A standard 1-1/4" inside diameter pipe carries 19.13 GPM (72.39 Liters/min) at standard design velocity (5 ft/s) and up to 30.6 GPM at maximum allowable cold water velocity (8 ft/s). Part of our master Pipe Flow Rate Calculator and Copper Flow Rate suite.
Flow results depend on internal pipe diameter and velocity. For actual installations, consider system head loss and friction fittings.
Volumetric water flow in US Gallons per Minute (GPM), Liters per Minute, and Liters per Second across fluid velocities (1 to 12 ft/s).
| Velocity (ft/s) | Flow Rate (GPM) | Liters / Min (L/min) | Liters / Sec (L/s) | Cubic Ft / Sec (cfs) | Hydraulic Status |
|---|---|---|---|---|---|
| 1 ft/s | 3.82 GPM | 14.5 L/min | 0.242 L/s | 0.0085 cfs | ✓ Quiet / Hot Water Safe |
| 2 ft/s | 7.65 GPM | 29.0 L/min | 0.483 L/s | 0.0170 cfs | ✓ Quiet / Hot Water Safe |
| 3 ft/s | 11.47 GPM | 43.4 L/min | 0.723 L/s | 0.0256 cfs | ✓ Quiet / Hot Water Safe |
| 4 ft/s | 15.30 GPM | 57.9 L/min | 0.965 L/s | 0.0341 cfs | ✓ Quiet / Hot Water Safe |
| 5 ft/s | 19.13 GPM | 72.4 L/min | 1.207 L/s | 0.0426 cfs | ✓ Design Standard (5 ft/s) |
| 6 ft/s | 22.95 GPM | 86.9 L/min | 1.448 L/s | 0.0511 cfs | ⚠ Cold Water Max (8 ft/s) |
| 7 ft/s | 26.77 GPM | 101.3 L/min | 1.688 L/s | 0.0596 cfs | ⚠ Cold Water Max (8 ft/s) |
| 8 ft/s | 30.60 GPM | 115.8 L/min | 1.930 L/s | 0.0682 cfs | ⚠ Cold Water Max (8 ft/s) |
| 10 ft/s | 38.25 GPM | 144.8 L/min | 2.413 L/s | 0.0852 cfs | ⛔ High Velocity / Erosive |
| 12 ft/s | 45.90 GPM | 173.8 L/min | 2.897 L/s | 0.1023 cfs | ⛔ High Velocity / Erosive |
Volumetric continuity equation $Q = A \times v$ applied to circular pipes.
Compute circular cross-sectional area in square inches:
A = \pi \times (0.625)^2 = 1.2272 sq in Divide square inches by 144:
A_sqft = 1.2272 / 144 = 0.00852 sq ft Multiply area by design velocity (v = 5 ft/s):
Q_cfs = 0.00852 \times 5 = 0.0426 cfs Multiply cubic feet per second by 448.831 (GPM conversion factor):
Q_gpm = 0.0426 \times 448.83 = 19.13 GPM Noise thresholds, maximum fixture capacities, and erosion limits.
Trade Hydraulic Advisory: High-demand residential and light commercial multi-family water distribution manifolds.
Explore volumetric GPM capacities across other plumbing supply and main line diameters.
Engineering questions and velocity guidelines for 1-1/4" piping.
At 5 ft/s design velocity, a 1-1/4" pipe delivers 19.13 GPM. At 8 ft/s maximum velocity, it delivers 30.60 GPM.