How ISO week numbers work
The ISO 8601 standard, used across the UK and Europe for payroll, production planning and diaries, numbers the weeks of the year with three simple rules:
- Weeks run Monday to Sunday.
- Week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January).
- Every date belongs to exactly one week, so a year has either 52 or 53 whole weeks, never a part week.
The Thursday rule sounds odd until you see what it does: it hands each week to whichever year holds most of it. A week split across New Year has at least four of its seven days, including the Thursday, in the year it gets numbered under. That is why the ISO year shown above can differ from the calendar year for dates between 29 December and 3 January.
In everyday UK usage you will more often hear "week commencing" with a Monday date than a bare number, and the tool gives you both: the number for systems that want it, and the Monday and Sunday for humans who want to know which actual week is meant.
Worked examples
ISO weeks vs the US system
If a colleague's spreadsheet or an American planner disagrees with the number here, it is almost certainly using the US convention: week 1 is the week containing 1 January, and weeks start on Sunday. Under that system the count can run one higher than ISO for much of the year, and "week 1" can be a stub of just a day or two. Excel's plain WEEKNUM defaults to the US style; ISOWEEKNUM matches this tool.
Week numbers matter more than they look. Manufacturing and retail plan stock by week, freight quotes lead times as "wk 36 delivery", schools and universities timetable by teaching week, and plenty of payrolls run on week numbers rather than dates. When two systems disagree by one, orders land seven days late. If a week number arrives without context, ask which convention it uses, or just look the date up here and compare.
52 weeks or 53?
A common year of 365 days is 52 weeks and one day, so most years hold 52 numbered weeks and the spare days get absorbed by the neighbouring years' weeks. But when a year starts on a Thursday (or a leap year starts on a Wednesday), it holds 53 Thursdays, and therefore 53 ISO weeks. 2026 starts on Thursday 1 January, so it is a 53-week year: week 53 runs from Monday 28 December 2026 to Sunday 3 January 2027. If you plan anything by week number across that boundary, quote the ISO year with it ("week 53 of 2026") to keep everyone on the same page.
Frequently asked questions
Why does Excel give me a different week number?
Excel's WEEKNUM function defaults to the US system, where week 1 is whatever week contains 1 January and weeks start on Sunday. Use ISOWEEKNUM (or WEEKNUM with return type 21) to match this tool and the ISO 8601 standard used across the UK and Europe.
How many weeks are there in 2026?
2026 has 53 ISO weeks, which is the rarer case. A year gets 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 starts on Thursday 1 January, so week 53 runs right up to Sunday 3 January 2027.
Can 1 January be in week 52 or 53?
Yes. ISO week 1 is the week containing the year's first Thursday, so up to three early January days can belong to the last week of the previous year. Friday 1 January 2027, for example, sits in week 53 of 2026.
What is the ISO year and why does it matter?
It is the year the week belongs to, which can differ from the calendar year for dates between 29 December and 3 January. Payroll and planning systems that number weeks need it, otherwise week 1 versus week 53 becomes ambiguous at the year boundary.
Does the week start on Monday or Sunday?
In ISO 8601, and in normal UK usage, the week runs Monday to Sunday, and that is what this tool shows. American calendars often start the week on Sunday, which is one reason week numbers from US software can disagree by one.