ISO Week Number — Free Online Tool

Find the ISO 8601 week number for any date, plus the day of year and week-year. ISO Week Number is a free online tool from the ToolsRift Everyday Tools collection. It runs entirely in your browser — nothing to install, no signup, and your data never leaves your device.

Loading ISO Week Number…

How to use ISO Week Number

  1. Open the ISO Week Number above — it loads instantly, no account needed.
  2. Enter or upload your input. Everything is processed locally on your device.
  3. Get your result immediately and copy or download it with one click.

Frequently asked questions

How is the ISO week number defined?

Under ISO 8601, weeks start on Monday and week 1 is the week containing the first Thursday of the year (equivalently, the week with January 4th).

Why can early January be week 52 or 53?

If the first days of January fall before that year's first Thursday, they belong to the last week of the previous ISO year — which is why the week-year can differ from the calendar year.

Does the week ever go to 53?

Yes. Years starting on a Thursday, and leap years starting on a Wednesday, have 53 ISO weeks.

More Everyday Tools

Day of the Week Finder
Find what day of the week any date falls on using Zeller
Moon Phase Calculator
See the moon phase, illumination and lunar age for any date with an emoji icon
Business Days Calculator
Count working days (Mon–Fri) between two dates, excluding weekends
Days in Month Calculator
Find how many days are in any month and year, including leap-year February
Easter Sunday Date Finder
Calculate the exact date of Easter Sunday for any year using the Computus algorithm
Unix Timestamp Converter
Convert Unix epoch timestamps to human dates and back, in UTC and local time

← View all Everyday Tools