it really depends on the AMP. But usually you can get a pretty good estimate by diviting the Max power which is usually put on the front and divide that by 2. For instance a 1400W amp would be around 700W RMS. BUT, this is just an estimate. Companies like Sony, pioneer, Audiobahn, ect will over-rate there amps. So a Sony 1400W is most likey about 3to 400W RMS. But compines like JL, RF, and others under rate for example a 2400W Rockford AMP is probably putting out about 900W RMS insted of 800W.