HJ hit the nail on the head the subject has never come up. Please explain what you are trying to do , we may be able to help.: I dont think the subject has ever come up, and I am also not sure if there is a correlation. The standard fixture unit is a lavatory, and the gpm is a function of the faucet flow rate, whereas the fixture unit is the drain rate. A bathtub can be rated as1 1/2 or 2 depending on the jurisdiction and a washing machine is usually 2. A water closet is 6 and that has not changed even though the gpf has gone from 6 down to 1.5.: : does anyone know how to convert gpm to fixture units?