How Flying Works
The KPIs That Actually Run an Airline
In 2025, Delta and American Airlines worked at roughly similar revenue scales, each taking in tens of billions of dollars. Delta’s net income came to around $5 billion. American’s came to around $111 million, down 87% on the year before. Same industry, same economy, wildly different results. The number of passengers each carried doesn’t explain the gap. A handful of standard metrics does, the ones every airline reports on its earnings calls and the ones compared in this site’s US majors comparison. They sound like finance jargon, but the formulas are simple. Once you know them, most airline strategy coverage starts to make sense.
| Term | Formula | What it actually tells you |
|---|---|---|
ASM Available Seat Miles | Seats × miles flown | The airline's total capacity — every seat, on every mile of every flight, whether it sold or not. The denominator underneath almost every other metric here. |
RPM Revenue Passenger Miles | Paying passengers × miles flown | The capacity that actually got used — a seat flying empty contributes to ASM but not RPM. |
Load Factor | RPM ÷ ASM | The percentage of available seats that were actually filled with paying passengers. A high load factor spreads fixed costs over more paying seats, directly lowering cost-per-passenger. |
CASM Cost per Available Seat Mile | Operating expenses ÷ ASM | What it costs the airline to fly one seat, occupied or not, one mile. The core efficiency number — lower is better, and it's what low-cost carriers are built around minimizing. |
RASM Revenue per Available Seat Mile | Total revenue ÷ ASM | How much revenue that same one seat-mile generates. If RASM exceeds CASM, the airline is profitable on a unit basis; if it doesn't, it's losing money on every seat-mile flown, regardless of headline revenue. |
Yield | Passenger revenue ÷ RPM | The average fare paid, per passenger, per mile actually flown. Where RASM blends in unsold seats, yield only counts revenue from seats that sold. |
Why a longer flight looks cheaper per mile
There’s a trap in comparing CASM between airlines: flight length. Many costs come once per flight, not per mile. Every departure pays a landing fee, a turnaround on the ground, and the fuel-hungry climb to cruising height. On a long flight those fixed costs are spread over many more miles, so the cost per seat-mile falls automatically, even if the airline is no more efficient.
That’s why analysts use stage-length-adjusted CASM. “Stage length” is the average distance of an airline’s flights. Before comparing, they convert every airline’s CASM to the same standard flight length. A common rule of thumb scales the figure by the square root of the ratio between the two distances. Without that step, a long-haul airline will always look leaner than a short-hop regional one, and the comparison says more about route maps than about management.
The same effect explains why aircraft utilisation, the hours a day each aircraft spends flying, matters so much. An aircraft costs money whether it flies or sits on the ground. Every extra hour in the air spreads its lease or loan, insurance and much of its maintenance over more seat-miles.
The one relationship that actually decides profitability
If RASM is greater than CASM, the airline earns more per seat-mile than it spends, so it’s profitable on each unit it sells. If RASM is less than CASM, it loses money on every seat-mile flown, however big the revenue or passenger numbers look. Tracked quarter by quarter, that one comparison tells you more about Delta and American’s 2025 gap than any single figure, load factor included. A full plane sold at low fares can still have RASM below CASM. Delta’s premium cabins and its American Express card revenue helped lift its RASM. American’s heavier debt and softer domestic fares were among the pressures that kept its CASM close to its RASM, leaving little room for profit.
Try it
Run a flight: does it make money?
A 186-seat jet flies 1,000 miles, producing 186,000 available seat-miles (ASM). Its cost per ASM (CASM) is fixed here at 12¢. Revenue per ASM (RASM) depends on how full it is and what people paid. When RASM beats CASM the flight makes money. At $160 average fare it needs 75% of seats filled just to break even. Illustrative numbers.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Which single number matters most?
None on its own. What decides profitability is how RASM compares with CASM. An airline can lead the industry on load factor and still lose money if its CASM is high enough. A low-cost carrier can run a lower load factor and still be very profitable if its CASM is low enough for RASM to beat it comfortably.
Why do airlines report "CASM-ex-fuel" separately?
Fuel is the most volatile big cost an airline has, and almost entirely outside its control. Taking fuel out of CASM shows how well the airline manages the costs it can control, such as labour, maintenance and airport fees, without oil-price swings hiding the picture.
Is a higher load factor always better?
Almost always, but not unconditionally. An airline that chases load factor by slashing fares can fill its seats and wreck its yield at the same time. That’s why airlines watch load factor and yield together, never one alone.
Why do these metrics use miles instead of just passenger counts?
A passenger flying 200 miles and one flying 5,000 miles use very different amounts of an airline’s resources. Measuring per mile lets a short-haul budget airline and a long-haul network airline be compared fairly.
Last reviewed: October 2026. Facts that change, such as fleets, airspace and airline ownership, are re-checked on a schedule. Spotted an error? Email contact@skyplus.news and we’ll fix it. See how we check facts.