Week number calculator

What is a week number calculator?

A week number calculator tells you which numbered week of the year a given date falls in. Instead of saying “the first Monday of June”, you can say “week 23”, which is a compact, unambiguous way of referring to a period of time. The calculator also shows the ISO week-numbering year, the weekday of the date, and — if you enter a second date — the number of whole weeks between the two dates.

Week numbers are widely used in business, logistics, manufacturing, and project planning, especially in Europe. A delivery scheduled “for week 14” or a sprint that runs “weeks 9 to 11” relies on everyone agreeing on the same numbering scheme.

How does the week numbering work?

This calculator uses the ISO 8601 standard, the most common international convention. Under ISO 8601:

• Weeks start on Monday and end on Sunday.
• Week 1 of a year is the week that contains the first Thursday of that year (equivalently, the week containing January 4).
• A year has either 52 or 53 weeks.

A consequence of this rule is that the first few days of January can belong to the last week (52 or 53) of the previous year, and the last few days of December can belong to week 1 of the next year. This is why the calculator reports the ISO week-numbering year separately: it can differ from the calendar year of the date.

Formula

The ISO week number depends on which Thursday the week contains. A convenient way to express it uses the ordinal day of the year and the weekday:

$W = \left\lfloor \frac{D_O - W_D + 10}{7} \right\rfloor$

Where:

• $W$ is the ISO week number.
• $D_O$ is the ordinal day of the year (January 1 = 1).
• $W_D$ is the ISO weekday (Monday = 1, …, Sunday = 7).

If the result is $0$, the date belongs to the last week of the previous year; if it exceeds the number of weeks in the year, it belongs to week 1 of the following year.

The number of whole weeks between two dates is:

$\text{Weeks Between} = \left\lfloor \frac{\lvert \text{End Date} - \text{Start Date} \rvert}{7} \right\rfloor$

Examples

Example 1: A date early in January

Take January 1, 2024. It is a Monday, and the week containing the first Thursday of 2024 starts on that day, so:

$W = \left\lfloor \frac{1 - 1 + 10}{7} \right\rfloor = 1$

January 1, 2024 falls in ISO week 1 of 2024.

Example 2: A date that rolls into the next year

December 30, 2024 is a Monday. Its week contains the first Thursday of 2025 (January 2, 2025), so the date belongs to week 1 of the ISO week-numbering year 2025 — even though the calendar date is still in December 2024.

Example 3: Weeks between two dates

To find how many whole weeks separate January 1, 2024 and January 15, 2024:

$\text{Weeks Between} = \left\lfloor \frac{14}{7} \right\rfloor = 2$

There are 2 whole weeks between the two dates.

Frequently asked questions (FAQs)

What standard does this calculator use?

It uses ISO 8601, where weeks start on Monday and week 1 is the week containing the first Thursday of the year (the week that contains January 4).

Why can a date in January belong to the previous year’s week?

Because ISO week 1 must contain the first Thursday of the year, up to three early-January days can belong to week 52 or 53 of the previous week-numbering year. The calculator shows this through the ISO week-numbering year.

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Most years have 52 weeks.

Is the week number the same in the US?

Not always. Some US conventions start the week on Sunday and define week 1 as the week containing January 1, which can produce different numbers. This calculator follows the international ISO standard.

How are weeks between two dates counted?

The calculator divides the absolute number of days between the dates by 7 and keeps the whole number, so it reports the count of complete weeks between them.