What is the flow rate?

How do I measure GPM?

Time how long it takes to fill a bucket of known size and divide: gallons ÷ seconds × 60. If a 5 gallon bucket fills in 30 seconds, the flow is 5 ÷ 30 × 60 = 10 gallons per minute. Open the tap all the way and use a bucket big enough to take at least 10 seconds.

How do I calculate flow in a pipe?

Flow = velocity × the inside area of the pipe. With the inside diameter in inches and the velocity in feet per second, GPM = 2.448 × velocity × diameter². Water moving 5 ft/s through a 1 inch pipe carries 2.448 × 5 × 1 = 12.2 GPM.

How fast should water move in a pipe?

The Copper Development Association recommends at most 8 feet per second for cold water and 5 ft/s for hot water in copper tube, to avoid noise and erosion. Faster flow needs a bigger pipe.

How many GPM does a shower use?

Federal law caps new showerheads at 2.5 gallons per minute, and WaterSense-labeled models use 2.0 GPM or less. Do the bucket test on yours to see its real flow.

How do I convert GPM to liters per minute?

Multiply by 3.785: 10 GPM is 37.9 liters per minute. To get gallons per hour, multiply GPM by 60 — 10 GPM is 600 GPH, the unit pond and fountain pumps use.

How long will it take to fill a tank or pool?

Divide the volume by the flow. A 50 gallon tank filling at 10 GPM takes 5 minutes. A 15,000 gallon pool filling at 10 GPM takes 1,500 minutes, or 25 hours. Measure the flow with the bucket test first, because the real flow at a hose bib is usually lower than the pipe could carry.

Why does my bucket test give less than the pipe formula?

The pipe formula tells you how much water the pipe carries at a given speed, not how fast the water actually moves. The real speed depends on the water pressure, the length and fittings of the run, and what the tap or fixture lets through. Use the bucket test for the flow you actually have, and the pipe formula to check whether a pipe is large enough for the flow you want.

Where the pipe formula comes from

Flow is the cross-section area of the pipe times the speed of the water. For a circular pipe the area is π ÷ 4 × d², where d is the inside diameter. With d in inches the area in square feet is that value divided by 144. Multiply by the speed in feet per second, by 60 to get cubic feet per minute, and by 7.48052 gallons per cubic foot (NIST). The constants collapse to 2.448, so GPM = 2.448 × speed × d², the formula on this page rounded to 2.448.

Because the diameter is squared, size matters a lot. At the same speed of 8 feet per second, the copper-tube limit for cold water, a pipe with a 0.75 inch inside diameter carries 11.0 GPM, and a 1.5 inch pipe, twice as wide, carries 44.1 GPM, four times as much.

Mistakes to avoid

Use the inside diameter, not the nominal size. Nominal pipe sizes are names, and the inside diameter differs with the material and the wall thickness, so check a table for your pipe type. A small error in diameter is squared in the result.

Keep the units straight: the bucket test is in gallons and seconds, so multiply gallons per second by 60 to get gallons per minute. Do not read a pipe speed limit as a flow limit; it limits the speed of the water to avoid noise and wear, and a larger pipe lets more flow through at the same speed. Last, do the bucket test with the tap fully open, and start the timer when the stream is steady.

Sources: 7.48052 gallons per cubic foot from NIST; the velocity limits for copper tube from the Copper Development Association's Copper Tube Handbook; showerhead limits from federal law and EPA WaterSense. Related tools: pipe velocity, pipe size and tank volume calculators.