Last updated: 2026-06-22

ISO Week Number

ISO 8601 defines a week as a seven-day period running from Monday to Sunday. Under this standard, week 1 of a year is the week containing the year's first Thursday — equivalently, the week that contains 4 January. Week numbers run from 1 to either 52 or 53, depending on the year.

ISO week-numbering year

Because ISO weeks always begin on Monday and end on Sunday, the first few days of a calendar year sometimes fall in week 52 or 53 of the preceding year, and the last few days of December sometimes fall in week 1 of the following year. ISO 8601 handles this by defining a separate week-numbering year (sometimes written as "ISO year") that may differ by one from the Gregorian calendar year.

For example, 1 January 2021 is a Friday. The Monday of that week is 28 December 2020, and Thursday is 31 December 2020 — still in 2020. ISO 8601 therefore assigns this week to 2020 as week 53. The week-numbering year for 1 January 2021 is 2020, not 2021.

Years with 53 weeks

Most years contain 52 ISO weeks. A year has 53 weeks when 1 January falls on Thursday (any year), or when 1 January falls on Wednesday in a leap year. This happens roughly every five to six years. Recent 53-week years include 2015, 2020, and 2026.

Practical use

ISO week numbers are widely used in manufacturing, logistics, and project scheduling across Europe and in international trade. Purchase orders, delivery schedules, and production plans frequently reference a week number rather than a specific date to simplify cross-border coordination. The standard is less common in North America and parts of East Asia, where weeks may start on Sunday or numbering conventions differ.

When integrating week numbers with external systems, confirm whether the system follows ISO 8601 or a local convention. Non-ISO systems may start weeks on Sunday, count week 1 as the week containing 1 January regardless of the day, or number weeks from 0.

For calculating the number of working days within a period, see Business Days Between Two Dates. For a general date-difference calculation in years, months, and days, see Date Difference Calculator.