Week Number Calculator — ISO Week Number for Any Date

The Week Number Calculator returns the ISO 8601 week number for any date. ISO week numbering is the standard used in most of Europe and in many international business contexts, where weeks are numbered 1–52 (or sometimes 53) and each week runs Monday to Sunday.

What it returns

ISO week number (1–53).
ISO week-year (which can differ from the calendar year at the boundaries).
The Monday and Sunday of that week.

The ISO 8601 rules

Weeks start on Monday.
Week 1 is the week containing the first Thursday of the year — equivalently, the week containing January 4.
Some years have 53 ISO weeks (about every 5–6 years).
Early January or late December dates can belong to the adjacent year's week numbering. For example, January 1, 2023 is in ISO week 52 of year 2022.

Worked example

Monday, January 1, 2024:

ISO week: 1
ISO year: 2024

Sunday, January 1, 2023:

ISO week: 52
ISO year: 2022 (because the week containing Jan 4, 2023 is the first week of 2023, and Jan 1 falls in the previous Mon–Sun span).

When this is useful

Project planning where teams reference "week 12" or similar shorthand, manufacturing and supply chain (where ISO weeks are common production-planning units), academic schedules in countries that use ISO weeks, and reporting periods.

Not the same as "ordinal week"

Some systems define "week 1" as the week containing January 1, with weeks starting Sunday — that is a different convention (used in some US contexts). ISO 8601 is specifically Monday-start, Thursday-anchored. This calculator reports the ISO value.

Frequently asked questions

Why is January 1 sometimes in last year's week?

Because ISO weeks always start on Monday and Week 1 is defined to contain January 4. If January 1 falls on a Friday, Saturday, or Sunday, it is in the last week of the previous ISO year.

Week Number