This manual on control valve sizing brings together the mathematical tools required to select Durco valves properly for control valve applications. The equations presented for liquids, gases, and steam are based on the ISA standard S75.01 and are divided into sections to simplify manual calculation for the more common sizing problems. Examples of each type are presented for further comprehension. The selection of a correct valve size as mathematically determined depends on accurate knowledge of the actual flowing data. Frequently, one or more of the operating conditions are arbitrarily assumed. Most errors in control valve sizing are due to incorrect assumptions. Generally speaking, the tendency is to make the valve too large to be on the “safe side”. Combining these so called “safety factors” can result in a valve which is oversized and one which contributes to poor control and system instability. There is no substitute for good engineering judgement. Only good common sense combined with experience can bring forth an acceptable solution in valve sizing. Control valve applications vary in degree from simple to complex. On occasion, guidance and assistance in selecting the proper control valve may be required.