What Week of the Year Is It

How ISO week numbers are counted

An ISO 8601 week runs from Monday to Sunday. Week 1 is the week that contains the first Thursday of January — which is the same as the week that contains 4 January. Every week belongs to the year its Thursday falls in, so a year has 52 or 53 numbered weeks, and the first and the last of them can reach a few days into the neighbouring years.

Step by step

1. Find the Monday-to-Sunday week your date is in and take its Thursday.
2. The year of that Thursday is the week's ISO year — usually the calendar year, but not always in the last days of December and the first days of January.
3. Take the Thursday's day number in that year, subtract 1, divide by 7 dropping the remainder, and add 1.

Week numbers by year

Between 2020 and 2040 the years with 53 ISO weeks are 2020, 2026, 2032, 2037. A year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year; every other year has 52.

Where the rule comes from

The rule is older than ISO 8601. It was first published as ISO 2015, «Numbering of weeks», in 1976, and ISO 8601 took it over in its first edition of 1988. The choice of Thursday is what makes the numbering fair: the week that holds the first Thursday has at least four of its seven days in January, so week 1 is always mostly in the new year. Most of Europe reads week numbers this way; in Germany the week is printed on calendars as «KW 41».

ISO week number vs US week number

The US numbering starts weeks on Sunday and calls week 1 the week that contains 1 January. The numbers often differ by one: Sunday, June 15, 2025 is in ISO week 24 (which began on Monday, June 9, 2025) but in US week 25 (which began that same day, Sunday, June 15, 2025).

Spreadsheets add a third variant. Excel's ISOWEEKNUM() gives the ISO number; WEEKNUM() counts from Sunday but cuts the week at 1 January, so December 31, 2025 is week 53 there, while in the week-based US numbering on this page it is already week 1 of 2026 — the week of 1 January.

Where week numbers are used

• Planning at work: sprints, releases and production plans are scheduled «in week 41» rather than by dates.
• Delivery and logistics: suppliers in Europe quote delivery weeks, not days.
• Shift rotas and payroll: alternating schedules are often written as odd and even weeks.
• Schools and universities: timetables that change by week number, such as odd and even weeks.
• Statistics: weekly reports — sales, traffic, health data — are labelled by ISO week so that years can be compared week by week.

Limits and caveats

• The ISO year of a week is not always the calendar year: up to three days at either end of January and December belong to the neighbouring year's week.
• The US and spreadsheet numberings differ from ISO and from each other at the edges of the year; say which one you mean when you write a week number.
• Years before 1583 are numbered on the proleptic Gregorian calendar, as ISO 8601 allows by agreement; nobody used those weeks at the time.
• How many weeks a year holds as whole days is a different question: 2026 is 52 weeks and 1 day long, and the 2026 calendar shows week numbers next to every month.