Work out cost per available seat mile instantly with clear inputs, formula shown and shareable results.
Cost per available seat mile is total operating cost divided by seats times miles flown. Airlines quote it both including and excluding fuel, because fuel is volatile and outside management control while the ex-fuel figure reflects structural efficiency such as fleet choice, labour cost and utilisation.
Unit cost
CASM = operating cost / (seats x miles); ex-fuel CASM = CASM x (1 - fuel share)
Fixed costs per departure — landing fees, ground handling, taxi fuel — are spread over many more miles, so unit cost falls with sector length.
Adding seats raises the denominator with little extra cost, which is the core of the low-cost carrier model.