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.
| 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 |
| PRESSURE | FLOW IN GPM THROUGH PIPE ID IN INCHES | ||||||
| PSI | 1" | 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 |
| Pipe Size | Maximum 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 |