Why do planes fly at 35,000 feet? Because that is roughly where a jet airliner gets the best trade-off between thin air, which cuts drag and saves fuel, and air that is still thick enough for the wings and engines to work. It also puts the plane above most weather, and air traffic rules sort jets into neat 1,000-foot layers, which is why you so often hear a round number like 35,000.
The exact figure changes from flight to flight. Typical airliner cruising altitudes run from about 31,000 to 38,000 feet, and the best level for any one plane depends on its weight, its direction and the winds that day. Here is how each piece fits together.
Reason 1: thin air means less drag
Air gets thinner as you climb. NASA’s standard model of the atmosphere shows temperature, pressure and density all falling with height through the lowest layer, the troposphere. Using NASA’s published formulas, air density at 35,000 feet works out to roughly a third of its sea-level value.
That matters because of how drag works. In NASA’s drag equation, drag is directly proportional to air density. Halve the density and, with everything else equal, you halve the drag. A plane pushing through thin air at cruise needs far less thrust, and so burns far less fuel, than it would at the same speed near the ground.
Thin air also lets jets fly fast. Narrow-body jets such as the Airbus A320 and Boeing 737 typically cruise at about Mach 0.78, while newer long-haul jets such as the A350 and Boeing 787 cruise at about Mach 0.85. Those speeds would be far more costly in fuel in the dense air at low altitude.
Reason 2: but not too thin
If thin air is good, why not fly at 50,000 feet? Because the same thin air that reduces drag also gives the wings less to work with and the engines less to breathe. Cruising altitude is usually the point where the benefit of higher speed is balanced against falling engine thrust and efficiency higher up.
Every aircraft has a ceiling. Most commercial jetliners have a certified service ceiling below 45,000 feet. The Airbus A380, for example, is listed with a service ceiling of 43,100 feet. Near its limits a plane needs a much higher true airspeed just to generate enough lift, and flying there is not economical.
Some aircraft are built to go higher. Certain business jets have service ceilings of about 51,000 feet, and the supersonic Concorde routinely flew at around 60,000 feet. Those are special designs. For an ordinary airliner, the sweet spot sits lower.
Reason 3: it is above most of the weather
According to the US National Oceanic and Atmospheric Administration (NOAA), almost all weather happens in the troposphere, the lowest layer of the atmosphere. Its depth varies with latitude:
| Location | Height of the troposphere (NOAA) | Compared with 35,000 ft (about 6.6 miles) |
|---|---|---|
| Near the poles | Just under 4 miles (6 km) | A cruising jet is well above it |
| Around 50° north or south | About 5.5 miles (9 km) | A cruising jet is above it |
| At the equator | 11 to 12 miles (18 to 20 km) | A cruising jet is still inside it |
So on many routes across the US, Canada, the UK and Europe, a jet at cruise is near or above the top of the weather layer. That is why the view from the window is so often clear sky with clouds far below. In the tropics, the weather layer reaches much higher, so a jet at the same altitude is still inside it.
It is also very cold up there. NOAA puts the temperature at the top of the troposphere at about -60°F (-51°C), and NASA’s model gives a similar figure of about -56°C from 36,000 feet upward.
Reason 4: the jet stream lives up there
Jet streams are narrow bands of strong wind that NOAA says typically occur around 30,000 feet and blow from west to east. They can exceed 275 mph and are strongest in winter.
Airlines use them to cut some flight times and fuel use. A plane flying east can ride the jet stream as a tailwind, while a plane flying west into it is slowed down. This is one reason the same route often takes less time eastbound than westbound.
Why a round number like 35,000?
Pilots do not pick any height they like. Above 18,000 feet in the US, altitudes are given as flight levels, which are pressure altitudes in hundreds of feet with every altimeter set to the same standard pressure. “Flight level 350” is a nominal 35,000 feet.
Between 29,000 and 41,000 feet, much of the world’s airspace now uses reduced vertical separation minimum (RVSM) rules, which keep aircraft 1,000 feet apart vertically instead of the older 2,000 feet. RVSM arrived over the North Atlantic in March 1997 and across the western hemisphere on January 20, 2005. It needs certified altimeters and autopilots, and it allows more planes to use the most efficient levels.
