Question 1

How do I measure GPM with a bucket?

Accepted Answer

Use a 5 gallon bucket and a stopwatch. Fully open the faucet and time how long it takes to fill. If it fills in 30 seconds, flow is 5 / 30 * 60 = 10 GPM. A 1 gallon bucket that fills in 6 seconds shows the same 10 GPM. For more accuracy, run the test 3 times and average the results, and make sure the water flows at the full normal pressure.

Question 2

What are typical flow rates for household fixtures?

Accepted Answer

Kitchen sink faucet: 2.2 GPM (8.3 L/min), bathroom sink: 1.5 GPM (5.7 L/min), low-flow showerhead: 2.0 GPM (7.6 L/min), standard showerhead: 2.5 GPM (9.5 L/min), standard toilet: 1.6 GPF per flush, bathtub filler: 4 GPM (15 L/min), dishwasher: 1 to 2.5 GPM (fills about 4 to 6 gallons per cycle), washing machine: 3 to 5 GPM (fills 15 to 30 gallons).

Question 3

How do I calculate GPM from pipe velocity?

Accepted Answer

Multiply velocity by cross-section area and a conversion factor. For imperial: GPM = velocity (ft/s) * pi * (diameter/24)^2 * 448.8, where diameter is in inches. A 1 inch pipe (ID 1.049") at 5 ft/s delivers 5 * pi * (1.049/24)^2 * 448.8 = 13.5 GPM. For metric: L/min = velocity (m/s) * pi * (d/2000)^2 * 60000, where d is in mm.

Question 4

What is a good water velocity for plumbing pipes?

Accepted Answer

Target 4 to 8 ft/s (1.2 to 2.4 m/s) for most copper supply lines. Above 8 ft/s you get noise, erosion, and water hammer - cold water should stay under 8 ft/s and hot water under 5 ft/s per Copper Development Association. PEX allows higher velocities (up to 12 ft/s). Drain lines target 2 to 4 ft/s minimum to keep solids moving.

Question 5

Is my well pump delivering enough GPM?

Accepted Answer

Residential wells typically need 6 to 12 GPM for a 2 to 3 bathroom home, and 12 to 20 GPM for larger homes with irrigation. Do a bucket test at the hose bib closest to the well: if you cannot fill a 5 gallon bucket in under 50 seconds (6 GPM), your pump, pressure tank, or well yield may need attention. Always check static vs. drawdown flow on a long continuous test.

Question 6

How do I convert GPM to LPM or GPH?

Accepted Answer

1 US gallon per minute = 3.785 liters per minute. So 10 GPM = 37.85 LPM. To convert to gallons per hour multiply by 60: 10 GPM = 600 GPH. 1 UK imperial gallon equals 1.201 US gallons, so be careful when reading international specs - most US product ratings use US gallons.

Question 7

What flow rate does a 3/4 inch garden hose deliver?

Accepted Answer

A standard 3/4 inch garden hose at 40 psi delivers about 23 GPM if unrestricted, but typical street pressure and hose length drops this to 10 to 17 GPM at the nozzle. A 5/8 inch hose at the same pressure delivers 8 to 12 GPM. Longer hoses (100+ ft) cut flow by 25 to 40 percent due to friction loss.

Fixture	GPM	L/min
Bathroom sink	1.5	5.7
Kitchen sink	2.2	8.3
Showerhead (standard)	2.5	9.5
Showerhead (low-flow)	2.0	7.6
Bathtub filler	4.0	15.1
Washing machine	4.0	15.1
Dishwasher	2.0	7.6
Garden hose 5/8"	10.0	37.9

Fixture	Max GPM (code)	Typical Actual	WaterSense
Showerhead	2.5	2.0-2.5	1.8 or less
Kitchen faucet	2.2	1.8-2.2	1.5
Bathroom faucet	2.2	1.0-1.5	1.0
Dishwasher	-	1-2 GPM avg	-
Washing machine	-	3-5 GPM peak	-
Garden hose	-	3-8 GPM	-

Fixture	GPM	L/min
Bathroom sink	1.5	5.7
Kitchen sink	2.2	8.3
Showerhead (standard)	2.5	9.5
Showerhead (low-flow)	2.0	7.6
Bathtub filler	4.0	15.1
Washing machine	4.0	15.1
Dishwasher	2.0	7.6
Garden hose 5/8"	10.0	37.9

GPM Calculator

Bucket Fill Inputs

Typical Fixture Flow Rates

How to Calculate Flow Rate (GPM)

Bucket Test Method

Velocity and Area Method

Typical Fixture GPM

Sizing Well Pumps and Pressure Tanks

Frequently Asked Questions

Bucket Fill Inputs

Typical Fixture Flow Rates