Who needs this?
Flow comes up whenever water or air has to get somewhere at a useful pace. These are the people who run into it most:
- HomeownersA new showerhead is stamped "1.8 gpm" and the old one isn't stamped at all. A bucket settles it.
- Plumbers and DIYersPipe size, pressure and fixture count all come down to how many gallons per minute the line has to carry.
- Well ownersA pump has to keep up with the house at its busiest without outrunning what the well can give.
- GardenersHow long it takes to fill a stock tank or soak a bed depends on the hose's real gpm, which is usually less than you'd guess.
- Fan and HVAC shoppersBathroom fans and air handlers are rated in CFM in the US and in m³/h nearly everywhere else.
- Anyone buying imported gearPumps, filters and aquarium equipment from Europe or Asia list L/min or L/h.
What flow rate actually measures
Think of a checkout line. Counting the people in the store tells you nothing about how fast the line moves; you need people per minute. Flow is the same idea for water and air: a volume, divided by the time it took to pass.
That gives you two families of units:
- Water: US gallons per minute (gpm) in American plumbing, liters per minute (L/min) almost everywhere else. Small pumps use per-hour units (gph, L/h), and big systems use cubic meters per hour (m³/h).
- Air: cubic feet per minute (CFM) in the US, cubic meters per hour (m³/h) or liters per second (L/s) in metric countries.
- The link: one US gallon is exactly 3.785411784 liters, so 1 gpm is exactly 3.785411784 L/min. One cubic foot is 28.317 liters, so 1 CFM is about 1.699 m³/h.
Inside a pipe, flow has a tidy geometric meaning. Each second, the water moves forward a certain distance (its speed, v). Everything inside that stretch of pipe has just passed you, and its volume is the pipe's cross-section A times that distance.
Double the speed and the flow doubles. Double the diameter and the area, and with it the flow, goes up four times. That square is why a 1 in line carries so much more than a 1/2 in one.
Converting and measuring, step by step
Gallons per minute to liters per minute
Multiply by 3.785411784 (3.785 is plenty for anything with a faucet on it). To go the other way, divide.
- Multiply by 3.785
2.5 × 3.785411784 = 9.46 L/min - Back to gpm
9.46 ÷ 3.785411784 = 2.5 gpm
Per-hour units are the same numbers times 60: 2.5 gpm is 150 gph.
The bucket test
This is how plumbers check a fixture without any gauges. Put a bucket with a known volume under the faucet or shower, open it fully, and time the fill. Use a 1-gallon bucket for showers and sinks and a 5-gallon bucket for hoses and tub spouts, so the fill takes long enough to time well.
- Divide volume by seconds
1 gal ÷ 24 s = 0.04167 gal per second - Multiply by 60
0.04167 × 60 = 2.5 gpm
Method: Bucket testBucket size: 1 galSeconds to fill: 24 s
Air: CFM to m³/h
A cubic foot is 0.0283168 m³, and an hour is 60 minutes, so multiply CFM by about 1.699. For liters per second, multiply CFM by about 0.472.
- Cubic feet to cubic meters
50 × 0.0283168 = 1.416 m³ per minute - Minutes to hours
1.416 × 60 = 84.95 m³/h
Real situations, worked out
A 1-gallon bucket fills in 24 seconds: 1 ÷ 24 × 60 = 2.5 gpm, right at the federal maximum. If it took 30 seconds, you'd have 2.0 gpm, which is the WaterSense limit. A 10-minute shower at 2.5 gpm uses 25 gallons; at 2.0 gpm it uses 20.
24 s per gallon = 2.5 gpm ≈ 9.5 L/minA 5-gallon bucket fills in 30 seconds: 5 ÷ 30 × 60 = 10 gpm. The tank needs 300 ÷ 10 = 30 minutes. Hose flow depends heavily on house pressure, hose length and diameter and any spray nozzle, so time your own hose instead of trusting a chart.
10 gpm · 300 gal in 30 minutesTake an inside diameter of 0.75 in and a water speed of 5 ft/s, a common comfortable speed for household lines. Area: π × 0.375² = 0.4418 in². Each second, 0.4418 × 60 in (5 ft) = 26.5 in³ passes, which is 26.5 × 60 ÷ 231 = 6.88 gpm. Real pipe varies: the inside of "3/4 inch" copper and PEX isn't exactly 0.75 in.
0.75 in at 5 ft/s ≈ 6.9 gpm ≈ 26 L/minMethod: Pipe size and water speedInside diameter: 0.75 inWater speed: 5 ft/s
The floor is 40 sq ft. The common rule of thumb from the Home Ventilating Institute (HVI) for small bathrooms is 1 CFM per square foot, so 40 CFM. The model residential code sets a floor of 50 CFM for an intermittent bathroom fan, so buy at least 50 CFM. A European catalog would list that as about 85 m³/h. Check local code; it's the one that counts.
