Pipe Velocity & Friction Checker

🔌 Pipe Inputs

Flow Rate (GPM)

Pipe Size (nominal, Sch 40)

Fluid Type

Application Type

📊 Results

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Velocity (ft/s)

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Friction Loss (ft/100ft)

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Reynolds Number

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Flow Regime

📚 ASHRAE Velocity Limits

Application	Max Velocity	Notes
Pump suction	3 ft/s	Prevent cavitation and noise
Branch / zone supply	4 ft/s	Low-noise terminal units
Main distribution headers	8 ft/s	ASHRAE recommended maximum
Condenser water	10 ft/s	Larger pipe, higher tolerance
Friction loss target	1–4 ft/100ft	Design target for sizing
Friction loss max	4 ft/100ft	Above this → velocity noise risk

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About This Calculator

This pipe velocity and friction checker computes water velocity, friction loss, and Reynolds number for any nominal Sch 40 pipe size and flow rate. Enter the flow in GPM, pick a pipe size and fluid, and choose an application; the tool returns velocity in ft/s and friction loss in feet per 100 feet, with pass/fail badges against ASHRAE recommended velocity limits.

Velocity that is too high causes noise and erosion-corrosion; too low risks sediment settling. The application selector applies the right velocity ceiling for branch lines, mains, pump suction, condenser water, and domestic cold, so you can confirm a size before committing it.

Formula & Method

Velocity	V (ft/s) = Q (GPM) ÷ 449 ÷ A A = π · d² ÷ 4 (ft)
Reynolds	Re = V · D ÷ ν
Friction f	Colebrook–White (turbulent); f = 64 ÷ Re (laminar)
Friction loss	h_L = f · (100 ÷ D) · V² ÷ (2 · g) (ft/100 ft)

Friction uses the Darcy–Weisbach equation with the Colebrook–White friction factor, solved iteratively, per ASHRAE Fundamentals hydronics. The constant 449 converts GPM to ft³/s (1 ft³/s ≈ 449 GPM); ν is the fluid kinematic viscosity (water at 60°F ≈ 1.22 × 10⁻⁵ ft²/s); steel roughness ε ≈ 0.00015 ft; g = 32.174 ft/s². Below Re ≈ 2,300 the flow is laminar and f = 64 ÷ Re.

Frequently Asked Questions

What velocity is too high for hydronic pipe?

It depends on the application. Pump suction is kept at or below about 3 ft/s to avoid cavitation, branch and zone supply at or below about 4 ft/s for low noise, main distribution headers at or below about 8 ft/s, and condenser water up to about 10 ft/s. This tool flags velocity against the limit for the application you select.

What is a good friction loss target?

A common design target is 1 to 4 feet of head per 100 feet of pipe. The tool shows green up to about 2 ft/100ft, amber from 2 to 4, and red above 4 ft/100ft, where velocity noise and pumping energy become a concern. Lower friction saves pump energy but needs larger pipe.

Why does the Reynolds number and flow regime matter?

The Reynolds number tells you whether flow is laminar (below about 2,300), transitional (2,300 to 4,000), or turbulent (above 4,000), and which friction equation applies. Almost all hydronic flow is turbulent, where the tool solves the Colebrook-White equation. In the rare laminar case it uses friction factor equal to 64 divided by Reynolds number.

Does glycol change the result?

Yes. Glycol is more viscous than water, which lowers the Reynolds number and raises friction loss for the same flow and pipe size. The fluid selector includes water at several temperatures and 25 to 40 percent glycol, each with its own kinematic viscosity, so the velocity and friction reflect the actual fluid.

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.

Pipe Velocity + Friction Quick Check