Week Number Calculator
The ISO week number of any date, with the US convention and day-of-year alongside.
How ISO week numbers work
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week of January 4). That means January 1 can fall in week 52 or 53 of the previous ISO year, and a year has 53 weeks when it starts on a Thursday (or a Wednesday in a leap year). Scandinavia, Germany and most European planners run on these numbers.
Why your calendar might disagree
The US convention starts weeks on Sunday and calls the week containing January 1 week 1, so it can differ from the ISO number by one either way. Outlook and Google Calendar both default to the local convention and can be switched to ISO; if two people quote different week numbers for the same date, this is almost always why.