Formula & worked example
v=Q/A; Re=ρvD/μ; hf=f(L/D)v²/(2g); ΔP=ρghf.
60 L/min through a 35 mm ID pipe gives about 1.04 m/s before friction and fitting allowances are applied.
HVAC & plant engineering calculator · Updated Sep 17, 2026
Hydronic Pipe Sizing Calculator — Flow, Velocity, Head Loss & Pump Head
Size a hydronic water pipe from flow and inside diameter, calculate velocity, Reynolds number, Darcy friction factor, straight-plus-equivalent-length head loss and pump pressure drop.
Inputs
Live engineering result
Velocity 1.0393792 m/s
Reynolds number 36,233.05
Darcy friction factor 0.02606736
Total design length 150 m
Head loss 6.1534356 m
Pressure drop 60.2239 kPa
Velocity is within a common hydronic screening band
Assumptions & limits
- Uses Darcy-Weisbach with Swamee-Jain turbulent friction approximation.
- Enter actual inside diameter, fluid density, viscosity and roughness at operating conditions.
Frequently asked questions
What does this Hydronic Pipe Sizing Calculator calculate?
Size a hydronic water pipe from flow and inside diameter, calculate velocity, Reynolds number, Darcy friction factor, straight-plus-equivalent-length head loss and pump pressure drop.
Can I use this for final design?
Use it as an auditable engineering sizing cross-check. Final equipment and piping selections should use project design conditions, manufacturer data, applicable standards and local code.
Why are the engineering assumptions shown?
Plant calculations are sensitive to fluid properties, pressure, temperature and operating hours. Exposing assumptions makes the result easier to verify instead of hiding them behind a single answer.