How this calculator works
ISO 8601 defines a week as Monday to Sunday, and week 1 as the week that contains the first Thursday of January. A date near New Year can therefore belong to week 52 or 53 of the previous ISO year, or to week 1 of the next one. The ISO week year is the year that owns the week, which is not always the calendar year of the date itself.
Formula
Let Q be the day number of the date and W its ISO weekday, where Monday is 1. Thursday of the same week is Q + 4 - W. The ISO week year is the calendar year of that Thursday, and the week number is floor((Q + 4 - W - O(January 1 of the ISO year)) / 7) + 1. Monday is Q - (W - 1).
Worked example
2026-11-01 is a Sunday. Its ISO week runs from Monday 2026-10-26 to Sunday 2026-11-01, in ISO week 44 of week year 2026. 2027-01-01 is a Friday and belongs to ISO week 53 of 2026, not to week 1 of 2027.
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Frequently asked questions
Why can the ISO year differ from the calendar year?
A week can straddle New Year. The ISO year follows the week, so the first days of January can belong to the previous year's last week, and the last days of December can belong to the next year's first week.
Why start weeks on Monday?
ISO 8601 uses Monday as the first day of the week for international business, payroll and scheduling. Canadian calendars often display Sunday first, but the ISO number is unaffected by that display choice.
What is week 53?
A year can have 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. 2026 is such a year. Week 53 is a real week and not an error.
Does this use local time?
No. The input is a civil date with no time or time zone, so no local-time rounding can move it to a neighbouring day.