Quick answer: Add up the CFM of all air tools and equipment that run at the same time, apply realistic usage factors, add 10–30% for leaks and growth, and choose a rotary screw compressor whose delivered CFM at your working pressure meets that total. Set pressure only as high as your equipment needs, and pair the compressor with a correctly sized receiver tank and dryer.
Step 1: List every air user
Make a list of each tool or machine that uses compressed air, with its air consumption in CFM and required pressure. You will find these figures in tool manuals or on product labels. Include blow guns, sanders, impact wrenches, tire changers, paint guns, blast cabinets and any machines with pneumatic cylinders.
Step 2: Apply usage factors
Few tools run 100% of the time. A sander might run 50% of the time it is in use; an impact wrench might run 10–20%. Multiply each tool’s CFM by its realistic usage factor, then add up the tools that run in the same period.
| Tool | CFM at 90 PSI | Quantity | Usage factor | Average CFM |
|---|---|---|---|---|
| Dual-action sander | 12 | 2 | 50% | 12 |
| 1/2 in. impact wrench | 5 | 3 | 20% | 3 |
| HVLP paint gun | 14 | 1 | 60% | 8.4 |
| Blow guns | 3 | 4 | 25% | 3 |
| Total average demand | 26.4 | |||
The figures above are example values; use your own tool data.
Step 3: Allow for leaks and growth
Every air system leaks at fittings, hoses and couplers. Add 10–30% to cover leakage and future tools. In the example, 26.4 CFM plus 25% is about 33 CFM. A compressor delivering around 38 CFM at 125 PSI would give comfortable headroom.
Step 4: Set the right pressure
Most air tools need 90 PSI at the tool. Compressors usually run at 110–125 PSI to cover pressure drop through dryers, filters and piping. Avoid running higher than needed: a common industry rule of thumb is that each 2 PSI of extra discharge pressure adds roughly 1% to energy use. Fix pressure drop problems with better piping instead of turning up the compressor.
Step 5: Receiver tank and dryer
- Receiver tank: Stores air and reduces compressor cycling. A common guideline is 3–5 gallons per CFM for load/unload screw compressors; follow the manufacturer’s recommendation.
- Dryer: Size the dryer for the compressor’s full output under your room temperature. See refrigerated vs desiccant dryers.
- All-in-one packages: Some screw compressors are tank-mounted with a built-in dryer, saving floor space and installation time.
Step 6: Check power and space
Confirm your supply voltage and phase. Many models in our rotary screw compressor range are offered in several voltages, such as 200V, 230V and 460V three-phase, and some 7.5 HP models in single-phase. Plan a compressor room with ventilation, service clearance and a level floor.
Common sizing mistakes
- Adding every tool at 100% use, which leads to an oversized compressor that runs unloaded most of the time.
- Ignoring future expansion, which leads to buying a second unit soon after.
- Comparing horsepower instead of delivered CFM at pressure.
- Forgetting the dryer’s capacity drop in hot rooms.
Frequently asked questions
What about variable speed drive (VSD) compressors?
VSD models adjust motor speed to match demand and save energy where demand varies widely. Fixed-speed units suit steady demand.
Is it better to buy two smaller compressors?
Two units give redundancy and handle varying demand well, but cost more up front. It suits facilities that cannot afford downtime.
How do I measure my current air demand?
Track how long your current compressor is loaded during a typical hour, or have an air audit done with data loggers.
Send us your tool list and shift pattern, and we will help you size a rotary screw compressor.