Week Number Calculator

When to use it

Week numbers are how operations teams talk about time: a delivery in week 34, a sprint that starts in week 12, a payroll cut-off in week 52. This page turns any date into its ISO week number, and adds the details that usually cause the argument — the ISO year, the day of the year, the quarter and the weekday.

The ISO year matters more than it looks. The last days of December and the first days of January can belong to the neighbouring year, so a date in 2027 may legitimately be week 53 of 2026. The page shows both, which is exactly where hand-counted spreadsheets go wrong.

How the maths works

Under ISO 8601 a week runs from Monday to Sunday, and week 1 is the week that contains the first Thursday of January. Equivalently, take the Thursday of the week the date falls in: the year of that Thursday is the ISO year, and the week number is how many Mondays have passed since the Monday of week 1, plus one.

The result is always between 1 and 53. Years where 1 January falls on a Thursday, or on a Wednesday in a leap year, have 53 weeks; all the others have 52.

A worked example

Which week is 21 October 2026?

1. 2026 begins on a Thursday, so week 1 runs from Monday 29 December 2025 to Sunday 4 January 2026.
2. 21 October is day 294 of 365.
3. 294 ÷ 7 = 42 exactly, so 42 full weeks have passed since the start of week 1.
4. Result: Week 43, ISO year 2026, fourth quarter, Wednesday.

Dates at a glance

The last two rows are the classic trap: the calendar has turned, but the working week has not.

Common mistakes

• Assuming the ISO year equals the calendar year. Around new year they disagree, which is why the page prints both.
• Counting from Sunday. ISO weeks start on Monday; a Sunday-start convention shifts every number by one.
• Dividing the day of the year by seven. That gives a rough count, not the ISO week, because week 1 rarely starts on 1 January.
• Assuming 52 weeks every year. Some years have 53, and payroll or delivery schedules must handle it.

Keep exploring

• Date Difference — the exact number of days between two dates.
• Add Days to a Date — move a date by a fixed number of days.
• Due Date Calculator — the same week counting applied to a pregnancy estimate.
• Guide: Working with Dates and Time.