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Weeks in that year
Find the ISO-8601 week number for any date of the year.
This tool converts any calendar date into its official ISO-8601 week number — the standardized 'Week 1' through 'Week 52' (or 53) numbering used across business, government and manufacturing calendars worldwide. It also shows how many total weeks fall in that particular year and how many remain after your chosen date.
There isn't just one way to number weeks: some conventions start counting from January 1st regardless of weekday, while others count from the first full Monday-to-Sunday week. ISO-8601, the international standard, defines Week 1 of a year as the week containing that year's first Thursday, and weeks always run Monday through Sunday. This has a useful side effect: it means the first few days of January can technically belong to Week 52 or 53 of the previous year, and the last few days of December can belong to Week 1 of the next year — which is exactly what this calculator correctly reflects rather than glossing over.
52 weeks only add up to 364 days, one or two short of a full year, so the extra day (or two, in a leap year) has to go somewhere. Roughly once every five to six years, that spillover is enough to create a 53rd ISO week rather than folding into the following year's Week 1. This calculator accounts for that automatically rather than assuming every year has exactly 52 weeks.
Manufacturing and supply chain schedules, payroll cycles for weekly-paid staff, sprint planning in software teams, and European business calendars all commonly reference a numbered week rather than a calendar date. If you've ever seen a code stamped on a product like 'Week 14' or been told a delivery is expected 'in week 27', this is the exact numbering system being referenced.
Under the ISO-8601 standard, Week 1 of a year is defined as the week containing that year's first Thursday. If January 1st falls on a Friday, Saturday or Sunday, the first few days of January actually belong to the last week of the prior year.
No, ISO-8601 is an international standard widely used in Europe and in many business and technical contexts, but some regions and religious calendars use different week-numbering conventions.
Monday. ISO-8601 weeks always run from Monday through Sunday, unlike some calendar conventions that start the week on Sunday.
Because 52 weeks (364 days) don't evenly divide a 365 or 366 day year, the leftover day or two occasionally accumulates into a 53rd week rather than being absorbed into the next year's Week 1.
Yes, you can select any date and the calculator will correctly compute its ISO week number regardless of how far in the future or past it is.