Week Number Calculator
Find the week number of the year for any date.
Which week of the year
The weeks of a year are sometimes numbered from one to fifty-two or fifty-three, a system used in business, planning and scheduling. This finds the week number for any date, useful when a schedule or report refers to weeks by number.
Enter a date to find its week number.
The surprisingly fiddly definition
Week numbering sounds simple but has a genuine subtlety: there is more than one convention for which week counts as week one. The widely-used international standard defines week one as the first week containing a majority of its days in the new year, or equivalently the week with the year's first Thursday, and weeks start on Monday — which means early January can fall in the last week of the previous year, and a year can have fifty-three weeks. Other systems, particularly in some regions, define it differently, starting weeks on Sunday or counting week one as the one containing the first of January. This is why two week-number systems can disagree by one around the turn of the year, so it is worth knowing which convention a given schedule uses.
Questions & answers
How is week one defined?
The common international standard makes it the week containing the year's first Thursday, with weeks starting Monday — so early January can fall in the previous year's last week.
Why do week numbers sometimes disagree?
Because there is more than one convention — some start weeks on Sunday or count week one differently — so systems can differ by one around the new year.
Is anything uploaded?
No. The calculation happens in your browser.