True airspeed (TAS) is your actual speed through the air: calibrated airspeed corrected for altitude and nonstandard temperature. Because the air thins as you climb, true airspeed runs higher than the airspeed indicator shows, by roughly 2 percent for every 1,000 feet. Use the calculator below to find your TAS, then learn the formula, the rule of thumb, and the E6B method you will use on the knowledge test.
Part of our Aircraft Performance guide. Need pressure altitude first? See pressure altitude, then come back.
The four airspeeds: IAS, CAS, TAS and ground speed
Pilots use four different airspeeds, and mixing them up is a classic knowledge test trap. Each one is the one before it with one more correction applied.
| Airspeed | What it is | Where you get it |
|---|---|---|
| Indicated airspeed (IAS) | The number on the airspeed indicator | Read it directly. V-speeds and airspeed limits in your POH are indicated airspeeds. |
| Calibrated airspeed (CAS) | IAS corrected for installation and instrument error | The airspeed calibration table in your POH. The error is largest at low airspeeds; in cruise, IAS and CAS are nearly the same. |
| True airspeed (TAS) | CAS corrected for altitude and nonstandard temperature | A flight computer, this calculator, or the 2 percent rule of thumb. TAS is the speed you plan with and file in a flight plan. |
| Ground speed (GS) | TAS adjusted for wind | The wind triangle before the flight, or GPS during it. A headwind lowers it and a tailwind raises it. |
The airspeed indicator works by comparing ram air pressure from the pitot tube with static pressure (see how the pitot-static system works). Thinner air at altitude produces less ram pressure at the same speed, so the needle reads low compared with your real speed through the air. True airspeed undoes that effect.
How to calculate true airspeed
The formula
True airspeed equals calibrated airspeed divided by the square root of the density ratio, where the density ratio is today’s air density at your altitude divided by standard sea level density. You find the density ratio from your pressure altitude and the outside air temperature, which is exactly what the calculator above and every flight computer do behind the scenes. It is the same air density that drives density altitude, so a high density altitude always means a true airspeed well above your calibrated airspeed.
The 2 percent rule of thumb
The FAA’s Pilot’s Handbook of Aeronautical Knowledge gives a quick estimate: add 2 percent of your calibrated airspeed for each 1,000 feet of altitude. At 110 knots and 6,500 feet that is 110 plus 13 percent, or about 124 knots. The rule ignores temperature and usually runs a few knots high at training altitudes, so it is a sanity check, not a planning number. The handbook calls the flight computer the more accurate method.
A worked example
You plan to cruise at 6,500 feet with a calibrated airspeed of 110 knots, and the forecast temperature at that altitude is plus 2 degrees Celsius, almost exactly standard. The calculator above opens with this example:
- Pressure altitude: 6,500 feet (altimeter set to 29.92).
- True airspeed: about 121 knots, 11 knots more than the airspeed indicator shows.
- Rule of thumb: about 124 knots, a few knots high, as expected.
Now change only the temperature to plus 25 degrees Celsius, a hot summer afternoon. True airspeed rises to about 126 knots at the same indicated airspeed, because the warm air is less dense. That 5 knot swing on a long cross-country adds up in time and fuel, which is why you plan with the forecast temperature, not a guess.
Check your plan in flight: once you level off, compare your GPS ground speed with the ground speed you planned. A big difference usually means the wind is not what was forecast, and it is a cue to update your times and fuel before the next checkpoint.
How to find true airspeed on an E6B
The manual E6B finds TAS on its calculator (slide rule) side, not the wind side. Here is the method:
- Find your pressure altitude: set 29.92 in the altimeter and read it, or calculate it from the field elevation and altimeter setting.
- Read the outside air temperature in degrees Celsius.
- In the airspeed correction window, line up the pressure altitude (in thousands of feet) with the outside air temperature.
- Find your calibrated airspeed on the inner scale and read true airspeed on the outer scale directly above it.
For the example above, set 6,500 feet against plus 2 degrees, find 110 on the inner scale, and read about 121 knots on the outer scale. The same window setting also gives you density altitude, so you can work both in one step. For more practice, see our step-by-step guide to using an E6B and CX-3.
On a CX-3
The ASA CX-3 electronic flight computer has a true airspeed function: enter pressure altitude, outside air temperature and calibrated airspeed, and it returns TAS along with density altitude. It is faster and removes reading errors, but learn the wheel too so you can spot a typo. Both are allowed at the FAA knowledge test, while phones and tablets are not. If you are deciding which one to buy, our E6B guide compares them.
Why true airspeed matters
- Flight planning: TAS goes into the wind triangle to give you heading and ground speed, and ground speed gives you time en route and fuel. Our wind correction angle calculator takes your TAS from here and finishes the job. See how to plan a cross-country flight for the whole process.
- Performance charts: POH cruise tables list true airspeed (KTAS) for each altitude and power setting, so you can plan with them directly.
- High, hot airports: you fly the same indicated approach speed everywhere, but at a high density altitude your true airspeed, and therefore your ground speed, is higher. Expect a faster-looking approach and a longer landing roll, and check the takeoff and landing performance numbers.
Want every flight computer function in one browser tab, from true airspeed to the wind triangle and fuel? Try PilotMall’s free online E6B flight computer.
What you'll need
Flight computers for finding true airspeed in training and on the knowledge test, from PilotMall.com.
Frequently asked questions
What is true airspeed?
True airspeed is the actual speed of the airplane through the air. It is calibrated airspeed corrected for altitude and nonstandard temperature, and it is the speed you use for flight planning and in a flight plan.
How do you calculate true airspeed?
Divide calibrated airspeed by the square root of the density ratio, or let a flight computer do it: enter pressure altitude, outside air temperature and calibrated airspeed. For a quick estimate, add 2 percent of calibrated airspeed for every 1,000 feet of altitude.
Is true airspeed higher than indicated airspeed?
Above sea level, almost always. Thinner air makes the airspeed indicator read less than your real speed through the air, so at 6,500 feet a calibrated airspeed of 110 knots is about 121 knots true on a standard day. At sea level on a standard day they are about the same.
What is the 2 percent rule for true airspeed?
Add 2 percent of your calibrated airspeed for every 1,000 feet of altitude. It comes from the FAA’s Pilot’s Handbook of Aeronautical Knowledge, ignores temperature, and usually runs a few knots high at training altitudes, so use a flight computer for planning.
Should I plan with true airspeed or indicated airspeed?
Plan with true airspeed: ground speed, time en route and fuel all come from TAS corrected for wind. Fly the airplane by indicated airspeed, because V-speeds and airspeed limits are indicated airspeeds.