Air Compressor Duty Cycle Calculator #

Work out how long a tool can run on stored air and how long the pump needs to refill it. Enter pump and tool CFM at 90 PSI plus tank size to get tool run time, pump recovery time and the actual duty cycle your machine has to sustain.

System Parameters

Units

Pump Delivered Airflow at 90 PSI (CFM)

Tool Air Demand at 90 PSI (CFM)

Tank Volume (gallons)

Cut-In to Cut-Out Band ΔP (PSI)Pressure swing between cut-in and cut-out; 30 PSI is a typical shop band.

Advanced settings

Rated duty cycle of your machine (%)Oil-free piston ≈ 50%; oil-lubricated piston 75–100%; industrial or rotary screw 100%. Take the figure from your manual.

About this calculator

This screen answers the time question rather than the volume question: how long the tool runs before the tank hits cut-in, how long the pump needs to refill it, and what duty cycle that cycle demands. Tank geometry and receiver sizing are handled by the air receiver tank sizing calculator, and pump selection by the air compressor sizing calculator. Browse all pneumatic calculators.

Results

Usable Stored Air

5.46 ft³

Tool Run Time

109.1 s

Pump Recovery Time

65.5 s

Actual Duty Cycle

62.5%

Required Pump Duty

100% (rated 100%)

Cycles per Hour

20.6

Usable stored air = 0.2728 ft³ per gallon at a 30 PSI band (ideal-gas approximation at 14.7 PSIA).

Planning estimate — ideal-gas approximation at 14.7 PSIA with a steady tool draw. Verify stored-air volume and duty cycle against the pump and tank documentation before relying on a continuous-duty claim.

Assumes 20.0 gal of storage, a 30 PSI cut-in to cut-out band, and a pump below tool demand.

Oil-free piston compressors are typically rated around 50% duty; oil-lubricated units 75–100%. Running a tool non-stop needs a 100%-duty machine regardless of tank size.

What to do with this result

Intermittent: the tool draws 8.0 CFM against a 5.0 CFM pump, so the tool can run 109.1 s before the tank reaches cut-in, then needs 65.5 s to refill. That cycle keeps the pump loaded continuously, so required pump duty is 100%, which your 100% rating allows. A larger tank only stretches the run and recovery times — it does not change the duty ratio. For genuinely continuous air, size the pump to at least 8.0 CFM, the tool demand at 90 PSI.

Continue Your Calculation

After duty cycle, size the pump so the tool never outruns it, then check what the tool draw does to distribution pressure.

Size compressor →

Quick Reference #

Usable stored air per gallon of receiver volume, for a cut-in to cut-out band at 72°F. Multiply by your tank gallons to get total usable air.

Pressure band ΔP (PSI)	Usable air per gallon (ft³)	20 gal tank (ft³)	Notes
10	0.0909	1.82	Narrow band, frequent starts
20	0.1819	3.64	Tight band, less reserve
30	0.2728	5.46	Typical shop band (this calculator's default)
40	0.3638	7.28	Wide band, fewer starts
50	0.4547	9.09	Very wide band, check tool pressure limits

Basis: V_air = gallons × 0.133681 ft³/gal × (ΔP / 14.7 PSIA). This corrects the 0.4 ft³/gal rule of thumb, which has no stated pressure basis.

Formulas and Method #

1. Usable stored air

V_air = V_gal × 0.133681 × (ΔP / P_atm)

Receiver volume is converted from US gallons to cubic feet with 0.133681, then multiplied by the pressure ratio between the storage band and atmosphere (14.7 PSIA). One gallon at a 30 PSI band yields 0.133681 × (30 / 14.7) = 0.27 ft³. Treating the release as isothermal ideal-gas expansion understates real recovery slightly and is a planning approximation only.

2. Tool run time and pump recovery time

t_run = V_air / (C_tool − C_comp)   (only when C_tool > C_comp)
t_recover = V_air / C_comp

While the tool runs, the pump still delivers C_comp, so the tank drains at the difference between demand and supply. Once the tool stops, the pump refills the same usable volume at its full output. If C_comp is greater than or equal to C_tool the tank no longer drains and the tool has continuous air.

