About the Week Number Calculator
Many businesses and planning systems refer to weeks by number, and the ISO 8601 standard defines exactly which week is week 1 (the one containing the year's first Thursday) and treats Monday as the first day. This tool gives the ISO week number and the day of the year for any date, which is not something most calendars show. Convert several at once, one per line, and use "now" for today.
The rules produce results that look wrong until you know them. A year holds 52 or 53 ISO weeks, so 31 December can belong to week 1 of the following year and 1 January can belong to week 52 or 53 of the previous one. Because week 1 is the week containing the first Thursday, it is also the first week with at least four of its days in the new year. The real trap is that other conventions exist: the common North American scheme starts weeks on Sunday and counts 1 January into week 1, so it disagrees with ISO by one for much of the year. A report that mixes the two produces an off-by-one that is very hard to spot.
It is useful for reconciling a "week 36" reference in a planning tool with an actual calendar date, for labelling sprint or manufacturing weeks, and for filing anything against a week-numbered schedule. For the full breakdown of a date, use the Unix Timestamp Converter; to see the month around the week you found, the Monthly Calendar Generator.
How to use
- Paste dates, one per line, or type "now".
- Read the ISO week number and day of year.
Common questions
- Which week is week 1?
- Under ISO 8601, the week containing the year's first Thursday, with Monday as the first day.
- Why might a January date be in week 52 or 53?
- Early January days can belong to the last ISO week of the previous year, by the standard's rules.
- What is the day of the year?
- The count of days from 1 January, so 5 September is day 248 (in a non-leap year).