TDH = PSI x 2.31 / S. G. or TDH = PSI x 144 / W TDH = Total dynamic head expressed in feet (total discharge head minus the total suction head). PSI = TDH x S.G / 2.31 or PSI = TDH x W / 144 PSI = Pounds per square inch (total discharge pressure minus the total suction pressure). BHP = Q x TDH x S.G. / 3960 x EFF or BHP = GPM x PSI / 1714 x EFF BHP = Brake Horsepower. S.G. = Specific Gravity of fluid. W = Specific weight in pounds per cubic foot of fluid (For example water W=62.32 lb per cu ft@68 degrees F).   GPM = Gallons per minute of flow EFF = Efficiency of the pump