Air compressors are sold with a confusing stack of numbers: horsepower, tank gallons, maximum PSI and an airflow figure in CFM. The one that decides whether your impact wrench keeps spinning or runs out of puff is the airflow, delivered at the pressure your tools need. This guide explains what each figure means and gives a method for sizing a compressor to your work.
Quick answer
Find the air consumption of the tool you use most, in CFM at 90 PSI, add about 30 percent, and choose a compressor whose delivered airflow at 90 PSI meets that number. For several tools used at once, add their consumption together. Tank size affects how long you can work between pump cycles, not how much air the compressor makes, and horsepower is a poor guide to anything.
In this guide
The numbers on the box
- CFM (cubic feet per minute) is the volume of air the compressor delivers. It is meaningless without a pressure, because air at 40 PSI and air at 90 PSI are very different things. Look for the figure at 90 PSI, which is what most tools run on.
- SCFM (standard CFM) is the same measurement corrected to a standard temperature, pressure and humidity, so that makers’ figures can be compared. Treat it as the honest version of CFM.
- PSI (pounds per square inch) is pressure. Most air tools need 90 PSI at the tool. A compressor’s maximum PSI, often 125 to 175, tells you how much air the tank can store, not how fast it is replaced.
- Tank size in gallons is storage. A bigger tank lets you run a hungry tool for longer before the pump has to catch up.
- Horsepower is the motor. Peak or developed HP figures are marketing; a motor on an ordinary 120 V, 15 A circuit cannot produce more than about 1.5 to 2 running horsepower, whatever the label says.
- Duty cycle is how much of the time the pump can run. A 50 percent duty cycle means five minutes on, five minutes off.
How much air do tools use?
Typical average consumption at 90 PSI, in CFM. Tools vary between makers, and the figures below assume intermittent use; a tool held on continuously uses more.
| Tool | Typical CFM at 90 PSI |
|---|---|
| Tire inflator | 1 to 2 |
| Brad or finish nailer | 1 to 2 |
| Framing nailer | 2 to 3 |
| Blow gun | 2.5 to 3 |
| Air ratchet, 3/8 inch | 3 to 5 |
| Impact wrench, 1/2 inch | 4 to 5 |
| Die grinder | 4 to 6 |
| Cut-off tool | 4 to 8 |
| Impact wrench, 3/4 inch | 7 to 9 |
| Orbital sander | 6 to 9 |
| Impact wrench, 1 inch | 10 to 12 |
| Dual-action sander | 11 to 13 |
| HVLP spray gun | 10 to 14 |
| Abrasive blaster | 10 to 20 or more |
The sizing method
- List your tools and find each one’s CFM at 90 PSI from its manual or label.
- Identify the biggest continuous user. Sanders, grinders and spray guns run for minutes at a time; nailers and impact wrenches run in bursts.
- Add 30 percent to that figure for headroom, or more if you will use the tool continuously.
- If tools run together, add their consumption; in a shop with several people, add everything that could run at once.
- Compare with delivered CFM at 90 PSI, not the maximum PSI, the tank size or the horsepower.
Examples
- Nailers and a tire inflator: 3 CFM plus 30 percent is about 4 CFM. A small pancake or hot-dog compressor of 2 to 6 gallons does it.
- A 1/2-inch impact wrench and an air ratchet in a home garage: 5 CFM plus 30 percent is about 6.5 CFM. A 20 to 30 gallon single-stage compressor delivering 5 to 7 CFM at 90 PSI is the usual answer, and the tank buffers the bursts.
- A dual-action sander used for bodywork: 12 CFM plus 30 percent is about 16 CFM, continuous. That needs a 60 to 80 gallon two-stage compressor on a 240 V circuit.
Why the tank matters, and why it does not
The pump makes the air; the tank stores it. If your tool uses more air than the pump makes, the tank drains until the pressure falls below what the tool needs, and you wait. A bigger tank just makes the wait less frequent. For burst tools such as impact wrenches, a decent tank lets a modest pump keep up. For continuous tools, no tank is big enough; the pump must match the tool. Our guide to single-stage vs two-stage compressors covers what makes a pump deliver more.
Pressure drop in hoses and fittings
A long, thin hose or restrictive quick couplers can lose 10 to 20 PSI between the tank and the tool, so a compressor set at 90 PSI delivers much less at the end of a 50-foot, 1/4-inch hose. Use 3/8-inch hose for anything beyond 25 feet, high-flow couplers, and set the regulator so the tool, not the tank, sees 90 PSI.
Electrical requirements
Compressors delivering more than about 5 to 6 CFM at 90 PSI generally need a 240 V circuit. Check the nameplate amperage and have a dedicated circuit installed if needed; a compressor sharing a circuit with other tools trips breakers and starves the motor.
Frequently asked questions
What CFM do I need for an impact wrench?
A 1/2-inch impact wrench typically uses 4 to 5 CFM at 90 PSI in intermittent use, so a compressor delivering 5 to 7 CFM at 90 PSI with a 20 gallon or larger tank runs it well. A 3/4-inch impact needs around 8 to 10 CFM.
Is a bigger tank better than more CFM?
Only for tools used in short bursts. The tank stores air; the pump makes it. For sanders, grinders and spray guns that run continuously, the pump's CFM at 90 PSI is the only figure that matters.
Why does my compressor not keep up even though it has enough horsepower?
Because horsepower does not measure air delivery. Two compressors with the same stated horsepower can deliver very different CFM, and peak horsepower figures on consumer compressors are often optimistic. Compare delivered CFM at 90 PSI instead.
What is the difference between CFM and SCFM?
Both measure airflow. SCFM is corrected to a standard set of conditions so that figures from different makers can be compared fairly. When a maker quotes both, SCFM at 90 PSI is the one to use for sizing.
