If you're interested in the technical details and implementation of the script please see my two blog posts Calculating week numbers in Google Spreadsheets and Calculating week numbers in Google Spreadsheets Revisited.

Google Apps Script for Spreadsheets that calculates ISO 8601 week numbers from date cells.

The script has no options or parameters, you simply invoke it with a reference to a date cell. For example:

The image above was taken from my test spreadsheet. Feel free to make a copy of it!

Don't hesitate to contact me!