Mathematics · calendars

Week Number Calculator

ISO 8601 week numbers, what dates a week covers, and the years with 53 of them.
ISO 8601 · Mon–Sun
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The long years

Which years have 53 weeks

payroll, retail and rota planning

An ISO year has 53 weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year — because week 1 is defined as the week containing the year's first Thursday, and those are the two ways to fit an extra one in. Everything else has 52.

Field notes

Where week numbers go wrong

How it works

One rule, and everything follows

ISO 8601 defines week 1 as the week containing the year's first Thursday — equivalently, the week containing 4 January, and equivalently again, the first week with most of its days in the new year. Weeks run Monday to Sunday. Every other property follows from that single rule, including the awkward ones.

Why a date's week-year is not always its calendar year

A week belongs entirely to one year, so the days at each end have to go somewhere. 29–31 December can sit in week 1 of the following year, and 1–3 January can sit in week 52 or 53 of the previous one. That is not an error — it is the rule working. It does mean "week 1 of 2027" and "the first week of January 2027" are different sets of days, which is where most calendar bugs start.

This is why ISO dates pair a week with its own year — 2026-W01, not just "week 1". Storing the week without the ISO year loses information for six days a year, every year.

Why does my spreadsheet give a different week number?

Because it is probably not using ISO. Excel's WEEKNUM defaults to weeks starting Sunday with week 1 being whichever week contains 1 January — a different rule that disagrees with ISO for several weeks most years. ISOWEEKNUM is the one that matches this tool.

Which years have 53 weeks?

Those where 1 January is a Thursday, or a Wednesday in a leap year. They arrive roughly every five to six years and matter more than they sound: weekly payroll gets a 53rd pay run, and retailers running 52-week financial calendars have to absorb an extra trading week.

Do all countries number weeks this way?

No. ISO 8601 is standard across Europe and much of business, but the US, Canada and parts of Asia commonly start weeks on Sunday and number from the week containing 1 January. Both are shown above, because agreeing which convention you mean is most of the work.

What is week 0?

Not an ISO concept. Some systems emit "week 0" for early-January days that ISO assigns to the previous year's final week — a symptom of implementing the numbering without the year pairing.

ISO 8601 throughout, with the US Sunday-start convention shown alongside for comparison. Dates are handled in UTC to avoid daylight-saving shifts moving a date across a week boundary.