Two conventions, and they disagree most years
ISO 8601 — used across Europe, in most project management and manufacturing systems, and in almost all software — says weeks start on Monday, and week 1 is the week containing the first Thursday of January. Equivalently, week 1 is the week containing 4 January.
The US convention — used by broadcast, retail and many US payroll systems — says weeks start on Sunday, and week 1 is the week containing 1 January.
These give different answers for most dates in early January, and they disagree by one for the rest of the year in many years. This page shows both, always, because "week 32" without a convention is not a well-defined date.
Where the ISO rule looks strange
Because week 1 must contain the first Thursday, the first days of January can belong to the last week of the previous year. 1 January 2027 is a Friday, so it falls in ISO week 53 of 2026, not week 1 of 2027. Under the US convention it is week 1 of 2027. Both are correct under their own rule and neither is a bug.
The reverse also happens: late December can belong to week 1 of the following year. 31 December 2029 is a Monday and falls in ISO week 1 of 2030.
Years with 53 weeks
Most years have 52 ISO weeks. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 has 53. This is why annual figures divided by 52 drift, and why retail calendars have a 53-week year every five or six years.
Worked example
18 August 2026 is a Tuesday. Under ISO 8601 it is in week 34 of 2026. Under the US Sunday-start convention it is in week 34 as well — they happen to agree here. Check 4 January 2027: ISO week 1 of 2027, US week 2 of 2027. Same day, two answers, both defensible.
Which one should you use?
If a system generated the number, match that system. Software, ERP and European business use ISO. US broadcast and retail calendars use the Sunday-start rule. If you are choosing for yourself, use ISO — it is a published standard and it never gives a year two week 1s.
Which years have 53 ISO weeks, and when
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Between 2024 and 2031 only 2026 qualifies: it starts on a Thursday. 2024, 2025, 2027, 2028, 2029, 2030 and 2031 all have 52. The pattern repeats on a 400-year cycle and works out at 71 long years per 400, so a little over one year in six.
This is the reason annual figures divided by 52 drift, and the reason retail and payroll calendars carry a 53-week year every five or six years — a 52-week year is 364 days, one or two short of a calendar year, and the shortfall has to be absorbed somewhere. If you are comparing this year's week 34 with last year's week 34, check first whether a 53-week year sits between them; if it does, you are comparing two different parts of the season.
Worked examples at the year boundary
The boundary is the only place the two systems dramatically part company, so here it is four times over:
- 1 January 2026 is a Thursday. ISO week 1 of 2026; US week 1 of 2026. They agree, because the year starts on the day the ISO rule pivots on.
- 1 January 2027 is a Friday. ISO week 53 of 2026; US week 1 of 2027. A date in January whose ISO year is the previous year.
- 4 January 2027 is a Monday. ISO week 1 of 2027; US week 2 of 2027. The systems are now a full week apart on the same date.
- 31 December 2029 is a Monday. ISO week 1 of 2030; US week 53 of 2029. A date in December whose ISO year is the following year.
Every one of those is correct under its own rule. The failure mode is not picking the wrong one, it is quoting a week number without saying which, at which point the number carries about six days of uncertainty.
Where week numbering goes wrong in practice
- Spreadsheets. The common spreadsheet week function defaults to a Sunday-start, 1-January rule — the US convention — and needs an explicit argument to produce ISO weeks. A column that looks right in January and drifts later is almost always this.
- The ISO year is not the calendar year. Storing a week number without its ISO year loses information at the boundary. Week 1 of 2027 begins on 4 January 2027; week 53 of 2026 contains 1 January 2027.
- Subtracting week numbers. Only valid inside one ISO year. For the number of weeks between two dates, use weeks between dates instead.
- Broadcast calendars. US broadcast weeks run Monday to Sunday but number months rather than the year, which is a third system again and does not match either of the two on this page.
Which one to use
If a system produced the number, match that system — the question is never which convention is better, it is which one generated the figure in front of you. Software, ERP, manufacturing scheduling and most of Europe use ISO. US broadcast, retail and many payroll calendars use the Sunday-start rule. If the choice is genuinely yours, use ISO: it is a published standard, every week has exactly seven days, and no year ever contains two week 1s. Related counts: how many weeks in 2026, weeks left in 2026, and the day-of-year number, which is the other common way of naming a position in the year. The full ISO rule is on the methodology page.