Gallons Per Minute (GPM) Calculator
Enter any two of the three variables (flow rate, volume, time) and the calculator solves for the third. Switch between US gallons, litres, cubic feet, and more. Results are shown in GPM plus seven other common flow-rate units. A fill/drain time chart updates live so you can see how long any tank will take at your flow rate.
What is GPM and why does it matter?
GPM stands for gallons per minute and is the standard US unit for volumetric flow rate in plumbing, irrigation, firefighting, and HVAC. It describes how much liquid passes a fixed point in one minute. A higher GPM means more water delivered in the same time, which affects fill times for tanks and pools, how many fixtures or sprinkler zones a pump can serve at once, pipe sizing (velocity must stay below roughly 5 ft/s to prevent noise and erosion), and energy use (pumps consume more power to push higher flow rates). Knowing your system GPM lets you size pipes correctly, check whether a pump is adequate, and diagnose low-pressure problems before spending money on equipment.
The GPM formula and how to use it
The fundamental relationship is: GPM = Volume (US gallons) / Time (minutes). Rearranged, Volume = GPM x Time and Time = Volume / GPM. This calculator handles all three modes. To measure GPM in the field, you can use the bucket-and-stopwatch method: hold a measured container (1 gallon works well) under the flow, time how long it takes to fill, then divide 60 by the number of seconds (or just enter volume = 1 gallon and time = the seconds you measured, switching the time unit to "seconds"). For a 1-gallon bucket that fills in 6 seconds, the flow rate is 60 / 6 = 10 GPM. For larger systems such as an irrigation main or a pump outlet, you can time how long it takes to fill a known tank instead.
GPM and pipe capacity: a quick sizing guide
Flow rate and pipe diameter are closely linked because velocity = flow rate / cross-sectional area. Most plumbing codes recommend keeping water velocity below 5 feet per second (ft/s) for cold water and 4 ft/s for hot, to limit noise, erosion, and water-hammer risk. As a rule of thumb: a 1/2-inch pipe handles roughly 2-4 GPM, a 3/4-inch pipe 4-8 GPM, a 1-inch pipe 8-15 GPM, and a 1.5-inch pipe 15-30 GPM. These figures assume standard residential pressure (40-80 psi) and schedule-40 PVC or copper. For precise sizing, use a pipe-flow calculator that accounts for pipe material, friction factor, and length.
Converting GPM to other flow units
Engineers and contractors often need flow in units other than GPM. The key conversion factors are: 1 US GPM = 3.785 L/min = 0.06309 L/s = 0.2271 m³/hr = 60 US gallons per hour (GPH) = 0.1337 ft³/min (CFM) = 0.8327 UK (imperial) gallons per minute. This calculator shows all eight simultaneously so you do not have to do the arithmetic yourself. Note that US and UK gallons differ by about 20 percent (1 UK gallon = 1.20095 US gallons), so always confirm which standard a specification or fixture rating uses.
Typical GPM by fixture or application
| Application | Typical GPM | Notes |
|---|---|---|
| WaterSense showerhead | 2.0 | EPA-certified low-flow |
| Standard showerhead | 2.5 | US federal max since 1992 |
| Bathroom faucet aerator | 0.5 - 1.5 | WaterSense max 1.5 GPM |
| Kitchen faucet | 1.5 - 2.2 | US federal max 2.2 GPM |
| Washing machine (fill) | 3 - 5 | Per fill cycle |
| Garden hose (3/4") | 9 - 17 | Depends on pressure |
| Bathtub spout | 4 - 8 | No federal standard |
| Fire sprinkler head | 10 - 26 | NFPA 13 design basis |
| Irrigation sprinkler (per head) | 0.5 - 3 | Rotary vs. fixed-spray |
| Residential well pump (small) | 5 - 10 | Shallow-well pumps |
| Residential well pump (deep) | 10 - 25 | Submersible pumps |
| Commercial booster pump | 50 - 200 | Varies by building size |
Reference flow rates for common residential and commercial uses. Actual values vary by model and pressure.
Frequently asked questions
How do I measure the GPM of my garden hose or faucet?
Use the bucket-and-stopwatch method. Hold a 1-gallon container under the flow and time how many seconds it takes to fill completely. Then set this calculator to "Solve for GPM", enter Volume = 1 US gallon and Time = the number of seconds (selecting "Seconds" as the unit). For example, if it takes 8 seconds, your flow rate is 60 / 8 = 7.5 GPM. For taps, you can also use a 2-litre bottle and convert: set volume to 2 litres and time to the seconds it took.
How long will it take to fill a swimming pool?
Use the "Solve for time" mode. Enter your pool volume in gallons (or litres) and your measured flow rate in GPM. The calculator gives you the fill time in minutes and hours. A typical garden hose delivers 9-17 GPM. A 15,000-gallon pool at 10 GPM takes 15,000 / 10 = 1,500 minutes, or 25 hours. Using a faster water source such as a 2-inch supply line at 50 GPM would cut that to 5 hours.
What does WaterSense GPM rating mean?
WaterSense is an EPA labelling program for water-efficient fixtures. A WaterSense showerhead is certified to use no more than 2.0 GPM (down from the 2.5 GPM federal maximum), and a WaterSense bathroom faucet aerator uses no more than 1.5 GPM. If you replace a 2.5 GPM showerhead with a 2.0 GPM model, the difference of 0.5 GPM adds up to 30 gallons over a 10-minute shower.
What is the difference between GPM and GPH?
GPM (gallons per minute) and GPH (gallons per hour) measure the same thing at different time scales. 1 GPM = 60 GPH. Fixtures are rated in GPM because typical usage lasts seconds to minutes, while irrigation controllers, hot-water heaters, and boilers often use GPH because they operate over longer cycles. This calculator shows both at the same time.
What is the difference between US gallons and UK (imperial) gallons?
A US liquid gallon is exactly 3.785411784 litres. A UK imperial gallon is exactly 4.54609 litres, roughly 20 percent larger. Flow-rate specifications in the United States almost always mean US gallons, but equipment imported from the UK or Commonwealth countries may use imperial gallons. This calculator offers both: select "UK (imperial) gallons" from the volume-unit menu or choose "UK gal/min" from the display-unit selector.