Week Number Calculator

Find the ISO-8601 week number of any date.

Week Number Calculator helps you iSO-8601 week number for any date. Enter date, review the breakdown, and use the guidance below to understand what the number means — and what it does not. What the Week number calculator does Week Number Calculator is built for dates, durations, ages, and schedules. ISO-8601 week number for any date. You enter date, then Simple Calculators computes the result instantly. Calendar math follows common civil-calendar rules used in everyday planning. How the math works Find the Thursday of the date’s Monday-to-Sunday week. Its year is the ISO year, and its day of the year tells you the week: week = whole part of (day of year − 1) ÷ 7, plus 1. The formula: Week = whole part of (day of year of that week’s Thursday − 1) ÷ 7, plus 1. Weeks run Monday through Sunday; week 1 is the week containing the year’s first Thursday.