Root Mean Square Speed of Gas Calculator. Online chemistry calculator to calculate root mean square (RMS) speed of gas, using gas molecular mass value.The root-mean-square speed (u), is the square root of the average speeds of the molecules in a sample of gas at a specific temperature and pressure. Or, solving for what we call the root mean square velocity, that better be equal to the square root of 3RT over M. That kinetic theory made a prediction for what the velocity ought to be. It took, of course, another hundred years before somebody could measure that. And, of course, it is correct.