Week Number Calculator

Enter a date to find its ISO week number, or enter a year and week number to get the corresponding start and end dates. The Week Number Calculator follows the ISO 8601 standard (used across Europe and internationally), where week 1 is the week containing the first Thursday of the year. Results include the week number, week start date, week end date, and day of year. Also try the Elapsed Time Calculator.

Calculation Mode *

Enter the date to find its ISO week number

Results

Week Number

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Week Year

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Week Start Date

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Week End Date

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Day of Year

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Day of Week

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Total Weeks in Year

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Results Table

Frequently Asked Questions

What is the ISO 8601 week numbering system?

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. See also our use the 30 Min Calculator.

How is Week 1 of the year determined?

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.

Why might a date in late December show a week number from the next year?

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.

How do I find what dates a specific week number covers?

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. You might also find our calculate Next Business Day Target Business Date useful.

Does every year have 52 weeks, or can there be 53?

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.

What is the difference between the ISO and US week numbering methods?

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.

Who commonly uses week numbers?

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.

Can I use this calculator for years other than the current year?

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.