📐 ISO 8601 Week Numbers: The Complete Guide

How the international week numbering standard works

ISO 8601 is the international standard for writing dates and times, published by the International Organization for Standardization (ISO). Its week numbering system is the most widely used way of counting weeks, especially across Europe.

📏 The Three Core Rules

Weeks start on Monday

Under ISO 8601, every week begins on Monday and ends on Sunday. That differs from the habit in the United States and Canada, where the week is usually thought of as starting on Sunday.

Week 1 contains the year's first Thursday

Week 1 of a year is the week that contains that year's first Thursday. Two other ways to say the same thing: week 1 is the week that contains January 4, or the first week with most of its days (at least four) in the new year.

A year has 52 or 53 weeks

A year with 53 weeks is called a long year. The test: a year has 53 weeks if its last day is a Thursday, or if it is a leap year and its last day is a Friday. An easier way to check: the year has 53 weeks if January 1 is a Thursday (in a common year) or a Wednesday or Thursday (in a leap year).

🔢 How to Calculate an ISO Week Number

To find the ISO week number of any date, follow these steps:

Find the Thursday of the week that date falls in (add or subtract the right number of days).
The year that Thursday falls in is the ISO year.
Count which Thursday of that year it is. That number is the week number.

JavaScript:

function getISOWeek(date) {
  // Create a copy and shift to Thursday
  const d = new Date(Date.UTC(
    date.getFullYear(), date.getMonth(), date.getDate()
  ));
  // Set to nearest Thursday: current date + 4 - current day number
  // Make Sunday = 7
  const dayNum = d.getUTCDay() || 7;
  d.setUTCDate(d.getUTCDate() + 4 - dayNum);
  
  // Get first day of year
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  
  // Calculate full weeks to nearest Thursday
  const weekNo = Math.ceil((((d - yearStart) / 86400000) + 1) / 7);
  
  return {
    week: weekNo,
    year: d.getUTCFullYear()
  };
}

// Usage:
console.log(getISOWeek(new Date('2026-06-20')));
// { week: 25, year: 2026 }

Python:

from datetime import date

def get_iso_week(d):
    """Return (ISO_year, ISO_week) for a given date."""
    return d.isocalendar()[0], d.isocalendar()[1]

# Usage:
print(get_iso_week(date(2026, 6, 20)))
# (2026, 25)

🇺🇸 ISO 8601 vs the US Week System

Feature	ISO 8601	US system
First day of the week	Monday	Sunday
Week 1 is	The week containing the first Thursday	The week containing January 1
Week numbers	1–53	1–54 (54 is rare)
Where it's used	Europe and international business	United States, Canada
Week 1 can start in the previous year	Yes (e.g. week 1 of 2026 began on 2025-12-29)	No (dates in late December keep the old year's numbers)
Official standard	ISO 8601-1:2019	No formal international standard

"US system" here means the common Sunday-start convention used by Excel's WEEKNUM System 1; it is not a single national standard.

📅 Which Years Have 53 Weeks?

These years between 2020 and 2040 have 53 weeks:

2020, 2026, 2032, 2037, roughly one every five or six years.

The rule: a year has 53 weeks if January 1 is a Thursday, or if it is a leap year and January 1 is a Wednesday.

2026 meets the first condition (January 1, 2026 was a Thursday), so 2026 has 53 weeks.

💼 Where Week Numbers Are Used

Payroll: In a 53-week year, employees paid every two weeks can get one extra paycheck, so finance teams need to plan ahead.
Project management (sprints): Scrum and agile teams use week numbers to define sprint cycles.
Academic calendars: Some universities, especially in Europe, use ISO week numbers to define terms and exam weeks.
Manufacturing: Factories use week numbers for production scheduling and quality traceability.
Shipping and logistics: Shipping and logistics teams often use ISO week numbers to plan departures and arrivals.

See the current week number →