```
HP = (CFM x PSI x 0.000578) / Compressor Efficiency
```
Where:
- CFM is the compressor's free air delivery in cubic feet per minute (CFM).
- PSI is the compressor's discharge pressure in pounds per square inch (PSI).
- 0.000578 is a constant that converts cubic feet per minute to horsepower.
- Compressor Efficiency is the efficiency of the compressor, typically ranging from 0.5 to 0.9.
To calculate the horsepower of a compressor, simply multiply the CFM by the PSI and multiply the result by 0.000578. Then, divide the result by the compressor efficiency to get the horsepower.
For example, let's say a compressor has a free air delivery of 20 CFM and a discharge pressure of 100 PSI, with an efficiency of 0.8. To calculate the horsepower, we would:
```
HP = (20 x 100 x 0.000578) / 0.8
= 1.44 HP
```
