Which week of the year is it?
Week numbers run payroll cycles, school terms, sprint planning, manufacturing schedules and half of Europe's diary system — but "week 42" only means something if everyone counts the same way. This calculator uses the international standard, ISO 8601, the system behind most printed calendars and planning software. Under ISO rules, weeks begin on Monday, and week 1 of a year is the first week with a Thursday in it (equivalently, the week containing January 4). A year has 52 or 53 ISO weeks — 2026, whose January 1 is a Thursday, has 53.
The ISO 8601 rules
Two consequences surprise people. First, the ISO year does not always match the calendar year printed on the date: December 29, 2025 already belongs to ISO week 1 of 2026, and January 1, 2027 still belongs to ISO week 53 of 2026. This calculator always reports the ISO year alongside the week number, because "week 1" alone is ambiguous at the boundary. Second, a 53rd week exists in roughly 71 years out of 400 — years starting on a Thursday, plus leap years starting on a Wednesday.
ISO weeks vs the US convention
Some US calendars number weeks from Sunday and call the week containing January 1 "week 1" even if it is mostly December. That system can disagree with ISO by one week for much of the year. If a colleague in Europe quotes a week number, it is ISO; if a US wall calendar shows one, check its convention. This tool follows ISO 8601 throughout, and also shows the week's exact Monday and Sunday dates so you can pin the week down unambiguously. The Date Pattern Calculator builds on this to find dates like "the third Monday of the month".
Worked example
October 12, 2026 falls in ISO week 42 of 2026, running Monday October 12 to Sunday October 18 — and 2026 has 53 ISO weeks in total.
How to use this calculator
- Pick any date — past or future.
- Read its ISO week number in the green panel.
- Check the table for the week's Monday and Sunday, the ISO year, and the day of the year.
Writing week dates
ISO also defines a compact week-date format: 2026-W42-1 means ISO year 2026, week 42, day 1 (Monday). Software logs, delivery schedules and European business documents use it heavily. With this calculator's ISO year, week and weekday, you can read or write any week date confidently.