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How Much Water Can Flow Through A Pipe (GPM/GPH)?

Water Flow (GPM/GPH) based on Pipe Size and Inside/Outside Diameters

Assume Gravity to Low Pressure. About 6 f/s flow velocity, also suction side of pump Assume Average Pressure (20-100PSI). About 12 f/s flow velocity Assume "High Pressure" PEAK flow. About 18 f/s flow velocity

Pipe Size (Sch. 40)I.D. (range)O.D.GPM(w/ min. PSI loss & noise)GPH(w/ min. PSI loss & noise)GPM(w/ min. PSI loss & noise)GPH(w/ min. PSI loss & noise)GPM(w/ min. PSI loss & noise)GPH(w/ min. PSI loss & noise)

1/2"0.5 - 0.6"0.85"7 420 14 840 21 1,260

3/4"0.75 - 0.85"1.06"11 660 23 1,410 36 2,160

1"1 - 1.03"1.33"16 960 37 2,200 58 3,480

1-1/4"1.25 - 1.36"1.67"25 1,500 62 3,750 100 6,000

1-1/2"1.5 - 1.6"1.9"35 2,100 81 4,830 126 7,560

2"1.95 - 2.05"2.38"55 3,300 127 7,650 200 12,000

2-1/2"2.35 - 2.45"2.89"80 4,800 190 11,400 300 18,000

3"2.9 - 3.05"3.5"140 8,400 273 16,350 425 25,500

4"3.85 - 3.95"4.5"240 14,400 480 28,800 700 42,000

5"4.95" - 5.05"5.563"380 22,800 750 45,000 1,100 66,000

6"5.85 - 5.95"6.61"550 33,000 1100 66,000 1700 102,000

8"7.96"8.625"950 57,000 1900 114,000 2800 168,000

Water Flow (GPM) Capacity Based on ID Size and Pressure

PRESSUREFLOW IN GPM THROUGH PIPE ID IN INCHESPSI1"1.25"1.5"2"2.5"3"4"

20 26 47 76 161 290 468 997

30 32 58 94 200 360 582 1240

40 38 68 110 234 421 680 1449

50 43 77 124 264 475 767 1635

60 47 85 137 291 524 846 1804

75 53 95 153 329 591 955 2035

100 62 112 180 384 690 1115 2377

125 70 126 203 433 779 1258 2681

150 77 139 224 478 859 1388 2958

200 90 162 262 558 1004 1621 3455

Water Flow Capacity in Steel Pipes (sch 40)

Pipe SizeMaximum Flow (gal/min)Velocity (ft/s)Head Loss (ft/100 ft)

2"45 4.3 3.9

2-1/2"75 5.0 4.1

3"130 5.6 3.9

4"260 6.6 4.0

6"800 8.9 4.0

8"1,600 10.3 3.8

10"3,000 12.2 4.0

12"4,700 13.4 4.0

14"6,000 14.2 4.0

16"8,000 14.5 3.5

18"10,000 14.3 3.0

20"12,000 13.8 2.4

24"18,000 14.4 2.1

We regularly get asked about the water flow capacity of different pipe sizes, and which is the best roof drain for a specific pipe size. Unfortunately, recommendations aren't that straightforward because you also need to account for water pressure, material friction and more.

That said, we put together the following tables to serve as general guides for estimating a pipe's water flow capacity through a pipe or roof drain. If you have questions, please call our Drain Wizard at 800-635-0384.