Week Number Calculator

A week number tells you which week of the year a date falls in — from 1 up to 52 or 53. This calculator uses the ISO 8601 standard, where weeks start on Monday and week 1 is the week containing the year's first Thursday. Week numbers are widely used in business, manufacturing, and project planning, especially in Europe.

Under ISO 8601, week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4). Weeks run Monday to Sunday. This means the first few days of January can belong to the last week of the previous year, and late December can fall in week 1 of the next year.

Most years have 52 weeks, but a year has 53 ISO weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year. This happens roughly every 5–6 years — 2026, for example, is a 53-week year in the ISO system.

ISO weeks start on Monday and use the first-Thursday rule. The common US convention starts weeks on Sunday and often counts week 1 as the week containing January 1. This calculator reports the ISO number, the international standard used in business and software.

Pick today's date and the calculator shows the current ISO week number, how many weeks remain in the year, the day of the year, and the current quarter — a quick snapshot of where you are in the calendar.