How to calculate GPM (water flow rate)
GPM = (gallons ÷ seconds) × 60. Fully open the tap, time how long a container of known size takes to fill, then divide the volume by the seconds and multiply by 60. Example: a 5-gallon bucket that fills in 30 seconds is (5 ÷ 30) × 60 = 10 GPM.
Prefer to skip the math? Use the GPM calculator → Enter the container size and seconds and it returns GPM and litres per minute.
The fill-time method, step by step
- Grab a container you know the volume of (a labelled bucket or jug).
- Open the tap or fixture fully.
- Start timing the moment water hits the container; stop when it's full.
- Flow rate = (volume ÷ seconds) × 60. Use gallons for GPM, litres for L/min.
Worked examples with real numbers
The formula handles any container and any timing — here are the three versions of it you'll actually use on a job, worked all the way through.
A container you know: a 5-gallon bucket under an outdoor tap fills in 45 seconds. (5 ÷ 45) × 60 = 6.7 GPM. That's a healthy outdoor flow — most garden taps land somewhere in the 6–12 GPM band.
A small jug, metric: a 2-litre jug at the kitchen tap fills in 14 seconds. (2 ÷ 14) × 60 = 8.6 L/min, which is 8.6 ÷ 3.785 = 2.3 GPM. Small containers work fine — they just reward careful timing, because a half-second of error matters more over 14 seconds than over 45.
From GPH: a pump is rated 300 gallons per hour. There are 60 minutes in an hour, so 300 ÷ 60 = 5 GPM. Pump and filter spec sheets mix GPM and GPH freely; dividing or multiplying by 60 moves between them, and the conversion is exact.
If the number comes back low
A measured flow well below what a fixture should deliver usually has an ordinary cause, and they're worth checking in this order because the first ones are free. A clogged aerator (the little screen on the tap outlet) is the most common culprit — unscrew it and look. Next, a supply valve under the sink that was never fully reopened after a repair. Then a flow restrictor doing exactly its job in a shower head. Only after those does it point to something bigger: scale build-up in older pipework, a pressure-reducing valve set low, or genuinely low supply pressure — which is worth measuring before you blame the pipes.
The fill-time test is also the standard quick check on a private well: run the test at a tap close to the pressure tank and you're measuring what the pump system actually delivers, which is the number that matters when sizing anything fed by it. One honest caveat about the method overall: it measures flow at the moment you test, with one outlet open. Open a second fixture and the available flow divides between them — which is exactly why whole-house demand is estimated by adding up fixtures, not by testing one tap.
Typical fixture flow rates
General reference figures — your actual fixtures may vary by model and pressure.
| Fixture | GPM | L/min |
|---|---|---|
| Bathroom faucet | 1.5 | 5.7 |
| Kitchen faucet | 1.5–2.2 | 5.7–8.3 |
| Shower head (modern) | 1.5–2.5 | 5.7–9.5 |
| Toilet (refill) | ~3 | ~11 |
| Dishwasher | 1–2 | 3.8–7.6 |
| Washing machine | 3–5 | 11–19 |
| Garden hose (½–¾") | 6–12 | 23–45 |
| Whole house (typical demand) | 6–12 | 23–45 |
Common questions
- What does GPM stand for?
- Gallons per minute — the volume of water that flows in one minute. Litres per minute (L/min) is the metric equivalent: 1 US GPM = 3.785 L/min.
- How do you measure GPM without a flow meter?
- The fill-time test: fully open the tap, time how many seconds it takes to fill a container of known volume, then GPM = (gallons ÷ seconds) × 60. No meter needed.
- How many GPM is 5 gallons in 45 seconds?
- (5 ÷ 45) × 60 = 6.7 GPM (about 25 L/min). The same three-step formula works for any container and any fill time.
- How do you calculate GPM from GPH?
- Divide by 60. GPH is gallons per hour, and there are 60 minutes in an hour, so 300 GPH ÷ 60 = 5 GPM. Going the other way, multiply GPM by 60 to get GPH.
- What is 1 GPM equal to?
- 1 US GPM = 3.785 litres per minute = 60 gallons per hour = 0.134 cubic feet per minute. (An imperial UK gallon is larger: 1 imperial GPM = 4.546 L/min.)
- Why does flow rate matter?
- It tells you whether a fixture, pump or supply line can keep up with demand — sizing a pump, choosing a shower head, checking a slow tap, or confirming a well's output.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-06.