How to calculate GPM
GPM (gallons per minute) is the volume of water divided by the time it took: GPM = gallons ÷ minutes. If a tap fills a 50-gallon drum in 10 minutes, the flow rate is 50 ÷ 10 = 5 GPM — equal to 300 gallons per hour.
How to use this calculator
Enter the gallons collected and the time in minutes. The result shows GPM and gallons per hour. A quick field method: time how long a known container takes to fill, then divide. For ratings quoted in metric units, convert them with the liters to gallons calculator.
Why flow rate matters
GPM sizes pumps, irrigation, and fixtures, and it's used to compare well output or hose performance. For the volume held inside a pipe rather than the flow through it, use the pipe volume calculator; for airflow, the CFM calculator.
Flow and pressure are not the same thing: a system can hold good pressure at rest and still deliver poor GPM if the bore is narrow or the run is long. When testing a well or a mains supply, take the reading with only one fixture open, then repeat it after several minutes. A source that starts strong and fades is telling you about its recovery rate rather than its peak flow.
Frequently asked questions
- How do I calculate gallons per minute?
- Divide the gallons by the time in minutes. 50 gallons in 10 minutes is 5 GPM.
- How do I measure GPM at home?
- Time how long it takes to fill a container of known size, then divide the gallons by the minutes. A 5-gallon bucket filled in 30 seconds is 10 GPM.
- What is a good water flow rate?
- Home fixtures range widely; a typical shower is about 2–2.5 GPM, and a garden hose is roughly 9–17 GPM depending on pressure.
- How do I convert GPM to gallons per hour?
- Multiply GPM by 60. 5 GPM is 300 gallons per hour.
- How do I convert GPM to litres per minute?
- Multiply GPM by 3.785. A 5 GPM tap delivers about 18.9 litres per minute, which is how most non-US fixtures and pumps are rated.