Week Number Calculator

How to use the week number calculator

Pick a date — today is selected by default — to see its ISO week number, the Monday and Sunday that bound that week, the US-style week number, how many weeks the year has, and the day of the year. Open the table for a complete list of every ISO week in the year with its start and end dates, which is handy for planning sprints, production schedules, school terms, payroll periods and delivery slots.

What is the ISO week number?

The international standard ISO 8601 defines a week-numbering system used throughout Europe, much of Asia, and in many businesses worldwide. In Germany it is the Kalenderwoche (KW), and in Sweden and the Nordic countries week numbers are part of everyday conversation. The rules are:

• Weeks start on Monday and end on Sunday.
• Week 1 is the week containing the year's first Thursday — equivalently, the week containing 4 January.
• A year has either 52 or 53 weeks. Years starting on a Thursday, and leap years starting on a Wednesday, have 53.

Because weeks never split across years, the first few days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. The ISO year is therefore sometimes different from the calendar year. ISO dates can be written as 2026-W40-6 — year, week and day of the week (1 = Monday).

The formula

Shift the date to the Thursday of its Monday–Sunday week. The year of that Thursday is the ISO year, and the number of whole weeks since 1 January of that year, plus one, is the week number.

Worked example

Take Saturday 3 October 2026. Its week runs from Monday 28 September to Sunday 4 October, and the Thursday is 1 October 2026 — day 274 of the year. Then 1 + ⌊273 ÷ 7⌋ = 1 + 39 = week 40. In ISO notation the date is 2026-W40-6.

A boundary case: Friday 1 January 2027. Its Thursday is 31 December 2026, so the date belongs to week 53 of 2026. 2026 is a 53-week ISO year because it began on a Thursday.

ISO vs US week numbers

In the United States and some other countries, calendars and software often use a different scheme: weeks start on Sunday and week 1 is simply the week containing 1 January, however few days of it fall in the new year. The two systems can differ by one week, and they disagree about which days belong to the first and last weeks. Spreadsheet functions reflect this — in Excel, ISOWEEKNUM() returns the ISO week, while WEEKNUM() defaults to the US system. When sharing schedules internationally, say which system you are using, or quote dates directly.