What week of the year is it?
The calculator defaults to today, so the ISO 8601 week number above is the answer to “what week is it?”, and you can check any other date. Two numbering systems are in real use: ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the year (standard across Europe and in software), while the North American convention starts weeks on Sunday and calls the week containing January 1 week 1. Around New Year the two can differ by a week, which is why planners and spreadsheets so often disagree.
Common questions
- Why do different calendars show different week numbers?
- They use different systems. ISO 8601 puts the year's first Thursday in week 1 and starts weeks on Monday; the US convention starts weeks on Sunday and puts January 1 in week 1. Both are shown above so you can tell which one a given planner uses.
- Can a date's ISO week belong to a different year?
- Yes. January 1 can fall in week 52 or 53 of the previous ISO year, and December 29–31 can fall in week 1 of the next. That's what the separate “ISO week-numbering year” row is for. In code, formatting a date with the wrong year field (YYYY vs ISO week year) is a classic New Year bug.
- How many weeks are in a year?
- An ISO year has 52 weeks usually, and 53 when the year starts or ends on a Thursday (about every 5–6 years; 2026 has 53).
- How do I get the ISO week number in code?
- Python: date.isocalendar().week. PostgreSQL: EXTRACT(WEEK FROM date). Java: get(WeekFields.ISO.weekOfWeekBasedYear()). JavaScript has no built-in; compute the week's Thursday, or use a library.