 1 being the week containing 1 January. So the two systems disagree on where a week starts and on which week is first, and around the turn of the year they can differ by a full week. Both are shown here, labelled, because a number without its convention is not an answer.

Where week numbers are used

Manufacturing and logistics schedule by week across Europe. Payroll and rota systems run on them. Retail and finance report weekly so that like is compared with like. In Sweden, Germany and much of northern Europe, people use week numbers in ordinary conversation - "let us meet in week 34" is a normal sentence.

Frequently asked questions

What week number is it today?

Shown at the top of the page on arrival, with the Monday-to-Sunday dates that week covers.

How many weeks are in a year?

52 in most years, 53 in a year that begins on a Thursday or is a leap year beginning on a Wednesday.

Why does my calendar show a different week number?

Almost always because it uses the US convention while you are expecting ISO, or the reverse. Compare both figures on this page and the difference will explain itself.

Does the week start on Monday or Sunday?

Monday under ISO 8601, which is the standard across Europe and most of the world. Sunday under the common US convention. Both are shown.

Related date tools

See days between dates, the date calculator, business days, and the meeting time planner.