Read your indicated airspeed (IAS) on your airspeed indicator. (In this case, we assume that the corrected airspeed (CAS) and the IAS are the same.) Read your altitude above Mean Sea Level (MSL) on your altimeter, based on the proper altimeter setting. Mathematically increase your indicated airspeed (IAS) by 2% per thousand feet of altitude to obtain the true airspeed (TAS).
For example, the indicated airspeed (IAS) of my Comanche at 8,500 ft. MSL is 170 knots. 2% of 170 kt. = 3.4 kt. (Depending on where you are, the temperature estimation can vary from about 1.5% to 2.5%, most often between 1.8% and 2.2%, but the 2% is a quick and easy way to do this calculation mentally while in your aircraft.) If you require a closer estimation (yes, it still an estimate), use the OAT formula and CAS correction for your aircraft.
The calculated (closely estimated) KTAS at 8,500 ft. MSL is 170 kts. + (8 x 3.4 kts.), or 197.2 knots. This is based on the 2% factor for OAT estimation.
We have entered the above hypothetical values as our defaults in the calculator, however, you may use any values you wish. Our calculator does the same calculation for you based on information entered. Enter the IAS (in either mph or knots) and the MSL altitude. If you wish, you may change the estimation percentage for the OAT addition to the IAS. Be sure that the entry is a percentage and base it on the percentage range given previously. Click on Calculate and the TAS (or KTAS) will be returned as whatever value you entered, either mph or knots. Our calculator rounds at 3 decimal places, in this case 197.200 knots. Keep 'em high and level! You can use our Distance and Speed Converter if you need to know the conversion between knots and miles per hour (or kilometers per hour).