How ISO week numbers work
This calculator follows ISO 8601, the international standard used across Europe and in most business software. ISO weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of January, which is also the first week with at least four days in the new year. A consequence of that rule is that 1 January does not always fall in week 1: if it lands on a Friday, Saturday, or Sunday, it belongs to the last week of the previous week-year. Some US calendars use a different convention, starting weeks on Sunday and calling the week containing 1 January week 1, so their numbers can differ from ISO by one.
Most ISO years contain exactly 52 weeks, but 52 weeks is only 364 days, so the odd day or two has to accumulate somewhere. Any year that begins on a Thursday, plus any leap year that begins on a Wednesday, picks up a 53rd week. This happens five or six times every 28 years. The calculator reports the total for your date's week-year and how many whole weeks remain after the current one, which is handy for production schedules and weekly planning.
Frequently asked questions
Why does my planner show a different week number?
This calculator uses ISO 8601: weeks start on Monday and week 1 contains the first Thursday of January. Many US calendars start weeks on Sunday and call the week containing 1 January week 1, so the two systems can differ by one, especially near the turn of the year.
What is a week-year and why can it differ from the calendar year?
The week-year is the year that owns the ISO week a date belongs to. Early January dates can still sit in week 52 or 53 of the previous week-year, and late December dates can already be in week 1 of the next, so the week-year can differ from the calendar year by one at both ends.
How many weeks does this year have?
52 in most years, 53 in years that start on a Thursday and leap years that start on a Wednesday. The calculator shows the total for the week-year of your chosen date.