ISO Week Numbers Explained

Under the ISO 8601 standard, weeks start on Monday and week 1 of any year is the week that contains that year’s first Thursday. An equivalent and easier test: week 1 is the week that contains January 4. Every ISO week has exactly seven days and belongs entirely to one week-numbering year, which is what makes the system reliable for payroll, manufacturing schedules, and databases.

The surprising consequence is that the first days of January do not always fall in week 1, and the last days of December do not always fall in the final week. Because a week is not allowed to be split across two numbering years, the calendar year and the ISO week-numbering year drift apart by a few days at each boundary. January 1 lands in week 1 only when it falls on Monday through Thursday; otherwise it belongs to the last week of the previous year.

The first-Thursday rule, and why Thursday

The rule sounds arbitrary until you notice what it guarantees: the year that owns a week is the year that holds the majority of that week’s days. A Monday-to-Sunday week has its midpoint on Thursday. So “the week containing the first Thursday” is precisely “the first week that has more days in the new year than the old one.” That single choice keeps week numbers stable and avoids stub weeks of one or two days.

Consider 2026. January 1, 2026 is a Thursday, so the week of Monday December 29, 2025 through Sunday January 4, 2026 contains that Thursday and becomes week 1 of 2026. December 29, 30, and 31 of 2025 therefore carry the label 2026-W01 even though they are calendar dates in 2025. Now compare 2028: January 1, 2028 is a Saturday. Its week (Monday December 27 through Sunday January 2) has its Thursday back in December, so that week is still week 52 of 2027, and January 1, 2028 is written 2027-W52.

Why some years have 53 weeks

A common year has 365 days, which is 52 weeks plus one day; a leap year has two spare days. Those leftover days accumulate until, occasionally, a year needs a 53rd week to cover them. The rule works out cleanly: a year has 53 ISO weeks when January 1 falls on a Thursday, or when January 1 falls on a Wednesday in a leap year.

2026 is the textbook case. Its January 1 is a Thursday, so it is a 53-week year, and December 31, 2026 sits in 2026-W53. The year 2020 shows the leap-year branch: January 1, 2020 was a Wednesday and 2020 was a leap year, giving it 53 weeks as well. Most years are ordinary 52-week years; the 53rd week appears only 71 times in every 400-year cycle, roughly once every five or six years.

December in next year, January in last year

Because a whole week must belong to one year, the boundary can spill in either direction, and it is worth seeing both:

• December belongs to the next year. December 30, 2024 is a Monday whose week contains January 1–2, 2025 and its Thursday in January, so it is 2025-W01. The date is in 2024; its week number is a 2025 week.
• January belongs to the previous year. January 1, 2021 was a Friday. Its week’s Thursday fell on December 31, 2020, so January 1, 2021 is written 2020-W53. Similarly January 1, 2022 is 2021-W52.

This is why serious systems record the full week-numbering year alongside the week number — 2025-W01, not a bare “week 1” — and never assume the week year matches the calendar year of the date.

Where the US convention differs

North American calendars usually do not follow ISO 8601 at all. The common US convention starts weeks on Sunday, and it defines week 1 as the week that contains January 1, no matter which weekday that is. Under that scheme January 1 is always in week 1, and the year always begins with a partial week rather than a clean Monday.

The two systems disagree on both axes — the first day of the week and which week gets the number 1 — so for the same date they frequently return different numbers. Neither is wrong; they answer different questions. The trouble starts when a spreadsheet built on one convention feeds a report that assumes the other.

Spreadsheet functions that quietly disagree

Spreadsheets ship both conventions, and the defaults are not the ISO ones. In Excel and Google Sheets, WEEKNUM defaults to the US style: weeks start Sunday and the week of January 1 is week 1. To get ISO numbering you need ISOWEEKNUM (or WEEKNUM with the return-type argument set to 21). The table shows how far apart they can land on a single date.

Before you trust a “week 1,” check which function produced it and which day the week started on. A mismatch of even one week throws off week-over-week comparisons and shift rosters.

Try it

Want the number for today without the arithmetic? See what ISO week number today is, or check what day of the week it is. If leap years are what brought you here, the sibling guide on leap year rules explains the extra days that give some years a 53rd week.