How to Use the ISO Week Number Calculator
Work reports, production schedules, and shipping timelines often reference "week 36" or "week 12" instead of a calendar date, but counting weeks by hand is easy to get wrong. This calculator takes any date and instantly returns its official ISO 8601 week number, along with the Monday-Sunday range that week covers. The page loads with today's date pre-filled, and you can switch to any past or future date to check its week number too.
The ISO 8601 standard defines each week as running Monday through Sunday, and designates the week containing the year's first Thursday as week 1. That means January 1st isn't always week 1 — depending on the day of the week it falls on, it can actually belong to the last week of the previous year. This is also why the "week number" a spreadsheet or calendar app shows sometimes disagrees with a simple day-count from January 1st.
The calculator also shows the day-of-year count, which is handy for attendance tracking and project planning. See the FAQ below for more on how the standard handles edge cases.
Frequently Asked Questions
This tool uses the international ISO 8601 standard. Each week runs Monday through Sunday, and week 1 is defined as the week containing the year's first Thursday.
No. If January 1st falls on a Friday, Saturday, or Sunday, that week counts as the final week (52 or 53) of the previous year, and the new year's week 1 starts the following Monday.
No. Most years have 52 ISO weeks, but years where more Thursdays fall within them can have 53 weeks instead.