Air Compressor HP to CFM Calculator
Estimate how much air a compressor of a given horsepower delivers at your working pressure, or how much horsepower you need for a target CFM. Includes the thermodynamic ceiling no compressor can beat.
Compressor horsepower tells you how much power goes in. CFM tells you how much air comes out, and that is what your tools care about. The two are linked by physics and by how efficient the compressor is, and the link changes with pressure. This calculator gives a realistic range for your compressor type and working pressure, plus the theoretical ceiling.
How Many CFM per Horsepower?
Industry guides commonly quote two working ranges at around 90 psi. Single-stage piston compressors deliver about 3 to 4 CFM per horsepower. Two-stage piston and rotary screw compressors deliver about 4 to 5 CFM per horsepower. Engineering rules of thumb for plant compressors put larger units at the top of that range and note that small compressors, especially those of 10 HP or less, often fall below it.
Pressure factor = ideal CFM/HP at your pressure ÷ ideal CFM/HP at 90 psi
The pressure factor is the part most charts leave out. The calculator derives it from the thermodynamics of compressing air, so the range moves correctly when you work at 40 psi for blowing off or at 150 psi for heavy tools.
The Physics Limit on CFM per HP
Compressing air takes a minimum amount of work, set by how much the pressure rises. In the ideal case of compression with no heat transfer and no losses (adiabatic compression), the work per cubic foot of free air is (k ÷ (k − 1)) × p₁ × [(p₂ ÷ p₁)(k−1)/k − 1], where k is 1.4 for air. Dividing one horsepower (33,000 ft·lb per minute) by that work gives the most CFM a perfect compressor could deliver per horsepower.
| Discharge pressure | Ideal single-stage | Ideal two-stage, intercooled | Isothermal limit |
|---|---|---|---|
| 40 psig | 9.8 CFM/HP | 10.8 | 11.9 |
| 90 psig | 5.9 CFM/HP | 6.9 | 7.9 |
| 100 psig | 5.6 CFM/HP | 6.5 | 7.6 |
| 125 psig | 4.9 CFM/HP | 5.9 | 6.9 |
| 150 psig | 4.5 CFM/HP | 5.4 | 6.5 |
| 175 psig | 4.1 CFM/HP | 5.1 | 6.1 |
The isothermal limit, where air stays at intake temperature throughout, is the absolute floor on work, and no real machine reaches it. The two-stage column shows why splitting compression with an intercooler between stages saves power: cooling the air halfway cuts the work needed for the second stage. Comparing the real-world 4 to 5 CFM per horsepower at 90 to 100 psi with the ideal single-stage 5.6 to 5.9 shows that real compressors deliver roughly 70 to 90% of the ideal, after mechanical friction, heat and leakage.
Why Pressure Matters So Much
Raising the working pressure from 90 to 125 psi cuts air delivery per horsepower by about 17%. That is the most practical lesson in the table. Running a whole shop at a higher pressure than the tools need costs real capacity, and it also increases leakage. Many tools are rated at 90 psi at the tool. Set the compressor high enough to cover pressure drop in hoses and fittings, but not higher than you need.
CFM, SCFM and ACFM: Comparing Ratings Fairly
Compressor delivery is quoted as free air: the volume the compressor draws in at atmospheric conditions, not the much smaller volume of compressed air in the tank. Makers may quote it as CFM, as ACFM (actual cubic feet per minute at the inlet conditions) or as SCFM (standard cubic feet per minute, corrected to a reference temperature, pressure and humidity). Different organizations define the standard conditions differently, so two SCFM figures are only directly comparable when they use the same reference.
Always check the pressure that a delivery figure is quoted at. A compressor rated at 40 psi will show a much larger number than the same machine rated at 90 psi, as the table above shows, so "CFM at 90 psi" is the figure to compare for most shop tools.
Beware Inflated Horsepower Labels
Small portable compressors are sometimes labeled with "peak" horsepower figures that a household outlet cannot supply. A standard 120 V, 15 A circuit delivers at most 1,800 W, about 2.4 HP of electrical input before any motor losses, and continuous loads are normally kept to 80% of a circuit's rating. A plug-in compressor labeled 5 or 6 HP cannot be drawing that much power continuously. Calculate from the true running horsepower, from the motor nameplate's amps and volts, or better still, measure the CFM directly.
How to Measure Your Compressor's Real CFM
A pump-up test measures delivered air without special equipment. Empty the tank partly, then time how long the compressor takes to raise the pressure between two readings:
For example, a 60 gallon tank that rises from 90 to 120 psi in 1.5 minutes delivers 60 × 30 ÷ (7.48 × 14.7 × 1.5) = about 10.9 CFM. If that compressor claims 5 HP, it is producing about 2.2 CFM per horsepower, well below the range for a healthy unit. Worn rings, a slipping belt or leaking valves are common causes. The test slightly overstates delivery while the air is still warm in the tank, so take readings soon after the motor stops.
Watch the units if you use other calculators. At least one published example puts tank gallons straight into a formula that needs cubic feet. Leaving out the 7.48 gallons per cubic foot conversion overstates delivery by a factor of 7.48.
Sizing a Compressor for Your Tools
Size from demand, not from the biggest tool. Brabazon, a compressor distributor, recommends multiplying each tool's CFM by the share of time it actually uses air, adding the results, then adding about 25% for leaks and growth. An 8 CFM blow gun used 30% of the time averages 2.4 CFM, and a 12 CFM cut-off tool used half the time averages 6 CFM. Together that is 8.4 CFM, or 10.5 CFM with the margin, which a true 2.1 to 2.6 HP two-stage or rotary screw compressor covers at 90 psi.
For tools that do run at the same time, add their full CFM at rated pressure. Piston compressors are often not designed to run continuously, so check the maker's duty cycle. A unit that must run most of the time is better served by a larger compressor or a rotary screw design. Use the result in reverse mode to find the horsepower your air demand calls for, then size the electrical supply with the HP to amps calculator and, for three-phase machines, the HP to kVA calculator.
Frequently Asked Questions
At about 90 psi, roughly 3 to 4 CFM per HP for single-stage piston compressors and 4 to 5 CFM per HP for two-stage piston and rotary screw units. Small compressors often fall below these ranges.
About 40 to 50 CFM at 90 psi for a two-stage piston or rotary screw unit, and about 30 to 40 CFM for single-stage. At 125 psi expect roughly 17% less.
A true 5 HP two-stage compressor delivers about 20 to 25 CFM at 90 psi, and a single-stage unit about 15 to 20 CFM. Many plug-in units labeled 5 HP use peak ratings and deliver much less.
At 100 psi, about 21 to 27 HP for a rotary screw or two-stage compressor. Even a perfect single-stage compressor would need about 18 HP, because physics sets a ceiling of about 5.6 CFM per HP at that pressure.
Compressing air to a higher pressure takes more work per cubic foot. Going from 90 to 125 psi reduces delivery per horsepower by about 17%.
Time how long it takes to raise tank pressure between two points, then use CFM = tank gallons × pressure rise ÷ (7.48 × 14.7 × minutes). A 60 gallon tank rising 30 psi in 1.5 minutes delivers about 10.9 CFM.