Pipe Flow Calculator
One pipe scenario, the whole picture: flow and velocity, Reynolds number and flow regime, Darcy-Weisbach pressure drop with fittings, and the pump horsepower to move it — all from a single set of inputs.
Add the equivalent length of elbows, tees and valves to include minor losses.
One Calculator, Four Answers
This pipe flow calculator takes a single scenario — pipe size, flow, length, fluid and pump head — and works the whole chain at once: flow and velocity, the Reynolds number and flow regime, the Darcy-Weisbach pressure drop including fittings, and the pump horsepower needed to move it. The modules feed each other exactly the way the physics does, instead of scattering across separate pages.
Flow Rate Formula: Q = A × V
Flow rate equals cross-sectional area times velocity, where A = π/4 × D² uses the inside diameter. The calculator auto-fills real inside diameters for each material and schedule, and works either direction — enter flow to get velocity, or velocity to get flow.
Reynolds Number and Flow Regime
The Reynolds number sets the flow regime and therefore the friction method: laminar below 2,300 (f = 64/Re), transitional from 2,300 to 4,000, and turbulent above 4,000 (Swamee-Jain approximation of Colebrook). Density and viscosity auto-fill from the fluid and temperature, and you can override them for any fluid not listed.
Pressure Drop: Darcy-Weisbach
This pressure drop calculator reports the loss in psi, kPa and Pa, plus head loss in feet and metres. Minor losses from elbows, tees and valves are included by adding their equivalent length to the pipe length — a quick, standard way to fold fittings into the same equation.
Pump Horsepower
With Q in GPM, total head H in feet, specific gravity SG and pump efficiency η as a decimal, the constant 3960 gives brake horsepower directly. The calculator also shows the hydraulic horsepower (the same formula without efficiency), the required motor HP after a 1.15 service factor, and the next standard motor size up. Total head should include both the friction head from the pressure-drop module and any static lift.
Worked example
50 GPM of water through 100 ft of 2 in Schedule 40 steel pipe (ID 2.067 in):
Velocity ≈ 4.8 ft/s · Reynolds ≈ 68,000 (turbulent) · f ≈ 0.023
Head loss ≈ 4.7 ft → pressure drop ≈ 2.0 psi. At 50 ft of total head and 70% efficiency, that is about 0.9 brake HP, or a 1.5 HP motor once a service-factor margin is added.
Recommended Pipe Velocities
| Application | Velocity |
|---|---|
| Domestic water | 4 – 8 ft/s |
| Copper pipe | under 8 ft/s |
| HVAC hydronic | 2 – 10 ft/s |
| Pump suction lines | 2 – 4 ft/s |
| Compressed air | 20 – 30 ft/s |
Frequently Asked Questions
What is a good water velocity in a pipe?
4-8 ft/s for water supply. Below 2 ft/s a line may not self-clean; above 8 ft/s copper erodes and noise and water hammer appear.
How do I include fittings in the pressure drop?
Enter the total equivalent length of the fittings (elbows, tees, valves) in the fittings field. It is added to the straight pipe length in the Darcy-Weisbach calculation.
What is the difference between hydraulic and brake horsepower?
Hydraulic HP is the useful power delivered to the fluid; brake HP is what the pump shaft must supply, larger by the efficiency. The motor is then sized above brake HP with a service factor.
Why does temperature change the result?
Hot water is far less viscous than cold, lowering friction; glycol antifreeze is much more viscous, raising it. The Reynolds and pressure-drop modules account for both.
Related calculators
- Water Flow Rate Calculator — flow for open channels and fixtures.
- Pipe Volume Calculator — how much fluid a length of pipe holds.
- Fabrication Calculators — weld and bolt strength for the pipe supports and flanges.
- Engineering Calculators — the full set, including more fluids tools to come.