A comprehensive theoretical analysis was made on the conversion methods of efficiency characteristics between prototype and model pumps and pumping installations. Most efficiency conversion formulas existed did not analyze the theory sufficiently, and the conversion precision is not adequate. The formula recommended by JIS B 8327—2002 and the new formula recommended are the ones expressed more detailedly. But there are too many empirical constants in former one. The formulas of efficiency conversion recommended are theoretical ones, and are applicable not only to the pumps but also to the pump installations. Quantitative expression for the influence of variety of factors upon the results of efficiency conversion can also be achieved by the formula.