What week is it?

Field notes

Whose week numbers are these?

ISO 8601 — the standard used across Europe and in most planning software. Weeks start on Monday, and week 1 is the week containing the year’s first Thursday (equivalently, January 4). That’s why January 1 can land in week 52 or 53 of the previous year.

Why do some years have 53 weeks?

A year has 53 ISO weeks when it starts on a Thursday — or on a Wednesday in a leap year. It happens roughly every 5–6 years; the tool shows the count for any year you pick.

Where does this run?

Entirely in your browser. Dates you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.