ISO 8601 weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday — the same as the week containing 4 January. Not simply the week containing 1 January.
A week belongs entirely to the year holding its Thursday, so 29–31 December can fall in week 1 of the next year and 1–3 January in week 52 or 53 of the last. That is why ISO pairs a week with its own year (2026-W01): six days a year belong to a different week-year than calendar year, and storing the number alone loses them. It also explains the long years — 53 weeks whenever 1 January is a Thursday, or a Wednesday in a leap year, which is when weekly payroll gets a 53rd run.
Estimate with the rule, then check it against the calculator that models it properly.
Open Week Number Calculator →ISO 8601 weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday — the same as the week containing 4 January. Not simply the week containing 1 January. A week belongs entirely to the year holding its Thursday, so 29–31 December can fall in week 1 of the next year and 1–3 January in week 52 or 53 of the last.