Does a Bigger Air Compressor Tank Give You More CFM?

The myth persists because a bigger tank changes what you notice: the tool runs longer before pressure sags, and the motor starts less often. Neither is more flow. Once the tank is down to the tool's pressure, the tool gets exactly what the pump makes.

Same pump, different tank: what the makers print

Two makers sell one pump on two tanks, which settles it:

The 8010 has 74 percent more tank than the 4610AC and the same flow at every pressure listed. The Quincy pair adds 20 gallons for the same 16.20 ACFM. Quincy's economy versions of both, with the pump turning 1,310 rpm instead of 1,399, drop to 15.20 ACFM on either tank, so the pump speed moved the number and the tank did not.

Ratings must also be read at a stated pressure. California Air Tools' pump delivers 3.10 CFM at 40 psi and 2.20 at 90, about 30 percent less. Quincy states its figure at 175 psig, Ingersoll Rand states its 2340L5 at 90 psi (14.3 cfm), and those two numbers are not on the same basis. Compare compressors at 90 psi, the pressure the impact wrench below is rated at, and treat a figure at 175 psig as an understatement of what the same pump delivers at 90.

What a tank actually holds

The Department of Energy's compressed air sourcebook, written with the Compressed Air Challenge, gives the receiver formula used across industry:

V = T x C x Pa / (P1 - P2)

V is tank volume in cubic feet, T the time in minutes, C the demand in cubic feet per minute of free air, Pa atmospheric pressure in psia (14.7 at sea level), and P1 and P2 the starting and ending tank pressures. Turned around, it says how much free air a tank releases as its pressure falls:

free air (cu ft) = (gallons / 7.48) x (P1 - P2) / 14.7

There are 7.48 gallons in a cubic foot. The formula assumes the tank is at ambient temperature, as the sourcebook notes.

For an air tool, P2 is the pressure the tool needs. Chicago Pneumatic rates its CP7748 1/2 in impact at a maximum of 6.3 bar (90 psi), so for that tool the air that counts is the air above 90 psi. Below that, the tool is running short of pressure.

Here are seven published compressors normalized to that one figure, using each maker's tank size and maximum pressure:

Worked for the DeWalt: 6 / 7.48 = 0.80 cu ft of tank, times (165 - 90) / 14.7 = 5.10, gives 4.1 cu ft of free air. The 6 gallon pancake holds more usable air than the 10 gallon 10020C because it is filled 40 psi higher.

The per gallon column is the normalization that makes tanks comparable: (max pressure - 90) / 110. A gallon at 175 psi holds 2.8 times the usable air of a gallon at 120 psi. The sourcebook makes the same point at plant scale: "a 500 gallon storage receiver with a 100 psi storage differential contains the same amount of air as a 5,000 gallon receiver with a 10 psi differential."

How long a full tank runs a 1/2 in impact

Put a tool on the outlet. Chicago Pneumatic's CP7748 manual lists 25 SCFM continuous, the flow while the anvil hammers. While the pump is off, the tank alone covers that, so seconds of run are 60 x free air / 25. When the pump is running, the sourcebook replaces C with C minus S, where S is the compressor's surplus, so the tank only has to cover the shortfall.

The truth sits between the two columns, because a tank compressor's pump does not start until pressure falls to its cut-in setting. The sourcebook gives the usual automotive setting: stop at 175 psig, restart at about 145 psig. Worked for the 60 gallon Quincy with that switch: from 175 to 145 psi the pump is off, and 16.4 cu ft / 25 CFM = 0.65 min. From 145 to 90 psi the pump runs, and 30.0 cu ft / (25 - 16.2) CFM = 3.4 min. Total, about 4.1 minutes of continuous hammering before the tank is at 90 psi.

The difference between the rows is mostly the pump. Once the tank is down, a 25 CFM tool on a 2.2 CFM pump gets 9 percent of its air; on a 16.2 CFM pump it gets 65 percent. Moving the 8010's 2.2 CFM pump onto a 60 gallon, 120 psi tank would add stored air, about 16 cu ft, and nothing to the flow. That is roughly 40 seconds of hammering before the wrench is on 2.2 CFM again.

