What velocity limit applies to compressed-air piping?

Keep compressed-air velocity at or below 30 ft/s and hydraulic velocity at or below 20 ft/s. Higher velocities raise pressure drop sharply and invite erosion, noise and water carry-over.

How do fittings affect pipe pressure drop?

Fittings add an equivalent length of straight pipe: L_eq = Σ (L/D)·D. A 90° elbow is about 30 diameters, a globe valve about 340 diameters, so a short run with many fittings can drop more pressure than the straight pipe.

Which friction factor model does the calculator use?

The calculator uses the Darcy-Weisbach equation with the Swamee-Jain explicit approximation of the Colebrook friction factor, valid across laminar and turbulent flow without iteration.

What velocity does 3/8 in aluminum airline run at?

At 50 SCFM and 100 PSI the 3/8 in aluminum pipe runs at 80.6 ft/s. Compressed air should stay at or below 30 ft/s, so this run is over the limit and should be upsized.

How much pressure does 3/8 in aluminum drop over 100 ft?

The straight run drops 12.78 psi over 100 ft at 50 SCFM. Fittings add equivalent length on top: a 90° elbow is about 30 diameters, so a short run with many fittings can drop more than the pipe itself.

Which friction factor model is used?

The calculator uses the Darcy-Weisbach equation with the Swamee-Jain explicit approximation of the Colebrook friction factor. At a Reynolds number of 5,114,541 the flow is turbulent.

3/8 in aluminum Airline Loss

Pressure drop and velocity for 3/8 in aluminum airline over a 50 to 500 ft run.

3/8 in aluminum airline loss

A 3/8 in aluminum Schedule 40 airline has a 0.493 in internal diameter. Carrying 50 SCFM at 100 PSI over a 100 ft run, the air moves at 80.6 ft/s and drops 12.78 psi. The Reynolds number is 5,114,541, so the flow is fully turbulent.

Velocity and pressure drop

Velocity and pressure drop is computed deterministically from the parameters encoded in this page's address, so the same inputs always produce the same result and the same OAS-XXXX verification hash.

Fitting equivalent length

Fitting equivalent length is computed deterministically from the parameters encoded in this page's address, so the same inputs always produce the same result and the same OAS-XXXX verification hash.

Worked example

Push 50 SCFM of air at 100 PSI through 100 ft of 3/8 in aluminum Schedule 40 pipe.

Worked example steps for this calculation
Step	Formula	Value
Internal diameter	D from the Schedule 40 table	0.493 in
Velocity	V = Q_actual / A	80.6 ft/s
Reynolds number	Re = ρVD / μ	5,114,541
Pressure drop	ΔP = f·(L/D)·ρV²/2	12.78 psi

The 3/8 in aluminum run holds 80.6 ft/s — above the 30 ft/s compressed-air limit — and drops 12.78 psi over 100 ft.

Export this calculation

OAS-2126

Copy the engineering spec, export it as CSV, or print a shop ticket. Every export carries this route's parameter outputs, a timestamp and its OAS-2126 verification hash.

Engineering summary

Governing parameters and computed results for 3/8 in aluminum Airline Loss.

Governing parameters and computed results for this calculation
Parameter	Value
Material	aluminum
Nominal	3/8
Fluid Velocity (V)	25.21 ft/s
Pressure Drop (ΔP)	178.84 psi
Reynolds Number (Re)	1,830
Friction Factor (f)	0.0350

Frequently asked questions

What velocity does 3/8 in aluminum airline run at?: At 50 SCFM and 100 PSI the 3/8 in aluminum pipe runs at 80.6 ft/s. Compressed air should stay at or below 30 ft/s, so this run is over the limit and should be upsized.
How much pressure does 3/8 in aluminum drop over 100 ft?: The straight run drops 12.78 psi over 100 ft at 50 SCFM. Fittings add equivalent length on top: a 90° elbow is about 30 diameters, so a short run with many fittings can drop more than the pipe itself.
Which friction factor model is used?: The calculator uses the Darcy-Weisbach equation with the Swamee-Jain explicit approximation of the Colebrook friction factor. At a Reynolds number of 5,114,541 the flow is turbulent.

Pipe Pressure Drop & Velocity Sizer

Deterministic bore, velocity and Darcy-Weisbach pressure-loss calculations for pneumatic and hydraulic shop piping.

Result

3/8 in Aluminum Schedule 40 pipe carries 25.21 ft/s at 178.84 psi total drop over 56.1 ft developed length.

25.21ft/s

Excessive velocity

25.21 ft/s exceeds the 20 ft/s limit for hydraulic service. Increase the pipe size or reduce the flow rate.

Fluid

Pipe

Material

Schedule

Nominal Size

ID 0.493 in · roughness 0.000005 ft

Flow & Fluid Properties

Flow Rate (Q)

gpm

Fluid Density (ρ)

lb/ft³

Dynamic Viscosity (μ)

Straight Pipe Length (L)

Fittings

Equivalent length uses Crane TP-410 L/D ratios. Total fitting allowance: 6.08 ft.

90° Elbow

qty

45° Elbow

qty

Tee (run)

qty

Tee (branch)

qty

Gate Valve (open)

qty

Globe Valve (open)

qty

Check Valve (swing)

qty

Ball Valve (open)

qty

Coupling

qty

Pipe Entrance

qty

Pipe Exit

qty