Pipe Flow Rate & Pressure Drop Calculator
Enter pipe material, diameter, flow rate, and length to instantly get velocity, flow regime, and Darcy-Weisbach pressure drop. Add multiple pipe runs for a full system schedule. Nothing uploaded.
Results
Full flow details
System run schedule
| Dia | Flow | Length | Velocity | dP |
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Learn more: pipe flow and pressure drop
How the pressure drop is calculated
The tool uses the Darcy-Weisbach equation. Wikipedia's article on it gives the loss as the friction factor times the pipe length over diameter times density times velocity squared over 2. It also gives f = 64 / Re for laminar flow, and the Swamee-Jain equation as a direct way to find the friction factor for turbulent flow in a full circular pipe, which is what the tool uses.
The tool works out velocity from flow and diameter, then the Reynolds number, and calls flow laminar below 2,300, transitional to 4,000 and turbulent above that. Wikipedia puts the laminar limit at 2,000, so sources differ on where the edge sits. Transitional flow is hard to predict, and the tool applies the turbulent equation there too, so treat that band loosely.
Why diameter matters so much
At a fixed flow, the Darcy equation makes the drop fall with roughly the fifth power of diameter, because velocity squared goes with 1/D^4 and the length term adds one more 1/D. In the tool, 30 L/min of 20 C water through 10 m of copper gives these results. A 22 mm pipe has a velocity of 1.32 m/s, a Reynolds number of 28,828 and a drop of 9.34 kPa. A 28 mm pipe drops 2.96 kPa, about a third as much, and a 44 mm pipe 0.35 kPa. Real values shift a little as the friction factor changes.
The tool takes the nominal sizes in its list as the inside diameter. A real pipe's inside diameter is usually smaller than its nominal size, and a small change in diameter has a large effect, so choose Custom and enter the true inside diameter when you know it.
Material roughness
The tool's roughness values are 1.5 micrometers for copper, PVC and CPVC, 3 for HDPE, 46 for new steel, 260 for cast iron and 300 for old rusty steel. Engineering Toolbox's roughness table lists 1 to 2 micrometers for drawn tubing, 1.5 to 7 for PVC, 45 to 90 for commercial steel and 250 to 800 for cast iron. It lists rusted steel at 1.5 to 2.5 mm, far above the tool's 0.3 mm, so a badly corroded pipe may be worse than the tool shows.
Roughness matters. For the same 30 L/min through 22 mm and 10 m, the tool gives 9.34 kPa for copper, 11.27 for new steel and 17.43 for old steel.
FAQ
What is the difference between laminar and turbulent flow?
Laminar flow moves in smooth layers, and the friction factor is 64 / Re. In turbulent flow the loss grows with velocity squared and the friction factor depends on roughness too. The tool picks the formula from the Reynolds number.
Why does hot water show a different drop?
Hot water is less viscous, so the Reynolds number is higher and the friction factor lower. For 30 L/min in 22 mm copper the tool gives 9.34 kPa at 20 C, 7.8 at 60 C and 7.1 at 90 C.
Is head loss the same as pressure drop?
They are the same loss in different units. Head loss is the pressure drop divided by density times 9.81, the height of a fluid column. The 9.34 kPa example is 0.95 m of water head.