Air Compressor Sizing Calculator

Work out air compressor sizing instantly with clear inputs, formula shown and shareable results.

About Air Compressor Sizing Calculator

Adiabatic compression power is (P1 Q / 229) x k/(k-1) x [(P2/P1)^((k-1)/k) - 1] with k = 1.4 for air. Dividing by compressor efficiency gives shaft power. The result explains the familiar rule of about four SCFM per horsepower at 100 psig, and why raising pressure costs power out of proportion to the air gained.

Formulas

Adiabatic power

hp = (P1 x Q / 229) x 3.5 x [(P2/P1)^0.2857 - 1]

P1: Inlet absolute pressure, psia
Q: SCFM
P2: Discharge absolute pressure

Shaft power

BHP = theoretical hp / efficiency

Worked example

With the default values below, Air Compressor Sizing Calculator returns Shaft power required of 8.97. Change any input above and every figure here is recalculated in your browser.

Inputs used

Air demandminimum 0.1 SCFM: 40 SCFM
Discharge pressureminimum 5 psig: 100 psig
Compressor efficiencyaccepts 30–95 %: 80 %

Results

Shaft power required: 8.97 hp
Theoretical air power: 7.18 hp
Motor power: 6.69 kW
Specific output: 4.46 SCFM/hp

Step by step

1Values used
2Adiabatic power
3Shaft power
4Shaft power required= 8.97 hp
5Theoretical air power= 7.18 hp
6Motor power= 6.69 kW
7Specific output= 4.46 SCFM/hp

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Air Compressor Sizing figure worked out?

hp = (P1 x Q / 229) x 3.5 x [(P2/P1)^0.2857 - 1]. Adiabatic compression power is (P1 Q / 229) x k/(k-1) x [(P2/P1)^((k-1)/k) - 1] with k = 1.4 for air.

Why reduce system pressure if possible?

Power rises with the pressure ratio term, so every 2 psi of unnecessary discharge pressure costs roughly 1 percent more energy for the same air delivered.

What values do I need to enter?

This calculator takes 3 inputs: Air demand, Discharge pressure, Compressor efficiency. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.