RT60 reverb time calculator.

The Sabine reverberation formula

RT60 is the time for sound to fall 60 dB after the source stops. Wallace Sabine found empirically that it depends on the room volume V and its total absorption A, measured in sabins: RT60 = 0.161 x V / A in metric units (V in cubic metres, A in square metres of absorption), where A = sum of (surface area x absorption coefficient). Wikipedia gives the constant as about 0.1611 s/m, and HyperPhysics uses 0.161 in its own reverberation calculator. In feet the constant works out to about 0.049 by unit conversion; this page converts feet to metres internally so you need not.

Worked example. A 6 m x 4 m room with a 2.7 m ceiling has a volume of 64.8 m3 and a surface area of 102 m2. With walls at 0.05, ceiling at 0.05 and floor at 0.1, the total absorption is 6.3 sabins and RT60 = 0.161 x 64.8 / 6.3 = 1.66 seconds. To bring that to 0.5 s the room needs 0.161 x 64.8 / 0.5 = 20.9 sabins, about 14.6 more.

The formula assumes sound is spread evenly through the room, which fits large, fairly reflective spaces best. Wikipedia notes that Eyring's equation was proposed to better estimate small, heavily absorbing ("dead") rooms. Coefficients vary with frequency, so a single number is a rough average. Absorption values for your materials are not on this page: take them from the product data sheet.