Wallace C. Sabine (Sabin) (1868 - 1919) came up with the formula in "Collected Papers on Acoustics" appeared in print 1922:
RT60 = k × V / A
with k = (24 × ln 10) / c20 = 0,161 (metric)
V = room volume in m3A = α × S = equivalent absorption surface in m2
RT60 = reverberation time in s S = absorbing surface in m2 A = α1×S1 + α2×S2 + α3×S3 + . . . c20 = 343 m/s at 20°C
RT60 is the reverberation time, the sound pressure level of a signal needs to decay to the value of (-) 60 dB.
k is a constant that equals 0.161 when the units of measurement are expressed in meters and 0.049 when units are expressed in feet.
A = S × α is the total surface absorption A of a room expressed in sabins. It is the sum of all the surface areas in the room multiplied by their respective absorption coefficients. The absorption coefficients α express the absorption factor of materials at given frequencies.
V is the volume of the enclosure. The figures used in this calculator come from the table of absorption coefficients for various materials.