Upselling Success Rate measures the percentage of upselling attempts or opportunities that are successful and sales are made.
Upselling Success Rate = (Number of Successful Up-Sales / Number of Upselling Attempts (or Opportunities)) x 100
This indicator will be included in my book: Key Performance Indicators - the 100 measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.