How many litres a minute for a good shower? Flow rate, shown in gallons per minute (GPM), measures how much water flows through a shower head. However, if you opt for a customized system with multi heads the flow rate can go up to 15 GPM or more. Standard showerheads have a rate of 2.5 GPM. The self-cleaning plunger inside the shower head helps to prevent the buildup from hard water so you don’t have to worry about clogging.