ISO Week Number Calculator

How to use

1. 1Open the calculator and the top panel shows today's ISO week, the Monday-to-Sunday date range, and how many weeks remain in the current ISO year.
2. 2Switch to Date to week to find the ISO week number for any date: pick a date or type YYYY-MM-DD and read the ISO week, week-year, weekday, and full week range.
3. 3Switch to Week to date to go the other way: enter the ISO week-year and week number and the seven calendar dates appear with their weekdays.
4. 4Switch to Year overview to list every ISO week in a year, see whether it is a 52-week common year or a 53-week long year, and find this week highlighted in the grid.
5. 5Use the Copy buttons to grab the ISO label (like 2026-W25), a single date range, the full summary, or the year's week table as TSV.

About this tool

ISO Week Number Calculator answers three questions that planners, finance teams, project managers, and engineers ask all the time: what ISO week is this date in, what dates belong to a given ISO week, and how many ISO weeks does a year contain. Use Date to week to drop in any calendar date and read back its ISO 8601 week number, week-year, ISO weekday (1 for Monday through 7 for Sunday), Monday and Sunday endpoints of that week, day of the year, and the canonical week-date strings YYYY-Www and YYYY-Www-D. Use Week to date to enter an ISO week-year and week number and get the seven calendar dates that make up the week, with both long and ISO formats and a per-weekday breakdown table. Use Year overview to scan every ISO week in a given year, see whether the year is a common 52-week year or a long 53-week year, and spot today's week highlighted in the grid. The header panel always shows the current ISO week, the week's date range, and how many weeks remain in the ISO year. ISO 8601 weeks run Monday to Sunday, and ISO week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4 or the week with the majority of its days in the new year). The tool surfaces the edge cases without hiding them: when a date in late December or early January belongs to a different ISO week-year than its calendar year, the result panel calls it out so dashboards, payroll exports, and ERP timesheets stay aligned. Output matches the numbering used by PostgreSQL, Excel ISOWEEKNUM, Python isocalendar, .NET ISO 8601, and Java WeekFields.ISO, so values from this calculator are safe to paste into any of them. Every calculation runs in UTC inside your browser, so the week never drifts because of your local timezone and nothing you type leaves your device.

Free to use. Works in your browser. No signup, no login.