40 sq ft → 50 CFM ≈ 85 m³/hWell pumps and garden supply, in general terms
A well pump is sized from two numbers: the peak flow the house draws (showers, washer and irrigation running together) and how fast the well refills, called its yield and measured in gpm. A pump that outruns the well will draw it down. A common shortcut counts about 1 gpm per fixture, but a well contractor sizes the real thing from a yield test.
Doing it in your head
Multiply gpm by 4, then knock off 5%. Plain × 4 is about 6% high, which is fine for a quick read; trimming 5% lands within half a percent. Going back, divide L/min by 4 and add 5%. For air, CFM × 1.7 gives m³/h within a tenth of a percent.
| gpm | Exact L/min | × 4 | × 4, less 5% |
|---|---|---|---|
| 1 | 3.79 | 4 | 3.8 |
| 2 | 7.57 | 8 | 7.6 |
| 2.5 | 9.46 | 10 | 9.5 |
| 5 | 18.93 | 20 | 19 |
| 10 | 37.85 | 40 | 38 |
| 20 | 75.71 | 80 | 76 |
Common mistakes
Mixing US and UK gallons. A UK (imperial) gallon is 4.54609 L, about 20% bigger than the US gallon. A British pump rated in "gallons per minute" means imperial gallons. All gpm figures on RocketCalc are US gallons.
Testing with the tap half open. The bucket test measures what the fixture delivers at full open with your pressure at that moment. Run it with nothing else using water, or a toilet refilling in the next room will skew it.
Using the nominal pipe size as the inside diameter. Pipe sizes are names. "1/2 inch" PEX has an inside diameter closer to 0.48 in, and since flow depends on diameter squared, small differences add up. Use the real inside diameter in Q = A × v.
Reading a fan's CFM as what it moves in your house. Fan ratings are measured at a set static pressure. A long, kinked duct run cuts real airflow well below the box number, so short straight ducts matter as much as the rating.
A bit of history
- 1628Benedetto Castelli, a student of Galileo, publishes a treatise on measuring running water, tying the amount a river carries to its cross-section and its speed.
- 1738Daniel Bernoulli publishes Hydrodynamica, which links the speed of a moving fluid to its pressure.
- 1992The US Energy Policy Act sets the national limit for showerheads at 2.5 gpm at 80 psi.
- 2006The EPA launches WaterSense, a voluntary label for water-efficient products.
- 2010WaterSense publishes its showerhead specification, capping labeled models at 2.0 gpm.
Reference table
| What | gpm | L/min | m³/h |
|---|---|---|---|
| WaterSense showerhead (max) | 2.0 | 7.57 | 0.45 |
| Bathroom faucet, federal max | 2.2 | 8.33 | 0.50 |
| US showerhead, federal max | 2.5 | 9.46 | 0.57 |
| Garden hose, example bucket test | 10 | 37.85 | 2.27 |
| 3/4 in line at 5 ft/s | 6.9 | 26.06 | 1.56 |
| 1 in line at 5 ft/s | 12.2 | 46.33 | 2.78 |
| Air | CFM | m³/h | L/s |
|---|---|---|---|
| Small bathroom fan | 50 | 85 | 23.6 |
| Larger bathroom fan | 80 | 136 | 37.8 |
| 100 sq ft bathroom (rule of thumb) | 100 | 170 | 47.2 |
| 3-ton central AC (≈400 CFM per ton) | 1,200 | 2,039 | 566 |
Questions people ask
How do I convert gpm to liters per minute?
Multiply by 3.785 (exactly 3.785411784). 2 gpm is 7.57 L/min. To go from L/min to gpm, divide by the same number.
How do I measure my shower's flow rate?
Hold a 1-gallon bucket under the head at full flow and time it. Divide 60 by the seconds: 24 seconds is 2.5 gpm, 30 seconds is 2.0 gpm.
What size bathroom fan do I need?
For bathrooms up to about 100 sq ft, HVI's rule of thumb is 1 CFM per square foot of floor, with 50 CFM as the usual minimum. Larger rooms and long duct runs need more, and local code has the final word.
What's the difference between gpm and gph?
Only the time unit. 1 gpm is 60 gph. Pond pumps and drip irrigation use gph because their flows are small.
What does CFM stand for?
Cubic feet per minute. One CFM is about 1.699 m³/h or 0.472 L/s. HVAC techs often plan around 400 CFM per ton of cooling as a starting point.