The equation for reverberation time is RT(60) = .05V/(SA). RT is the reverberation time it takes sound to decrease 60 decibels. Ideally, this should be around half a second. Enter the appropriate numbers into this equation.
V stands for total room volume. Measure the space and calculate its volume. For a square or rectangular space, this is length times width times height.
S stands for surface area. This is the area of each different type of surface in the room, such as concrete, wood or brick. Calculate the surface area for each of these surfaces.
A stands for absorption coefficient. This expresses how sound-absorbent each type of material you found the surface area for is. A list of absorption coefficients can be found at many different websites.
Multiply each surface area (S) by its absorption coefficient (A). Add these all together to find the total SA. Solve the equation.