What it means
Airlines operate networks with aircraft of different sizes and flight lengths, so a total operating-cost figure cannot show how much capacity those costs supported. CASM divides the expense by available seat miles, or ASM, to express cost for one unit of seat capacity flown.
A flight with 100 available seats flying 500 miles creates 50,000 available seat miles, even if only 60 people buy seats. The calculation uses seats available, not passengers actually carried, which is a different demand and utilisation measure.
The US Bureau of Transportation Statistics reports airline operating expenses per available seat mile in cents, and its published unit-cost tables demonstrate how to compare a defined airline group and reporting period. Historical table values describe those periods, not today's ticket prices.
The ordinary numerator is operating expense within the chosen scope, so an analyst should check whether the carrier reports systemwide flights, a mainline segment or another group, since a number from one network cannot safely be compared with a competitor's narrower segment. Fuel is a major and volatile cost, so some presentations use total CASM while others show CASM excluding fuel to examine costs management may control more directly.
Excluding fuel can be useful, but it is not the same measure and it still excludes a real operating cost. If available seat miles rise faster than operating expenses, CASM falls, which may reflect better use of aircraft and fixed resources but can also result from longer average flights, route changes or an aircraft mix with more seats, so a low number alone does not prove service is profitable.
An empty seat counts in ASM even though it earns no passenger fare, so a carrier could cut CASM by adding capacity, but if too few seats sell, revenue per unit of capacity may deteriorate. Cost efficiency has to be read with load factor and revenue, and revenue per available seat mile, or RASM, uses operating revenue divided by ASM when the definitions align.
Comparing RASM and CASM can help explain the operating spread, but non-operating costs, taxes and accounting scope still affect final profit, so do not equate the spread automatically with net income. A seat-mile measure is not a cost per passenger, because a passenger-mile counts travel actually sold to or used by passengers while a seat-mile counts capacity made available, and confusing the denominators can make an airline with empty flights look artificially efficient.
The denominator can also change when routes are cancelled or aircraft are grounded; expenses do not always fall at once, so CASM may rise even if total cost declines, and capacity shocks make year-over-year comparisons misleading. The best use is a like-for-like trend or peer comparison with the same time basis, fuel treatment, currency and network scope, paired with revenue, utilisation and service context before judging performance.
In practice
Real-world examples.
Example
An airline records 500 million of operating expenses and offers 5 billion available seat miles. CASM is 0.10 per available seat mile, usually reported as 10 cents.
Example
Two carriers both report 10-cent CASM, but one excludes fuel and the other includes it. Their published numbers are not a valid direct cost comparison until the definitions match.
Example
A 150-seat flight travels 400 miles and generates 60,000 available seat miles. Selling only 90 seats changes revenue and load factor, not the ASM denominator for that flight.
Formula
Calculation
CASM = operating expenses / available seat miles. ASM = sum over flights of available seats x miles flown. With 500 million of operating expenses and 5 billion ASM, CASM is 0.10 in the same currency, or 10 cents if the figures are in dollars. Identify whether fuel and other costs are included.Case study
Seen in the real world.
Fictional example: Amir compares two airlines' annual presentations. One reports total CASM and the other highlights CASM excluding fuel, making the second appear much cheaper. He retrieves their full expense definitions and matches the network scope, then compares like-for-like quarters. He also checks RASM and occupancy. The adjusted comparison shows that the original headline difference mostly reflected the excluded fuel, not necessarily stronger operating performance.
Watch out
Common mistakes.
- Confusing available seat miles with paying passenger miles or seats actually sold.
- Comparing fuel-excluded CASM with total CASM as if the numerators were identical.
- Assuming low unit cost guarantees net profit without checking revenue, occupancy and other costs.
Questions
People also ask.
Do empty seats count in available seat miles?
Yes. ASM is offered seat capacity multiplied by distance, regardless of whether the seat sells.
Why do carriers publish CASM excluding fuel?
It can isolate part of the cost base from fuel-price swings, but the variant must be labelled and is not total cost.
Is CASM a ticket price?
No. It is an operating-cost measure per unit of seat capacity, not what a passenger pays.
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