ISO 8601 week
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Week Number Calculator
Which week of the year are we in? Any date, any year — ISO 8601 (international) and US convention side by side.
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US week
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Weeks remaining
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ISO week count this year
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ISO 8601 vs US
ISO 8601 (international standard): weeks start Monday, week 1 contains January 4 (equivalently, has the first Thursday). This means week 1 can begin in late December and week 52/53 can spill into early January. US convention: weeks start Sunday, week 1 always contains Jan 1.
Why it matters
International business, project management (weekly sprints, OKRs), and healthcare (epidemiological tracking) all use ISO 8601. Saying "week 38" can mean different things to different nationalities — this calculator shows both so you always know which is which.
Frequently asked questions
Why does ISO week 1 start in December?
ISO week 1 = week containing January 4. If Jan 1 is Fri/Sat/Sun, the first 3–6 days belong to the last week of the previous year.
Why 53 weeks some years?
365÷7=52.14, so most years = 52 weeks. A year starting on Thursday (or Wed in leap year) gets 53 weeks. Last: 2020. Next: 2026.
Which to use?
ISO 8601 for anything international. US numbering only when audience is exclusively US. In doubt: "ISO week X of YYYY".
Fiscal weeks?
Retailers use 4-5-4 calendars for financial reporting — different from both ISO and US. Not covered here.
Christmas week number?
Dec 25 is almost always ISO week 52 or 53. Enter Dec 25 of any year to see.
Last updated
For general calculations only. Not financial, tax, or legal advice — see our disclaimer.