Airliners cruise at around 35,000 feet, roughly 10.7 km or 6.6 miles up, because it's the sweet spot for jet engines and wings. The air there is thin enough that drag is low and fuel goes much further, but still dense enough for the wings to lift the plane and the engines to breathe. It's also above most weather. The exact height varies from about 30,000 to 41,000 feet with the aircraft, its weight and the route.
Thin air, less drag
Air gets thinner as you climb. At 35,000 feet it's about a third as dense as at sea level, so a plane moving through it pushes far fewer air molecules out of the way. Less drag means the same engines move the plane much faster on much less fuel per kilometre. Jet engines are also more efficient in the cold air up high.
| Altitude | Air density | Temperature | Speed of sound |
|---|---|---|---|
| Sea level | 100% of sea level | 15 °C | 661 knots |
| 10,000 ft | 74% of sea level | -5 °C | 638 knots |
| 20,000 ft | 53% of sea level | -25 °C | 614 knots |
| 30,000 ft | 37% of sea level | -44 °C | 589 knots |
| 35,000 ft | 31% of sea level | -54 °C | 576 knots |
| 40,000 ft | 25% of sea level | -56 °C | 574 knots |
In the International Standard Atmosphere, the reference model used in aviation.
So why not fly higher still?
- Lift. Thinner air also means less lift. To stay up in thinner air, a plane must fly faster, or at a higher angle.
- The speed of sound. Airliners are designed to cruise just below it, around Mach 0.78 to 0.85. Higher up, the minimum speed needed to stay airborne creeps toward the maximum speed the wing can handle. Pilots call the narrow gap between them the “coffin corner”.
- Engines. They need oxygen, and there's less of it with every thousand feet.
- Weight. A heavy plane, full of fuel at the start of a long flight, can't climb as high. That's why long-haul flights often step up to higher levels as they burn fuel and get lighter.
- Certification. Each type has a maximum certified altitude, typically around 39,000 to 43,000 feet for airliners, set partly by how quickly the cabin could be made safe if it lost pressure.
Above the weather
Most clouds and storms live in the troposphere, the lowest layer of the atmosphere, which reaches up to around 36,000 feet at mid-latitudes. Cruising near its top keeps planes above most cloud and rain, although big thunderstorms can tower higher, and are flown around rather than over. The jet stream lives up there too, which is why eastbound flights are faster.
Why the altitude ends in odd or even thousands
To keep traffic apart, cruising levels follow a rule based on direction: in most of the world, aircraft heading roughly east fly at odd thousands of feet (35,000, 37,000) and those heading west at even thousands (34,000, 36,000). Pilots call these flight levels, so 35,000 feet is FL350.
Short flights fly lower
On a short hop there's no point climbing all the way: by the time you'd get there, you'd need to start down again. Short flights often cruise in the 20,000s. The flight time calculator uses a simple version of the same idea, and the knots converter shows the Mach number at different heights. In Flight, your focus flight holds its cruising altitude from take-off to landing.