The amount of air in two containers, even if the containers are of the same dimensions, can be different if they are stored under different pressures or temperatures. To accurately compare the amount of air in one container with another, calculate the air volume that would exist under a standard temperature and pressure. The Ideal Gas Law is the basis for this calculation. Several different standards are in use, such as 0 degrees Celsius and 100 kilopascals or 60 degrees Fahrenheit and 14.696 psi. Choose the units most relevant to your situation. By reporting air volume at standard temperature and pressure, or STP, the amount of air in a container can be reliably compared across a range of actual conditions.
Choose the standard conditions to be used in the calculations. In the United States, pressure is most often measured in psi, but temperature can be measured with either Celsius or Fahrenheit. For this example, standard conditions of 60 degrees Fahrenheit and atmospheric pressure, 14.696 psi, will be used.
Use a pressure gauge and thermometer to measure the actual conditions of your air sample so that you know the pressure, temperature and volume of the air. This calculation will assume a pressure of 3000 psi, a temperature of 75 degrees Fahrenheit and a container volume of 0.4 cubic feet.
Convert your volume at actual conditions to what the volume would be at the standard pressure, assuming the temperature would remain constant. The Ideal Gas Law, rearranged in a form known as Boyle's Law, says the volume at a new pressure is equal to the initial pressure multiplied by the initial volume and divided by the new pressure. Calculate the new volume by multiplying 3000 by 0.4, then divide the result by 14.696, which yields a result of 81.7 cubic feet of standard air at the actual temperature of 75 degrees Fahrenheit.
Convert your volume at actual temperature and standard pressure to what the volume would be at the standard temperature. The Ideal Gas Law, rearranged as Charles' Law, says that the volume at a new temperature is equal to the initial volume divided by the initial temperature and multiplied by the new temperature. To calculate the air volume at standard temperature and pressure, divide 81.7 by 75 and then multiply the result by 60; this gives you 65.3 cubic feet of air at the standard temperature and pressure of 60 degrees Fahrenheit and 14.696 psi.
TL;DR (Too Long; Didn't Read)
Always reference which standard pressure and standard temperature are being used. This gives people looking at your volumes enough information to easily convert that volume to a different standard, if they need to.
Real gases do not behave ideally at high pressures. To accurately use the Ideal Gas Law at high pressures, compressibility factors should be used whenever they are known.