GPM Calculator
Measure real flow with a bucket and a stopwatch. Enter the container size in gallons, quarts, or liters and the seconds it took to fill, and get gallons per minute, gallons per hour, and liters per minute.
Example: with Container size 5 · Container unit US gallons · Time to fill (seconds) 30 → Flow rate: 10 GPM.
- Per hour600 gal/h
- Metric37.9 L/min
Computed by the calculator below using its default values. Change any input to see your own numbers.
GPM = gallons ÷ seconds × 60. A 5-gallon bucket filling in 30 seconds is 10 GPM — strong flow for a residential line.
The bucket test
The most reliable way to measure flow at a spigot, well, or pump discharge is also the simplest: time how long a known container takes to fill, then scale to a minute. GPM = volume ÷ seconds × 60. A 5-gallon bucket filling in 30 seconds means 10 gallons per minute; the same bucket taking 75 seconds means 4 GPM. Open the valve fully, let flow stabilize for a few seconds, and start timing with the container already under the stream.
For low-flow fixtures like showerheads, a smaller container timed precisely beats a big bucket timed sloppily — two quarts filling in 5 seconds is a clean 6 GPM reading.
What the number is used for
Flow rate drives real decisions: irrigation zones are designed around available GPM, tankless water heaters are rated by the GPM they can lift to temperature, and well systems are judged by sustained yield. A common benchmark is that 5 GPM sustained supports an average household, though storage tanks let weaker wells work fine. Remember that flow at one fixture with everything else closed is the best case — simultaneous demands share it.
How it’s calculated
GPM = container volume (US gal) ÷ fill time (s) × 60. Quarts convert at 4 qt/gal; liters at 3.785411784 L/gal (exact NIST factor). Gal/h = GPM × 60; L/min = GPM × 3.785411784.
A bucket test measures flow at that fixture and pressure at that moment — flow drops when other fixtures run, and well yield can fall during sustained draw.
Typical fixture flow rates
| Fixture | Flow |
|---|---|
| Bathroom faucet (WaterSense) | 1.5 GPM max |
| Showerhead (federal max) | 2.5 GPM |
| Showerhead (WaterSense) | 2.0 GPM max |
| Kitchen faucet (federal max) | 2.2 GPM |
| Garden hose, 5/8 in | 10–17 GPM, pressure-dependent |
US federal fixture maximums and EPA WaterSense specifications; hose flow varies with supply pressure and length.
Common mistakes
- Timing with the valve partially open or before flow stabilizes, which understates true capacity.
- Using a big bucket for a weak flow: a ±1 second stopwatch error on a 10-second fill is a 10% error.
- Confusing quarts and gallons — a 2-quart pitcher filling in 5 seconds is 6 GPM, not 24.
- Treating a single-fixture reading as whole-house capacity while other fixtures are running.
Frequently asked questions
What is the GPM formula?
GPM = gallons collected ÷ seconds elapsed × 60. Any container works if you know its volume: quarts divide by 4, liters divide by 3.785.
How many GPM is a 5-gallon bucket in 30 seconds?
Exactly 10 GPM. Each 6-second-per-gallon pace equals 10 GPM, so a 5-gallon bucket in 60 seconds would be 5 GPM.
What is a good GPM for a well?
5 GPM sustained is the common benchmark for a typical household without storage; 10+ is comfortable. Weaker wells are workable with a storage tank sized to buffer peak demand.
Is GPM the same as PSI?
No — PSI is pressure, GPM is flow. They interact (more pressure usually pushes more flow through the same pipe), but a high-pressure line can still be flow-starved by a narrow or clogged pipe. Measure flow directly with the bucket test.