One gallon of fresh water  =  8.33 Lbs. 
One cubic foot of water  =  7.48 Gallons 
One cubic foot of water  =  62.4 Lbs. 
One U.S. Gallon  =  231 Cubic inches 
%Efficiency  =  GPH x 8.33 x Temp. Rise
BTU/Hr. Input 

BTU Input  =  GPH x 8.33 x Temp. Rise
%Efficiency* 
*%Efficiency for gas is 76% (as of January 1, 1994) Electric is 100% 

BTU Output  =  GPH x 8.33 Lbs/Gal. x Temp. Rise  
kW  =  GPH x 8.33 x Temp. Rise
3413 
or  GPH x Temp. Rise
410 
GPH  =  kW x 3413
Temp. Rise x 8.33 
or  kW x 410
Temp. Rise 
Rise (°F)  =  kW x 3413
GPH x 8.33 
or  kW x 410
GPH 
1 kW  =  3413 BTU  =  4.1 GPH at 100° Temp. Rise 
1 kW  =  1000 Watts  BTU x 0.293 = Watts  
1 BTU  =  Amount of heat required to raise the temperature of 1Lb of water 1° 