Convert pressure in psi to atm, the international standard unit of pressure, keeping in mind that the atmosphere exerts a constant pressure of 14.696 psi at sea level -- in other words, one atm is equal to 14.696 psi. If the psi reading, for example, is 10.05, figure this as follows: 10.05 divided by 14.696 = .684 atm.
Extrapolate altitude, keeping in mind the equation P = 0.9877^a. To counteract the exponent "^," take the logarithm of the pressure. Enter pressure on a scientific calculator, then hit the "log" key. For the example of pressure, this value is -0.165. To counteract the fact that .9877 is slightly less than one, divide the logarithm by -0.012, for the example pressure, this is -0.165 divided by -0.012, or 13.75 hundred meters.
Convert altitude to the units of choice. The example pressure occurred at a height of 13.75 hundred meters --13.75 times 100 meters, or 1,375 meters. Multiply the initial value by 100 to obtain the result in meters. To convert the resultant value to feet, multiply by 3.28 -- 1,375 times 3.28 = 4,511 feet.