Drop-off rate is calculated by dividing abandoned applications by applications started, usually tracked through analytics on the careers site or ATS. If 1,000 people click apply and 600 submit, the drop-off rate is 40%. Where people leave is often more useful than the headline figure: a spike at a CV upload step or a long questionnaire shows exactly what to fix.
Every person who abandons an application is someone the team paid to attract and never got to assess. Hourly and frontline applicants often apply on a phone during a break, so long forms, forced account creation and repeated data entry hit them hardest. Asking only for what is needed at that stage, designing for mobile and stating how long the application takes all help more people finish.