Week Number Calculator

What week is Christmas? Christmas (Dec 25) falls in ISO week in .

What week does Q3 start? Q3 (Jul 1) begins in ISO week in .

What week does the US fiscal year start? The US federal fiscal year (Oct 1) lands in ISO week in .

The ISO 8601 standard, first published in 1988 by the International Organization for Standardization, defines a consistent method for numbering weeks within a year. Unlike common calendar conventions that vary by country, ISO 8601 provides a single, unambiguous format that works across borders, languages, and cultures. It is the foundation of week-numbering systems used in global commerce, logistics, and software development.

The defining rule of ISO 8601 is that Week 1 of any year is the week containing the first Thursday. This seemingly simple rule — often called the "Thursday rule" — has profound implications for calendar consistency. Because Thursday is the middle day of the ISO week (Monday through Sunday), requiring the first Thursday to fall in Week 1 ensures that no week ever spans across more than one year, and that every year has either exactly 52 or 53 full weeks. A year has 53 ISO weeks when January 1 falls on a Thursday (in a common year) or on a Wednesday (in a leap year), totaling roughly one occurrence every five to six years.

The Thursday rule also elegantly handles end-of-year transitions. If December 29, 30, or 31 falls on a Monday, Tuesday, or Wednesday, those days belong to Week 1 of the following year rather than the final week of the current year. Conversely, the first few days of January can belong to the final week of the previous year if January 1 lands on a Friday, Saturday, or Sunday. This design ensures that every ISO week has exactly seven days and belongs unambiguously to a single ISO year — a property that makes it invaluable for financial reporting, manufacturing scheduling, and payroll systems where partial weeks cause errors.

In practice, ISO 8601 week notation takes the form YYYY-Www-D. For example, 2026-W26-2 represents Tuesday of the 26th week of 2026. Most enterprise resource planning (ERP) systems, including SAP and Oracle, use this format internally. The standard is also the basis for calendar functions in programming languages: Python's datetime.date.isocalendar(), JavaScript date libraries like Luxon and date-fns, and built-in date functions in .NET and Java all implement ISO 8601 week numbering.

Retail fiscal calendars rely heavily on week numbers. The National Retail Federation (NRF) in the United States uses a 4-5-4 calendar system where each quarter contains two 4-week months and one 5-week month. This ensures every quarter has exactly 13 weeks and aligns weekly sales data for accurate year-over-year comparisons. Major retailers like Walmart, Target, and Amazon plan promotions, inventory, and staffing around these week-numbered fiscal periods. A 53-week fiscal year occurs approximately every five to six years, requiring careful accounting adjustments to normalize annual comparisons.

Manufacturing and supply chain operations use ISO week numbers for production planning and delivery scheduling. Automotive manufacturers, for example, label parts with their production week and year — the well-known "DOT date code" on tires includes the week number of manufacture. Pharmaceutical companies use week numbers to track batch production, expiration dating, and regulatory compliance across facilities located in different countries. Without a standardized week-numbering system, coordinating just-in-time manufacturing across continents would be significantly more error-prone.

Payroll and HR departments in many European countries process salaries by week number. In Germany, Austria, Switzerland, and all of Scandinavia, it is common for employment contracts, timesheets, and payslips to reference calendar weeks ("Kalenderwoche" or "KW"). Employees say "I'm on vacation in Week 32" rather than using specific dates. The European road freight industry uses week numbers for tachograph inspections and driver scheduling. Even in the United States, federal payroll calendars map pay periods to week numbers, with agencies like the Department of Defense using ISO-compatible week numbering for multi-national operations.

What week number is it today?

Today's ISO 8601 week number is —. This means we are in the — week of the year according to the international standard. The ISO week runs from Monday to Sunday, and Week 1 is always the week containing the first Thursday of the year. You can see the US and Middle Eastern week numbers for today in the Current Week card at the top of this page.

How does the ISO 8601 week numbering system work?

ISO 8601 defines Week 1 as the week containing the year's first Thursday, with weeks starting on Monday and ending on Sunday. This means that the first few days of January can belong to the last week of the previous year (if January 1 falls on a Friday, Saturday, or Sunday), and the last few days of December can belong to Week 1 of the following year (if December 29-31 falls early in the week). The system guarantees that every week has exactly seven days and belongs unambiguously to one ISO year.

What is the difference between ISO weeks and US weeks?

The two main differences are: (1) start day — ISO weeks start on Monday while US weeks traditionally start on Sunday, and (2) Week 1 definition — ISO uses the first Thursday rule while the US convention makes Week 1 the week containing January 1. This means a given date can have a different week number depending on which standard you apply. For example, when January 1 falls on a Friday, the ISO system places those first three days in the previous year's final week, while the US system counts them as Week 1 of the new year.

Why do some years have 53 weeks?

A calendar year has 365 days (366 in a leap year), which divides into 52 weeks plus one extra day (or two in a leap year). The ISO 8601 system defines Week 1 around the first Thursday to prevent partial weeks from spanning years. When those extra days accumulate in the right alignment, the year ends up with 53 full ISO weeks. This happens when January 1 of the year falls on a Thursday (in a common year) or on a Wednesday (in a leap year) — roughly once every five to six years. The next 53-week ISO years are listed in the detector section above.

Which countries use week numbers in daily life?

Week numbers are widely used in daily life across Germany, Austria, Switzerland, the Netherlands, Belgium, Luxembourg, Denmark, Norway, Sweden, Finland, and Iceland. In these countries, calendar weeks ("Kalenderwoche," "vecka," "uke," "viikko") appear on printed calendars, school schedules, and business planning documents. People routinely schedule meetings and vacations by week number. Outside Europe, week numbers are common in corporate settings in Japan, South Korea, and Brazil, where ISO-compatible planning systems are standard in manufacturing and logistics.

How are week numbers used in business and payroll?

Businesses use week numbers for fiscal calendars (notably the 4-5-4 retail calendar), production scheduling, delivery tracking, and performance measurement. Payroll systems in many countries use week numbers to define pay periods — a bi-weekly payroll cycle maps cleanly to two ISO weeks. In Europe, employment contracts and timesheets routinely reference calendar weeks. The automotive, pharmaceutical, and food industries all use week numbers for batch tracking and expiration dating. ERP systems like SAP and Microsoft Dynamics 365 use ISO week numbers as their native calendar structure for all modules from finance to warehouse management.