The short answer
CFM = V × (P2 − P1) ÷ (14.7 × T) — that's the pump-up formula. Fill your receiver from pressure P1 to P2 with no air being used, time it, and you get the compressor's true delivered CFM. To size a compressor instead, add up your tools' CFM (adjusted for duty cycle) and multiply by 1.25–1.5 headroom.
Method 1 — Pump-up test (measures real CFM)
This is the field-standard way to find out what a compressor actually delivers — nameplates describe a new machine, not yours.
- 1Isolate the tank. Close the discharge valve so no air is being consumed; you want 100% of pump output going into the receiver.
- 2Record start pressure (P1) and start the stopwatch. Common choice: 0 psig from empty, or 90→120 psig for a loaded-condition test.
- 3Record stop pressure (P2) and elapsed time (T). Note the cut-out pressure and stop the clock.
- 4Apply the formula.
CFM = V(cu ft) × (P2 − P1) ÷ (14.7 × T minutes)V(cu ft) = tank gallons ÷ 7.48
80 ÷ 7.48 = 10.7 cu ft → 10.7 × 100 ÷ (14.7 × 4.0) ≈ 18.2 CFMThat's healthy for a 5 HP two-stage (rule of thumb ≈ 4 CFM per HP at 100 psig ⇒ ~20 CFM new).Skip the math: the pump-up calculator has a built-in stopwatch and does this formula for you.
Method 2 — Tool demand (sizes a compressor)
- 1List every tool that runs at the same time. Use the CFM rating at your working pressure (usually 90 psig for shop tools).
- 2Apply a duty factor to each. Continuous tools (sanders, grinders, blasting): ×1.0. Intermittent tools (impacts, nailers, ratchets): ×0.25–0.5.
- 3Sum and add headroom. Multiply the total by 1.25–1.5 for leaks, wear, and future tools.
Rules of thumb @ 100 psig
| Compressor type | CFM per HP | 5 HP delivers ≈ |
|---|---|---|
| Single-stage piston | 3–4 | 15–20 CFM |
| Two-stage piston | ~4 | ~20 CFM |
| Rotary screw | 4–5+ | 20–25 CFM |
Convert between ACFM and SCFM at your altitude with the CFM converter.
Frequently asked questions
What is CFM in an air compressor?
CFM (cubic feet per minute) is the volume of air a compressor delivers per minute. It's the single most important sizing number: if a compressor's CFM at your working pressure is lower than your tools' combined demand, pressure sags and tools starve. Always compare CFM at the same pressure — a '10 CFM' rating at 40 PSI is much less air than 10 CFM at 125 PSI.
What is the difference between SCFM, ACFM, and FAD?
SCFM is flow corrected to 'standard' conditions (14.7 psia, 68 °F, 36% RH) so numbers compare apples-to-apples. ACFM is the actual volume at the compressor's inlet conditions — it changes with altitude, temperature, and humidity. FAD (free air delivery) is the measured air the machine actually puts out, which is what matters in the field. Manufacturers advertise SCFM or FAD; the pump-up test measures your real FAD.
How do I calculate CFM with the pump-up (tank) method?
Time how long the compressor takes to raise the receiver from a start pressure to a stop pressure with nothing using air. Then: CFM = V × (P2 − P1) ÷ (14.7 × T), where V = tank volume in gallons ÷ 7.48 (to get cubic feet), P1 and P2 are start/stop pressures in psig, and T is the fill time in minutes. Example: an 80-gallon tank going from 0 to 100 psig in 4 minutes delivers 80 ÷ 7.48 × 100 ÷ (14.7 × 4) ≈ 18.2 CFM.
How much CFM do my air tools need?
Add the rated CFM of every tool that runs at the same time, multiply continuous-use tools (sanders, grinders, blasting) by 1.0 duty, and intermittent tools (impacts, ratchets, nailers) by about 0.25–0.5 duty. Then multiply the total by 1.25–1.5 for headroom and future growth. Typical values: ½-inch impact wrench 4–5 CFM, die grinder 4–6 CFM, DA sander 10–15 CFM, HVLP spray gun 10–14 CFM, media blasting cabinet 10–20+ CFM — all at 90 psig.
How many CFM per horsepower does a compressor make?
Rule of thumb at 100 psig: a single-stage piston makes about 3–4 CFM per HP, a two-stage piston about 4 CFM per HP, and a rotary screw about 4–5 CFM per HP. Energy-efficient screws can exceed 5. If your measured CFM per HP is well below these ranges, the pump is worn, valves are leaking, or the drive is slipping.
Does altitude affect compressor CFM?
Yes. Higher altitude means thinner air at the inlet, so the machine moves less mass of air per revolution. Expect roughly 3–4% capacity loss per 1,000 ft of elevation, plus a similar hit to motor cooling. At 5,000 ft, plan on needing a compressor about 15–20% larger than the sea-level sizing math suggests.
What size compressor do I need for a shop?
Total your simultaneous tool demand, apply duty factors, add 25–50% headroom, then pick a compressor whose CFM at your required pressure meets that number. A home garage running impacts and a blow gun is fine at 5–10 CFM @ 90 psig (a 20–30 gallon, ~2 HP unit). A body shop sanding and painting needs 15–25+ CFM (a 60–80 gallon two-stage, 5–7.5 HP). Continuous industrial demand belongs on a rotary screw sized at 70–100% load.
Why does my compressor lose pressure when a tool runs?
The tool is consuming air faster than the pump replaces it — demand CFM exceeds delivered CFM. Confirm with a pump-up test what the machine actually delivers, check for leaks and restrictive filters/hoses/fittings (a ¼-inch fitting can strangle a tool that needs ⅜-inch), and verify the tool's rated CFM against the compressor's rating at the same pressure.