In 1898, Wallace C. Sabine (Sabin) (1868 - 1919) came up with the formula in
"Collected Papers on Acoustics" appeared in print 1922:

with k = (24 · ln 10) / c₂₀ = 0.161 (metric)

V = room volume in m³    A = α · S = equivalent absorption surface or area 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 which can be found in any book that talks about room acoustics.