Definition
A standard airline capacity measurement equal to one seat flown one mile, whether or not that seat is occupied. Total ASMs for a flight are calculated by multiplying the number of seats available for sale by the distance flown in miles. ASMs across a fleet or network are summed to express total system capacity over a given period.
Plain English
It is a way airlines measure how much flying capacity they offered. One seat carried one mile equals one ASM. If a 150-seat airplane flies 500 miles, it produced 75,000 ASMs, regardless of how many seats actually had passengers in them.
Context Anchor
Seen in airline operations, route planning, airline performance reports, and aviation business discussions rather than normal cockpit procedures.
Derivation
The name describes itself in plain terms: the seats that were available for sale, multiplied by the miles flown. It is a supply measure, not a sales measure — the seat counts whether or not anyone bought it.
Why Pilots Care
ASMs drive route planning, fleet decisions, and ultimately what airplanes get flown where. Pairing ASMs with revenue (RASM) or cost (CASM) per mile is how airlines judge whether a route is worth operating, which affects pilot schedules and bases.
Intuition Check
Do not read “available” as “occupied.” An available seat mile counts a seat the airline offered for the flight, even if no passenger sat in it.
Example Sentence 1
The airline reported a 6 percent year-over-year increase in available seat miles after adding two new long-haul routes.
Example Sentence 2
Load factor compares revenue passenger miles to available seat miles to show how full the flights were.