Using RMS level - (root mean squared) - is a very loose (inaccurate) way of measuring perceived loudness. A statistical measurement of the magnitude of a varying quantity.
To complicate matters more there are a couple different standards to measure rms. One uses the sine wave (aes - 17) for calibration and most daw meters use the square wave.
There is peak rms and there average rms and then there is the dynamic range "crest factor" which is the difference of average peak levels and average rms levels.uare wave. These are 3.01 dB off from each other. There are also different weightings.
One song can have a higher rms than another and the lower numbered one can still be perceived to be louder.
In the end, imo/e rms level won't tell you much at all about perceived loudness, only your ears do.