What is the difference between the ISO and US week number?

ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the year. The US convention starts weeks on Sunday and counts the week containing January 1 as week 1, so the two numbers can differ near the new year.

Why does the ISO year sometimes differ from the calendar year?

Early January dates can belong to the last ISO week of the previous year, and late December dates can belong to week 1 of the next year. The tool shows the correct ISO year alongside the week number.

Is my date uploaded anywhere?

No — the week number is computed entirely in your browser and nothing is ever uploaded.

Week Number Calculator

ISO 8601 and US week number for any date.

Week number calculator

Find out which week of the year any date falls in. The calculator shows the ISO 8601 week number (the European standard, weeks starting Monday) and ISO year, plus the US week number, the day of the year and the weekday for reference.

Everything runs entirely in your browser — nothing is uploaded.