Quick answer
Find the Monday of the week containing 4 January — that is week 1. Every other week is a fixed number of weeks from there, and the week-year can differ from the calendar year at both ends.
The rule in one sentence
ISO week 1 is the week containing the first Thursday of January — equivalently, the week containing 4 January. Weeks run Monday to Sunday, and every week belongs entirely to one week-year. Those three rules determine every ISO week date without exception.
The consequence people find surprising is that week 1 often starts in December. If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous week-year, and week 1 begins on the following Monday. If it falls Monday to Thursday, week 1 has already started in late December.
Use this guide with the right tool: Open the ISO Week to Date Converter for the calendar dates covered by a given ISO-8601 week number and week-year. If the question shifts, compare it with the Week Number Calculator or the Weekday Calculator.
Week-year is not calendar year
This is the single largest source of ISO week bugs. A date in early January can carry a week-year of the previous year, and a date in late December can carry the next. Writing "week 1 of 2026" is ambiguous unless you mean the ISO week-year — and reporting tools frequently pair an ISO week number with a calendar year, which produces off-by-one-year errors once a year, every year.
The ISO designation format exists to remove the ambiguity: 2026-W33-7 means week-year 2026, week 33, day 7 (Sunday). If a system stores week and year as separate integers, make sure the year field is the week-year and label it as such.
53-week years
Most years contain 52 ISO weeks; some contain 53. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Roughly 71 years in every 400 have 53 weeks.
Systems that assume 52 weeks break on those years — usually by dropping the final week of data or by wrapping it into the next year. If you build a rota, a payroll cycle or a reporting calendar on week numbers, test it explicitly against a 53-week year before it goes live.
Where ISO weeks are worth using
ISO weeks are excellent for anything that repeats weekly and needs to be compared year over year: manufacturing schedules, retail reporting, project sprints, academic timetables and rota planning. Because every week has exactly seven days and always starts on the same weekday, week-to-week comparisons are clean in a way that month-to-month comparisons never are.
They are a poor fit for anything anchored to a calendar month or to a personal date, because the two grids do not align. Do not use ISO weeks to describe a person's age, a monthly billing cycle or a legal deadline defined in calendar days.
Worked example
Swap in your own figures and the method carries over unchanged. What does not carry over is the source of authority, which is specific to your situation.
Common mistakes to avoid
- Pairing an ISO week number with the calendar year. Use the ISO week-year. They differ at the start and end of most years, which creates an annual off-by-one error.
- Assuming every year has 52 weeks. Some have 53. Test any week-based schedule against one before relying on it.
- Assuming weeks start on Sunday. ISO weeks start on Monday. US-style week numbering starts on Sunday and uses a different week-1 rule, so the two systems disagree.
Use the right calculator
Pick the one whose inputs match the information you actually have. Each page explains what its result does and does not cover.
Frequently asked questions
Why 4 January and not 1 January?
Because 4 January is always in week 1 under the first-Thursday rule, which makes it a reliable anchor to calculate from.
Do all countries use ISO weeks?
No. ISO numbering is standard in Europe and in most technical systems; North American business calendars often use a Sunday-start convention with a different week-1 definition.
What is week 0?
There is no week 0 in ISO-8601. Dates before week 1 belong to the final week — 52 or 53 — of the previous week-year.