The Java source code of this applet and its documentation are freely available.
RT60 = k * (V/Sa)
k is a constant that equals 0.161 when the units of measurement are expressed in meters and 0.049 when in feet. V is the volume of a room, Sa is the total surface absorption of a room which is a sum of all the surface areas multiplied by their repective absorption coefficients. The absorption coefficients can be obtained from the Table of absorption coefficients.