# Airline unit economics: CASM, RASM and the breakeven load factor
A budget carrier flies a 180-seat A320 from Barcelona to Milan, 79% full, and loses money on the flight. Its rival flies the same route, same aircraft, 81% full, and books a healthy profit. Two percentage points of load factor separate a loss-maker from a cash generator. That gap is not luck. It is the mechanics of airline unit economics, and once you can calculate it, you can read any airline's earnings call like an insider.
Airlines operate at massive scale with wildly different aircraft sizes and route lengths, so raw costs and revenues are useless for comparison. A $10,000 cost on a 500-mile flight means something totally different from a $10,000 cost on a 2,000-mile flight.
The industry solves this by standardizing everything to ASM: Available Seat Mile, one seat flown one mile, regardless of whether it was occupied. Europeans often use ASK: Available Seat Kilometre, same logic, metric units. This lesson uses ASM/mile convention (US-style) since it is the more common teaching standard; the math is identical with kilometres.
From ASM comes the two ratios that run the industry:
Let's build the numbers for a realistic short-haul A320 flight.
Route facts (illustrative, built from typical short-haul parameters):
Cost side:
This is in a realistic range. As of 2024 estimates, US low-cost carriers report system-wide CASM around 8 to 11 cents, while European short-haul budget carriers report similar or slightly lower figures in euro-cent terms, per data aggregated by IATA's economics reports and airline 10-KKThe average number of new users each existing user generates through referrals. Above 1.0, growth compounds on itself and becomes exponential.Voir la définition complète → filings.
Revenue side, scenario A (79% load factor):
CASM (9.0) > RASM (8.94). This flight loses roughly $64. Small, almost invisible in absolute terms, but it is a loss.
Revenue side, scenario B (81% load factor, same fare):
Now RASM (9.19) > CASM (9.0). Profit of roughly $208 on the flight.
Two extra passengers, worth $136 in fares, swing the flight from a $64 loss to a $208 profit, a $272 swing. This is the core insight: short-haul flying has high fixed costs (crew, fuel to move the aircraft regardless of load, airport fees, aircraft ownership costs) and low marginal cost per extra passenger. Once fixed costs are covered, almost every additional fare drops straight to the bottom line.
Breakeven load factor is the percentage of seats that must be filled, at a given average fare and cost level, for revenue to equal cost. Below it, the flight loses money; above it, it earns a profit.
Formula:
Breakeven load factor = CASM / (RASM at 100% load factor)
= CASM / (average fare ÷ average trip length in miles)Or more simply, using our flight:
Breakeven load factor = CASM / Yield per ASM at full capacityUsing the numbers above: at $68 average fare over 600 miles, revenue per passenger mile (this is called yield, revenue per revenue passenger mile) is $68 / 600 = 11.33 cents.
Breakeven load factor = CASM / yield = 9.0 / 11.33 = 79.4%
That is why 79% was a loss and 81% was a profit. The breakeven point sat almost exactly between them. This is not a coincidence in the lesson, it is the everyday reality of short-haul flying: airlines constantly operate within a few points of breakeven, and small shifts in fuel price, fare levels, or booking demand tip individual flights between loss and profit.
For context, actual industry breakeven load factors, as estimated by IATA and airline investor materials, typically sit in the 75 to 80% range for full-service and budget short-haul carriers in the US and Europe, while actual achieved load factors for healthy carriers run 80 to 85%. That gap of a few points is the airline's margin of safety, and it is thin by the standards of most industries.
A more targeted revenue measure is PRASM (Passenger Revenue per Available Seat Mile), which strips out cargo and ancillary revenue (bags, seat selection, onboard sales) to isolate ticket revenue per seat mile. Analysts compare PRASM growth to CASM growth quarter over quarter: if CASM is rising faster than PRASM, margins are compressing even if the airline looks busy.
Airlines also track CASM-ex fuel (CASM excluding fuel costs) to separate what management controls (labor efficiency, maintenance contracts, airport fee negotiation) from what it does not (global oil prices). When a CEO says "CASM-ex is down 2% year over year," they are claiming operational efficiency gains, not benefiting from cheaper fuel.
Vérification des acquis
1. Why do airlines standardize costs and revenues to an 'available seat mile' basis rather than comparing total flight costs directly?
2. An airline's CASM is 8.5 cents and its RASM is 8.2 cents on a given route. What does this tell you?
3. Two competing airlines fly the identical route with the identical aircraft, but one is profitable at a lower load factor than the other needs just to break even. What does this most directly reflect?
4. Select ALL correct answers about Available Seat Miles (ASM) as a metric.
Sélectionnez toutes les réponses correctes.
5. Select ALL correct answers about the relationship between RASM, CASM, and load factor.
Sélectionnez toutes les réponses correctes.
When Ryanair or easyJet report quarterly results, or when Delta or Southwest hold earnings calls, listen for these exact terms. A carrier with CASM-ex around 6 to 7 cents (2024 estimates, ultra-low-cost model) is playing a fundamentally different game than one with CASM-ex above 10 cents (full-service, higher labor costs, more legroom, more free services). Neither is "wrong": Ryanair's model targets high load factor at rock-bottom fares and thin per-seat margins across huge volume, while a full-service carrier like Lufthansa targets fewer, higher-yielding passengers with business class and connecting traffic.
The unit economics also explain why airlines obsess over ancillary revenue (baggage fees, seat selection, priority boarding). Ancillaries lift RASM without needing extra ASMs, directly narrowing the CASM-RASM gap without the airline needing to raise base fares or add seats.
🎬 [VIDEO: "Airline Economics 101: Why Are Airlines So Unprofitable?" — youtube.com — search for this title from Wendover Productions or similar aviation-economics channels for a visual walkthrough of load factor, CASM and thin margins]