Direction matters too. US federal rules (14 CFR 91.179) set out the pattern for RVSM airspace:
- Heading broadly east (magnetic course 000 to 179 degrees): odd flight levels such as 29,000, 31,000, 33,000, 35,000, 37,000, 39,000 and 41,000 feet.
- Heading broadly west (180 to 359 degrees): even flight levels such as 30,000, 32,000, 34,000, 36,000, 38,000 and 40,000 feet.
- In controlled airspace: whatever level air traffic control assigns.
So 35,000 feet is an eastbound level. On the way back west, the same aircraft is more likely to be at 34,000 or 36,000 feet.
Why planes climb during the flight
A long-haul jet is at its heaviest at takeoff. As it burns fuel it gets lighter, and the most fuel-efficient altitude rises. A slow, continuous climb would be ideal, but it does not fit with fixed flight levels, so pilots ask air traffic control for a “step climb” instead: a jump of 1,000, 2,000 or 4,000 feet, depending on the rules for that airway.
This is why a long flight can finish its cruise several thousand feet higher than it started. Small piston and turboprop aircraft are different. Their most efficient altitude can be far lower, so step climbs generally do not apply to them.
How can you breathe at 35,000 feet?
The cabin is pressurized. US certification rules (14 CFR 25.841) require a pressurized cabin to hold a cabin pressure altitude of no more than 8,000 feet in normal operation, and the crew must get a warning if it goes above 10,000 feet. In other words, at cruise the air inside feels like being on a mountain no higher than 8,000 feet, not more than six miles up.
Newer designs go further. Airbus says its A350 has a lower cabin altitude of 6,000 feet for comfort.
What this means for you as a passenger
- Expect different flight times each way. Eastbound flights often get a jet stream tailwind. When you plan connections, use the scheduled time for your direction, not the time of the outbound leg.
- Do not worry about mid-flight climbs. A climb a few hours into a long flight is usually a planned step climb to save fuel.
- A different number is normal. If the moving map shows 32,000 or 38,000 feet, nothing is wrong. The best level depends on the aircraft, its weight, its direction and what air traffic control assigns.
- Check the moving map. Most seatback screens show the current altitude, so you can see which level your flight was given and when it steps up.
If you are getting ready for a trip, our guides to carry-on size limits on US airlines, bringing a power bank on a plane and choosing noise-cancelling headphones for flying cover the practical side.
FAQ
Do all planes fly at 35,000 feet?
No. It is a typical figure, not a rule. Airliners usually cruise somewhere between about 31,000 and 38,000 feet, and the level changes with weight, direction, winds and air traffic control instructions. Small propeller aircraft cruise much lower.
Why don’t planes fly even higher to save more fuel?
Engine thrust and efficiency fall off in very thin air, and the wings need a higher true airspeed to produce lift. Most jetliners are certified to less than 45,000 feet, and flying close to an aircraft’s limit is not economical.
Is 35,000 feet the plane’s exact height above the ground?
Not exactly. At cruise, altitude is a flight level based on a standard pressure setting of 29.92 inches of mercury that all aircraft share. It keeps planes safely separated from each other, but the true height above sea level varies a little with the weather.
Bottom line
Planes fly at around 35,000 feet because it is the efficient middle ground: the air is thin enough to cut drag sharply, yet thick enough for jet engines and wings to perform well. It keeps flights above most weather, lets airlines use the jet stream, and fits the 1,000-foot flight level system that keeps traffic apart. The number on your seatback screen is simply the best available level for that aircraft, at that weight, in that direction, on that day.
Sources
- NASA Glenn Research Center: Earth Atmosphere Model
- NASA Glenn Research Center: The Drag Equation
- NOAA JetStream: Layers of the Atmosphere
- NOAA JetStream: The Jet Stream
- 14 CFR 91.179: IFR cruising altitude or flight level (Cornell Legal Information Institute)
- 14 CFR 25.841: Pressurized cabins (Cornell Legal Information Institute)
- Airbus: A350 family
- Wikipedia: Cruise (aeronautics), Step climb, Ceiling (aeronautics), Reduced vertical separation minima, Flight level and Airbus A380
