Authoritative Source & Methodology

This calculator provides week numbers based on established international and regional standards. The primary source for this methodology is:

  • Source: ISO 8601:2004 (and subsequent revisions)
  • Reference: "Data elements and interchange formats – Information interchange – Representation of dates and times"
  • Link: iso.org

All calculations are based strictly on the rules defined by these standards. We use UTC-based date logic to ensure calculations are consistent and independent of your local time zone.

The "Formula" Explained

Unlike a simple algebraic equation, week numbering is based on a set of rules. The most common formula, used for the **ISO 8601** standard, can be expressed mathematically. This formula finds the week number for a given date's ordinal day number ($N$) and its ISO weekday ($W$, where Monday=1, Sunday=7).

$$ \text{WeekNumber} = \left\lfloor \frac{10 + (N - W)}{7} \right\rfloor $$

Where:
$N$ = The ordinal day of the year (e.g., Jan 1st is 1, Feb 1st is 32).
$W$ = The ISO-8601 weekday number (1 for Monday, 7 for Sunday).

However, a rule-based explanation is often clearer:

  • ISO 8601 Standard: Weeks start on **Monday**. Week 1 of any year is the one that contains the **first Thursday** of that year. This also means it's the first week with 4 or more days in the new year.
  • US Standard: Weeks start on **Sunday**. Week 1 is the week that contains **January 1st**.
  • Middle Eastern Standard: Weeks start on **Saturday**. Week 1 is the week that contains **January 1st**.

Glossary of Variables

Selected Date
The specific date (day, month, year) for which you want to find the week number.
Week Numbering Standard
The set of rules used for the calculation. This calculator supports the three most common systems.
Week Number
The sequential number (1-53) of the week within its year, as defined by the selected standard.
Week Starts On / Ends On
The first and last day of the calculated week. This changes depending on the standard (Saturday, Sunday, or Monday).

How It Works: A Step-by-Step Example

Let's see why the same date can have two different week numbers. A perfect example is **January 1, 2023**.

Case 1: Using the US Standard (Weeks start Sunday)

  1. Input Date: January 1, 2023 (which was a Sunday).
  2. Rule: The US standard states that weeks start on Sunday, and Week 1 is the week containing Jan 1st.
  3. Analysis: Since Jan 1, 2023, *was* a Sunday, it became the first day of Week 1 for 2023.
  4. Result: Week 1 (running from Sunday, Jan 1 to Saturday, Jan 7).

Case 2: Using the ISO 8601 Standard (Weeks start Monday)

  1. Input Date: January 1, 2023 (a Sunday).
  2. Rule: The ISO standard states that Week 1 is the week containing the *first Thursday* of the year. Weeks start on Monday.
  3. Analysis: The first Thursday of 2023 was January 5th. The week containing this Thursday *started* on Monday, January 2nd. Therefore, the date January 1st (the Sunday before) belongs to the *previous* week.
  4. Result: Week 52 (of the year 2022, running from Monday, Dec 26, 2022 to Sunday, Jan 1, 2023).

This calculator handles these complex edge cases for you.

Frequently Asked Questions (FAQ)

What is the current week number?

To find the current week number, simply load this page or click the "Reset to Today" button. The calculator automatically defaults to today's date and shows the current week number based on the ISO 8601 standard.

Why are there sometimes 53 weeks in a year?

A year has 365 days, and $365 / 7 = 52.14$. This extra 0.14 days (or 0.14 + 1 for a leap year) adds up. An ISO year has 53 weeks if January 1st is a Thursday, or if it's a Wednesday in a leap year. In the US system, any year that starts on a Saturday has 53 weeks.

What is the ISO 8601 week numbering standard?

It's the international standard for date and time representation. It's widely used in business, logistics, and government to avoid ambiguity. Its key rules are: weeks start on Monday, and Week 1 is the week containing the year's first Thursday.

How is the US week number different from the ISO standard?

There are two major differences:
1. Start of Week: US weeks start on Sunday, while ISO weeks start on Monday.
2. Week 1 Rule: The US Week 1 is simply the week containing January 1st. The ISO Week 1 is the week with the year's first Thursday. This can cause the week numbers to be different by 1 around the new year.

Can a year start in Week 52 or 53?

Yes. In the ISO 8601 system, if January 1st falls on a Friday, Saturday, or Sunday, those days are considered part of the last week (52 or 53) of the *previous* year. The calculator shows this automatically, as in the example of Jan 1, 2023.

Which standard should I use?

If you are in North America, you might be used to the US standard (Sunday start). For international business, planning, or any technical application, **ISO 8601 is strongly recommended** as it is unambiguous and globally recognized.

Does this calculator handle leap years?

Yes. The calculations are based on the actual calendar date, so leap years (like 2024 or 2028) and their extra day (February 29th) are automatically factored in correctly.

Tool developed by Ugo Candido. Contents verified by the CalcDomain Editorial Board.
Last accuracy review: .