3. Duty cycle

tool duty = t_run / (t_run + t_recover) = C_comp / C_tool
required pump duty = C_tool / C_comp

Both follow the compressor-industry definition of duty cycle, on time divided by on time plus off time. Because V_air appears in both time terms it cancels, so tank volume drops out of the ratio entirely. Note the asymmetry: a pump that is smaller than the tool leaves the tool running only part of the time, yet the pump itself must stay loaded throughout that cycle, which is why duty rating and pump size must be judged together.

Worked example (this page's defaults): a 20 gallon tank on a 30 PSI band holds 20 × 0.133681 = 2.674 ft³ of volume, giving V_air = 2.674 × (30 / 14.7) = 5.46 ft³. With a tool drawing 8 CFM against a 5 CFM pump, t_run = 5.46 / (8 − 5) = 1.82 min = 109.1 s and t_recover = 5.46 / 5 = 1.09 min = 65.5 s. The resulting tool duty cycle is 109.1 / (109.1 + 65.5) = 62.5%, which matches C_comp / C_tool = 5 / 8. Required pump duty is C_tool / C_comp = 160%, capped at 100%. Doubling the tank to 40 gallons doubles both times to 218 s and 131 s but leaves the duty cycle at 62.5%.

Rated duty cycle by compressor type #

Compressor type	Typical rated duty	Practical implication
Oil-free piston, 1–6 gal (portable)	50%	Run/rest roughly 1:1 in a 10 minute window; long tool runs need breaks
Oil-lubricated piston, 20–60 gal	75–100%	Tolerates long loaded periods; check the manual for the exact figure
Industrial piston / rotary screw	100%	Continuous duty by design; intended for always-on plant air
VSD / variable speed	100%	Throttles instead of cycling; changes start frequency, not tool demand

Duty cycle ratings describe the pump and motor, not the tool. Verify the rating for your exact model, and remember that a cycle which drains the tank faster than the pump refills will hold the pump loaded for the whole cycle.

Frequently Asked Questions #

How do you calculate air compressor duty cycle?

Duty cycle is the share of a cycle that the equipment is running, written as on time divided by on time plus off time. For a tank served by a pump, the tool can run for t_run = V_air / (C_tool - C_comp) before the tank reaches cut-in, and the pump needs t_recover = V_air / C_comp to refill. The tool duty cycle is therefore t_run / (t_run + t_recover), which simplifies to C_comp / C_tool.

How long can an air compressor run before the tank empties?

It depends on usable stored air and on how much faster the tool draws than the pump delivers. Usable stored air is V_air = V_gal x 0.133681 x (dP / 14.7). A 20 gallon tank with a 30 PSI band holds about 5.46 cubic feet, so a tool drawing 8 CFM against a 5 CFM pump drains it in about 109 seconds, while a 10 CFM pump would keep up and the tank would not empty at all.

Why does a bigger tank not change the duty cycle?

Tank volume scales run time and recovery time together because V_air appears in both formulas. Cancelling V_air leaves t_run / (t_run + t_recover) = C_comp / C_tool, which contains no tank term. A larger tank stretches each cycle and reduces starts per hour, but only a higher pump CFM changes the duty ratio.

What duty cycle does an oil-free compressor have?

Oil-free piston compressors are typically rated around 50 percent duty, meaning roughly 5 minutes running for every 5 minutes of cooldown in a 10 minute window. Oil-lubricated piston units are commonly rated 75 to 100 percent, and industrial or rotary screw machines are generally rated 100 percent continuous. Always use the duty cycle printed in the manual for the specific model.

Is the tool duty cycle the same as the compressor duty cycle?

No, and they are reciprocal views of the same balance. Tool duty cycle = C_comp / C_tool describes how much of the time the tool can run, while required pump duty = C_tool / C_comp describes how hard the pump must work. With a 5 CFM pump and an 8 CFM tool the tool runs about 62.5 percent of the time, but the pump inside that cycle must run 100 percent of the time, which is why intermittent tools still need a duty-rated machine.