Week Number Calculator Find ISO & Custom Week Numbers Free
Find the ISO 8601 week number, US week number, or any custom week for any date. Set your own week start day or fiscal year start month and get the full week date range instantly.
How This Week Number Calculator Works
This week number calculator supports three distinct systems used worldwide. ISO 8601 is the international standard: weeks start on Monday, and Week 1 is the week containing the year’s first Thursday. A year can therefore have 52 or 53 weeks, and Week 1 may begin in late December of the prior year.
The US system treats weeks as starting on Sunday, with January 1 always in Week 1. This produces different numbers from ISO in January and December. The two systems can diverge by up to two weeks at year boundaries.
Custom mode lets you define the week start day and fiscal year start month, essential for retail 4 to 5 to 4 calendars, UK fiscal years starting in April, US federal years starting in October, or any internal reporting system that doesn’t follow the Gregorian calendar year.
What this tool does
How to use it
Pick a date
Use the date picker or click Today to auto fill the current date.
Choose your system
Select ISO 8601, US (Sunday start), or Custom for fiscal and project calendars.
Read the results
See the week number, year, date range, quarter, day of year, and progress bars.
Navigate and copy
Use prev/next week buttons to browse. Copy any individual value with one click.
Questions About the Week Number Calculator
ISO week numbers are defined by the ISO 8601 international standard. Weeks run Monday to Sunday, and Week 1 is the week that contains the year’s first Thursday. Because of this, a year can have 52 or 53 weeks, and Week 1 may start in late December of the prior calendar year. ISO week numbers are used in most of Europe, in logistics, and in most ERP and calendar systems.
The US system starts weeks on Sunday and always treats January 1 as belonging to Week 1, regardless of what day it falls on. This can make Week 1 as short as one day. The ISO and US systems can differ by up to two week numbers at the start and end of the year. Check both outputs in the calculator to compare them side by side.
Click the Today button at the top of the calculator. It instantly shows today’s week number in ISO 8601, US, or your custom format, along with the full date range for the current week, the day of year, and both week and year progress bars.
Yes, under ISO 8601. A year has 53 ISO weeks when January 1 falls on a Thursday, or when it is a leap year and January 1 falls on a Wednesday or Thursday. This happens roughly every 5 to 6 years. 2015, 2020, and 2026 are recent examples of 53 week years.
A fiscal week counts weeks within a financial year that does not start on January 1. UK companies often start in April, US federal agencies in October, and retail companies may use a 4 to 5 to 4 calendar starting in February. Switch to Custom mode and set the fiscal year start month to match your organization’s calendar year.
The ISO 8601 week date format is YYYY-Www-D. For example, 2024-W24 to 3 means the year 2024, week 24, Wednesday (day 3, with Monday as day 1). The calculator shows this formatted value in the results, and you can copy it with one click.
Under ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, that week does not contain a Thursday in the new year, so it belongs to the last week of the previous year. Week 1 of the new year then starts on the following Monday, which falls in December of the prior year.
No. All calculations run entirely in your browser using JavaScript. No date you enter is sent to any server or stored anywhere. You can use this tool without an internet connection after the page initially loads.