Week Calculator

This works in weeks rather than days. It gives the ISO week number for a date, which is the numbering used in payroll runs, manufacturing schedules, project plans and a great deal of European business correspondence, and it counts the weeks between two dates. The ISO rule is worth knowing because it is not the obvious one. Week 1 of a year is the week containing the first Thursday of January, equivalently the week containing 4 January, and weeks run Monday to Sunday. That means the first days of January can belong to the final week of the previous year, and the last days of December can belong to week 1 of the next. A year has either 52 or 53 ISO weeks. If someone refers to week 40 and you count forty times seven days from 1 January you will usually land in the wrong place, which is exactly the kind of error that produces a schedule that is a week out and nobody notices until it matters. The calculator also gives the weeks between two dates as a whole number and as a decimal, since a fortnightly pay cycle or a rental period usually needs the exact fraction rather than a rounded one.

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ISO week 1 is the week containing the first Thursday of January, and weeks run Monday to Sunday. Early January can fall in the last week of the previous year.