For TDM ,there exists for formula to calculate bandwidth...for 'm' channels 'n ' number of bits ,the expression is 2fmn that 2 is for sampling....
similarly is there formula for finding bandwidth for FDM......
The bandwidth required for a FDM system varies from that in a TDM system in several regards. In TDM you can calculate the total bandwidth used with a simple equation as you state in your question. For FDM the total bandwidth is usually channel specific and will be specified. Calculating the amount of BW needed by any one component of the FDM system is no different than calculating the bandwidth for a that single signal as if it where traveling all by itself over the transmission medium. An FDM system simply shifts the signals to ride on a higher carrier wave instead of letting them travel as baseband data. A TDM system allows a small portion of each signal to be sent over the same medium at a higher rate than the signal would normally be transmitted.