How do I calculate gallons per minute?

Measure the volume that flows out (in gallons) over a timed interval (in minutes), then divide volume by time: GPM = gallons / minutes. Timing 30 seconds and doubling the result is a common shortcut for a quick fixture check.

How many GPM does a shower or garden hose use?

US EPA WaterSense-labelled showerheads are capped at 2.0 GPM, with older or standard showerheads running around 2.5 GPM; a typical garden hose delivers roughly 9-17 GPM depending on its diameter and the supply pressure.

What is the WELS rating and how does it relate to flow rate?

Australia's Water Efficiency Labelling Scheme (WELS) rates showerheads, taps and other fixtures from 0 to 6 stars based on their flow rate in litres per minute -- a higher star rating means a lower flow rate and better water efficiency, the opposite direction to how star ratings usually work for performance.

Gallons Per Minute (GPM) to Litres Per Minute Calculator

Gallons Per Minute (GPM) Calculator: Flow Rate, Fill Time and Volume

GPM -- gallons per minute -- is how flow rate is usually described for showers, garden hoses, faucets and pumps. This calculator solves any one of volume, time or flow rate from the other two, so you can work out how long a tank takes to fill at a known flow rate, or the flow rate of a fixture from how long it took to fill a container of known volume. Enter the two values you know, in whichever unit you have them (US or UK gallons, litres, cubic feet or metres), and read off the third.

How to use the GPM calculator

Pick what you want to solve for -- flow rate, volume, or time -- by leaving that field blank.
Enter the other two values: volume (in gallons, litres, cubic feet or cubic metres) and time (seconds, minutes or hours), or volume and flow rate, or time and flow rate.
Optionally use a fixture preset -- low-flow shower, standard shower, garden hose, kitchen faucet, bathtub -- to fill in a typical flow rate instead of measuring your own.
Read off the solved value, converted automatically between US gallons, UK gallons, litres and other flow units.

The GPM formula, explained

Flow rate = volume / time. Rearranged, that also gives volume = flow rate x time, and time = volume / flow rate -- the same relationship solved for whichever quantity is missing.

Worked example: a bathtub holds 40 US gallons and takes 20 minutes to fill. Flow rate = 40 / 20 = 2 GPM. The same tub at a stronger 4 GPM flow would fill in 40 / 4 = 10 minutes instead.

Tips

As a typical Australian example, having a licensed plumber diagnose low water pressure or a low flow rate at a fixture typically costs around AUD 110-400 for the callout and assessment.

GPM measures flow rate, not water pressure (PSI) -- a low-pressure line can still deliver a high flow rate through a wide pipe, and vice versa, so don't use the two interchangeably.
1 US GPM = 3.785 litres per minute; 1 UK (imperial) GPM = 4.546 litres per minute -- check which gallon a fixture's spec sheet uses before comparing figures.