Which week of the year is it?

ISO 8601 vs US week numbering

The two systems give different answers for the same date and there is no single correct one. ISO 8601 starts the week on Monday and defines week 1 as the week containing the first Thursday of the year, which is the same as saying week 1 contains January 4. Most of Europe, the UK, and a lot of international business use this. The US convention starts the week on Sunday and counts week 1 as the week containing January 1, which means the first week of the year can be very short. Both are useful; pick the one your spreadsheet or HR system expects.

When week numbers matter

Manufacturing schedules, retail buying calendars, agile sprint planning, magazine publishing, and payroll systems all key off week numbers. If your team uses "see you in W43" or "this ships in W22", you need a quick lookup to avoid arguments about Monday-vs-Sunday counting. Some ERP systems use a custom "4-5-4" retail calendar that does not match ISO or US conventions at all; if you need that, drop it into the days between dates calculator to convert into raw date ranges instead.

Edge cases at year boundaries

ISO weeks can spill across a calendar year. For example, January 1 in some years belongs to W52 or W53 of the previous year, and December 31 can belong to W1 of the next year. This calculator follows the standard faithfully, so the year shown for an ISO week is the ISO week year, which occasionally differs from the calendar year of the same date. You can also look up other date-related questions with the what day was it calculator.