How do you calculate CFM from pump-up time?

CFM = tank volume (ft³) × pressure rise (psi) ÷ 14.7 ÷ time (minutes). A gallon is 0.1337 ft³, so a 120 gal tank is about 16 ft³.

What pump-up result indicates a worn compressor?

Delivered flow more than roughly 10–15% below nameplate usually points to worn rings or valves on a reciprocating unit, or a worn airend and leaking check valve on a screw.

Do I need to close the system first?

Yes. Any open drain, leak or tool draw during the test shows up as lost capacity and makes a healthy compressor look weak.

Air Compressor Pump-Up Test — Measure Actual CFM

All calculators

Formula

Actual CFM = (Tank gal × ΔPSI × 0.536) / Time sec

How to run the test:

Close service valve so no air leaves the tank.
Drain tank to start pressure (or note current).
Start compressor, time how long to reach end pressure.
Enter values below.

Stopwatch

times the pump-up for you

00:00.00

Units

Tank size

gal

Start pressure

psi

End pressure

psi

Time

sec

Nameplate rating (optional)

cfm

Target spec (optional)

Compare against

Actual delivered flow

42.9cfm

20.24 L/s · 72.9 m³/h · ΔP 40.0 psi

What the result tells you

Way below nameplate: worn rings, leaking reed/discharge valves, failed check valve, clogged intake filter.
Builds fine but runs constantly: system leak downstream, or demand exceeds supply.
Short-cycles (< 1 min load/unload): leaks, tank too small, or pressure band set too narrow.

Constant 0.536 comes from converting tank volume × pressure rise to free air (1 ft³ at 14.7 psia per 7.48 gal × 60 s/min ÷ 14.7 psi).

How to use this calculator

Time how long a compressor takes to fill a receiver and convert that into delivered CFM.

Isolate the system so no air is being consumed downstream.
Start the built-in stopwatch when the compressor loads and stop it at the cut-out pressure.
Enter receiver volume in gallons and the starting and ending pressures.
Read the delivered CFM and compare it with the compressor nameplate rating.

Frequently asked questions

How do you calculate CFM from pump-up time?: CFM = tank volume (ft³) × pressure rise (psi) ÷ 14.7 ÷ time (minutes). A gallon is 0.1337 ft³, so a 120 gal tank is about 16 ft³.
What pump-up result indicates a worn compressor?: Delivered flow more than roughly 10–15% below nameplate usually points to worn rings or valves on a reciprocating unit, or a worn airend and leaking check valve on a screw.
Do I need to close the system first?: Yes. Any open drain, leak or tool draw during the test shows up as lost capacity and makes a healthy compressor look weak.