CASK (Cost per Available Seat Kilometre): Definition and Formula

CASK: Cost per available seat kilometre

CASK (cost per available seat kilometre) is an airline's operating cost divided by its available seat kilometres (ASK). It is the standard measure of airline unit cost; comparing it with RASK, the revenue per available seat kilometre, shows whether the airline covers its costs.

Updated 16 October 2026 · Wireum team

The formula

CASK = operating cost ÷ ASK

CASK ex-fuel = (operating cost − fuel cost) ÷ ASK

Example

An airline has 375 million in operating costs on 5 billion ASK.

CASK = 375,000,000 ÷ 5,000,000,000 = 0.075, or 7.5 cents per ASK.

Comparing CASK fairly

CASK depends heavily on the network. Longer average flights, denser seating and higher aircraft utilisation all lower CASK. When comparing airlines or routes, adjust for stage length, or compare CASK ex-fuel, before drawing conclusions.

How airlines use it

Breaking CASK down by cost category (fuel, crew, maintenance, airport and handling, ownership) to see where costs move.
Tracking fuel cost per block hour and per ASK.
Comparing CASK with RASK by route to see which routes cover their costs.

CASK in Wireum Insights

Wireum Insights shows CASK/CASM broken down by cost category, fuel cost per block hour and per ASK, and ground-handling costs by station, alongside RASK and yield.

Frequently asked questions

How is CASK calculated?

Divide total operating cost by available seat kilometres (ASK). It is usually expressed in cents per ASK. The miles equivalent is CASM.

What is CASK excluding fuel?

CASK with fuel costs removed. Because fuel prices swing widely, airlines and analysts use CASK ex-fuel to see how well the costs the airline controls are managed.

Why do longer flights have lower CASK?

Many costs occur per flight rather than per kilometre — airport and handling charges, take-off and landing, turnaround time. Spread over a longer flight, they add less per seat kilometre, so CASK falls as average stage length rises.