Sizing an Air Compressor for Your Shop

Buying a compressor on horsepower or tank size alone is the usual mistake. What counts is how many cubic feet of air per minute it delivers at the pressure your tools need. This guide shows how to read the ratings, add up your tools and choose a compressor that does not run out of air in the middle of a job.

CFM, SCFM and PSI

CFM (cubic feet per minute) is the volume of air the compressor delivers. SCFM is that volume corrected to standard conditions, which is the figure that matters for comparing models. Compressors are rated at a given pressure, nearly always 90 psi for shop tools and sometimes 40 psi for smaller figures. A compressor that makes 6 SCFM at 90 psi might claim 8 or more at 40 psi, so compare like with like and read the small print.

Which tools use how much air

Tools list their air requirement in CFM at 90 psi. The values below are typical and vary by model, so read the label on your own tools.

Tool	Typical use (CFM at 90 psi)	Duty
Brad nailer, tire inflator	0.5 to 2	Intermittent
Impact wrench (1/2 in)	4 to 5	Intermittent
Die grinder, cut-off tool	4 to 6	Often continuous
Ratchet, small air tools	2 to 4	Intermittent
HVLP spray gun	8 to 15	Continuous while spraying
Dual-action sander	8 to 12	Continuous
Media blaster	10 to 30 or more depending on nozzle	Continuous

Intermittent versus continuous tools

An impact wrench uses air in short bursts, so the tank covers the gaps and a modest compressor will do. A sander or spray gun draws air continuously. For those, the compressor must be able to supply the full CFM all the time, because the tank only delays the moment when pressure drops. A common guide is to take the CFM of the most demanding continuous tool and add 30 to 50 percent for leaks, hose losses and the fact that the compressor will not be able to run at its rating all day.

Sizing for several tools

If two people will use air tools at once, add the demands together. If you run one tool at a time, take the highest single demand. A worked example: you plan to run a DA sander (10 CFM) and occasionally an impact wrench (5 CFM). You will not use both at once, so the peak demand is 10 CFM. Add 40 percent for headroom and you need about 14 CFM at 90 psi. That points to a two-stage compressor in the 5 horsepower range with a tank of 60 gallons or more. The Air Compressor CFM Calculator adds up your tool list and applies a safety margin for you.

Painting and blasting need the most

These jobs are where small compressors fail. A conventional or HVLP gun plus a sander will push you well above 12 CFM, and a media blaster with a 3/16 inch nozzle will want far more than most home compressors deliver. The Sandblaster Compressor CFM Calculator gives you the demand for each nozzle size and pressure. If your compressor cannot keep up, use a smaller nozzle, work in short bursts or consider an alternative such as a soda blaster that suits a lower air volume.

Tank size

The tank is a buffer. A bigger tank lets the compressor cycle less often and gives you a reserve for short bursts, but it does not create air. A 60-gallon tank on a 5 CFM compressor still cannot run a sander for long. Think of CFM as the size of the tap and the tank as a bucket.

Single-stage or two-stage

Single-stage compressors compress the air in one stroke and generally top out around 125 to 135 psi, suiting hobby and light duty. Two-stage compressors compress in two steps, run cooler, are more efficient and are meant for longer duty, which makes them the usual choice for a serious garage. They are larger, need a dedicated circuit and are noisier.

Duty cycle

Duty cycle is the share of each period a compressor can run without overheating. A 50 percent rating means it can run about five minutes in every ten. Oil-lubricated cast-iron pumps tolerate long runs better than oil-free, lighter pumps. If you plan to use continuous tools, look for a high duty cycle or a model rated for continuous operation.

Power supply

Large compressors draw a lot of current, and many need a 240-volt circuit. The motor's current draw is on the nameplate. Size the wire and breaker to suit, and use the Wire Gauge Calculator to check the run. Have a qualified electrician install new circuits where code requires.

Plumbing and drying the air

Use an air line big enough to avoid pressure loss. A 1/2 inch hose beats a 1/4 inch one over any distance.
Install a water separator or desiccant dryer when painting. Moisture ruins finishes.
Drain the tank regularly to prevent internal rust.
Use a regulator at the tool, and never exceed the pressure rating of the hose, fittings or tool.

Safety

Compressed air is dangerous. Wear eye protection, never point air at skin, and do not use compressed air to clean clothes. Make sure the safety relief valve works and never tamper with it. Have the tank inspected if it shows rust or damage.

Frequently asked questions

How many CFM do I need for a DA sander?

Most dual-action sanders use around 8 to 12 CFM at 90 psi. Because they run continuously, aim for a compressor that delivers 30 to 50 percent more than the tool's rating.

Is a bigger tank better?

A bigger tank gives you a longer buffer and fewer motor starts, but it does not increase the air the compressor makes. CFM at 90 psi is the number that decides whether it can keep up with a tool.

Do I need a two-stage compressor?

For occasional impact wrenches and inflating tires, a single-stage unit is fine. For painting, sanding or blasting for longer periods, a two-stage unit is usually the better choice because it delivers more air at pressure and runs cooler.

Tools used in this guide

More guides

Figures in this guide are general guidance for planning. Always follow your vehicle, tool and product manufacturers' instructions and local codes, and read our disclaimer.