Week Number Calculator

ISO 8601 is the international standard for week numbering, widely used across Europe and in many industries worldwide. Under this system, weeks always start on Monday and end on Sunday. Week 1 of the year is defined as the week containing the first Thursday of January, which means it always includes January 4th. As a result, a few days in late December may belong to Week 1 of the following year, and a few early January days may belong to the last week of the previous year.

Under ISO 8601, Week 1 is the week that contains the first Thursday of the year, or equivalently the week that contains January 4th. This ensures that Week 1 always has the majority (at least 4 days) of its days in the new year. In the US system, Week 1 simply begins on the Sunday on or before January 1st.

Because ISO 8601 defines Week 1 as the week containing the first Thursday of the year, some days in late December (typically December 29–31) can fall into Week 1 of the following year. For example, December 31, 2018 was in Week 1 of 2019 under ISO 8601. This is why the week year (ISO year) can differ from the calendar year for dates near the start or end of a year.

Switch the calculator to 'Week Number → Dates' mode, enter the year and the week number, and the calculator will return the Monday start date and Sunday end date for that week. This is useful for project planning, payroll processing, or scheduling tasks by week number.

Most years have 52 ISO weeks, but some years have 53. A year has 53 weeks if January 1st falls on a Thursday, or if it is a leap year and January 1st falls on a Wednesday or Thursday. For example, 2015, 2020, and 2026 are 53-week years. The calculator displays the total weeks in the selected year.

The main differences are the start day of the week and the definition of Week 1. ISO 8601 (European) weeks start on Monday, and Week 1 is the week with the year's first Thursday. The US method starts weeks on Sunday, and Week 1 simply begins on the Sunday on or before January 1st. These differences can result in the same date being assigned a different week number depending on which system is used.

Week numbers are widely used in business, manufacturing, logistics, project management, and payroll. European and Scandinavian countries routinely reference ISO week numbers in scheduling and planning. Industries like retail (for sales periods), construction (project timelines), and HR (payroll cycles) rely on week numbers for consistent date referencing across teams and systems.

Yes — the calculator works for any year between 1900 and 2099. Simply enter any date in the date picker, or enter any year and week number in the reverse lookup mode, and the results will be calculated accurately for that year. The full week schedule table also updates to reflect all weeks in the selected year.