ISO Week Number Calculator
The ISO 8601 standard gives every week of the year a single agreed number, used worldwide in business, manufacturing, and logistics. ISO weeks always start on Monday, and week 1 is the week containing the first Thursday of the year, which can place a few days of early January into the previous year's final week or push late December into week 1 of the next year. This calculator takes any date and returns its ISO week number, the ISO weekday from 1 (Monday) to 7 (Sunday), and the ISO week-year that the week belongs to, all derived strictly from the standard.
ISO 8601 week-date rule
ISO weekday: Mon=1 ... Sun=7
Move date to the Thursday of its week (date + 4 - ISO weekday)
ISO week-year = calendar year of that Thursday
week 1 = the week containing 4 January (first Thursday)
week number = floor((Thursday - Jan 1 of week-year) / 7 days) + 1
The Thursday rule assigns each week to the year that holds most of its days. Shifting to the in-week Thursday before counting guarantees the correct year boundary and week number near the start and end of the year.
Notes on ISO weeks
- Weeks always begin on Monday and end on Sunday.
- Week 1 always contains 4 January and the first Thursday of the year.
- A year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year.
- Early January can belong to week 52 or 53 of the prior ISO year.
- The ISO week-year may differ from the calendar year for dates near the boundary.
ISO week numbers: frequently asked questions
What is an ISO week number?
An ISO week number is the position of a week within its year under the ISO 8601 standard. Weeks run from Monday to Sunday and are numbered 1 to 52 or 53. Week 1 is defined as the week containing the first Thursday of the year, which is the same as the week containing 4 January.
Why can a date in early January belong to the previous year's week 52 or 53?
Under ISO 8601 the week belongs to whichever year holds the majority of its days, decided by the Thursday rule. So 1 January can fall in week 52 or 53 of the previous ISO year, and late December can fall in week 1 of the next ISO year. This calculator reports the ISO week-year separately for that reason.
How is the ISO weekday numbered?
ISO 8601 numbers the days of the week from 1 to 7 starting with Monday, so Monday is 1, Tuesday is 2, and Sunday is 7. This differs from the common programming convention where Sunday is 0, so the calculator converts to the ISO numbering for you.
When does a year have 53 ISO weeks?
A year has 53 ISO weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. Such years are sometimes called long years. All other years have 52 ISO weeks. The calculator derives this from the date itself rather than assuming 52.
Where are ISO week numbers used?
ISO week numbering is common in business and manufacturing planning, payroll, logistics, and across Europe in everyday scheduling. It gives every week a single agreed number worldwide, avoiding confusion between different national conventions for the first week of the year.
Official sources
- International Organization for Standardization: ISO 8601 date and time format.
- National Institute of Standards and Technology: NIST home.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.