The average figure decides whether a tank can keep up at all

An impact wrench does not hammer continuously. The CP7748 manual's other air figure is 6.2 SCFM average, about a quarter of the continuous figure, and it is the one that tells you whether the compressor can sustain the work over a job rather than a single fastener. The air hose guide covers why the two numbers differ by four times; see whether 3/8 hose is big enough for a 1/2 in impact.

The rule that follows is simple and has no tank in it:

• Pump CFM at 90 psi below the tool's average draw: the compressor loses ground on every cycle, and any tank only delays the moment the tool starves. The 2.20 CFM California Air Tools pump against the CP7748's 6.2 SCFM average is in this case. So is DeWalt's 2.6 SCFM pancake.
• Pump CFM above the average draw: the compressor keeps up over the job, and the tank only has to carry each burst until the pump refills it between triggers. The Quincy and Ingersoll Rand pumps are in this case for a single 1/2 in impact.

Sizing a tank for the burst you actually make

Turn the sourcebook formula back to size a tank. Tank volume is the burst length times the shortfall, times 14.7, divided by the usable pressure band:

gallons = 7.48 x T x (C - S) x 14.7 / (P1 - P2)

For a 30 second burst at 25 CFM with the tank falling to 90 psi:

Worked for the last row: 7.48 x 0.5 x 22.8 x 14.7 / 30 = 41.8 gal. A big pump needs very little tank to carry a burst, and a small pump needs a very large one, filled as high as the tank and pump allow. Pump CFM and cut-out pressure are both on the spec sheet; the gallons follow from them.

The sourcebook also retires the shortcut that usually replaces this arithmetic. For industrial primary receivers it notes that the old rule of 1 to 3 gallons per cfm of compressor capacity "is no longer regarded as good practice." The formula above is what replaced it.

What a bigger tank does buy: fewer motor starts

At a steady demand below the pump's output, the pump fills the tank to cut-out, stops, and the tool drains it to cut-in. Each half of the cycle is a division:

• Pump off: free air in the band / demand
• Pump on: free air in the band / (pump CFM - demand)

Worked for a 6.2 CFM average load on the Quincy QT-54's 16.2 ACFM pump, with the sourcebook's 175 to 145 psi switch band:

The last column is the one people do not expect. The share of time the pump runs is demand divided by pump CFM, 6.2 / 16.2 = 38 percent, and the tank size cancels out of it. A bigger tank makes the cycles longer and less frequent, which the sourcebook describes as the purpose of the wide band on small tank compressors: it "reduces the frequency of motor starts, thereby protecting the motor." It does not make the pump work less.

That matters when you read a duty cycle. Ingersoll Rand lists the 2340L5 at "100% continuous duty". California Air Tools lists the 8010 at a 70/30 duty cycle. Your running share is demand divided by pump CFM, and that is the number to hold against the maker's duty rating. A bigger tank cannot lower it.

Checking a maker's recovery time against the arithmetic

The same formula checks recovery figures, which some makers publish instead of a cut-in pressure. Pump-up time is the free air in the band divided by the pump's CFM, the sourcebook's formula with C replaced by S:

The 8010 matches its CFM rating to the second, and the 4610AC is close. The 10020C's 25 seconds needs about 6.5 CFM across 95 to 125 psi, more than its own 6.40 CFM rating at 40 psi, so plan on the 5.30 CFM at 90 psi instead.

Where this answer stops applying

• The tool's pressure is not 90 psi. Everything here counts air down to 90 psi. A tool that runs on less, such as a brad nailer or a blow gun turned down, can draw the tank further and gets more out of the same gallons.
• Hose and regulator losses. The figures are at the tank. A hose loses pressure while air flows, about 30 psi over 50 ft of 3/8 in hose at 25 CFM as read from the hose industry chart, so the tank has to stay further above 90 psi than this page assumes. The air hose guide has the numbers by length.
• Temperature. The receiver formula assumes ambient temperature. A tank pumped up hot loses pressure as it cools, so it holds less than the gauge read at shutoff.
• Unpublished switch settings. Where the maker does not publish cut-in pressure, this page uses the band the maker quotes recovery over, or the sourcebook's typical 175 to 145 psi. Check your own pressure switch for the real band.
• Industrial systems. For plant receivers, the sourcebook says recommended size varies with compressor type and capacity control. This page is about tank-mounted shop compressors.

Placing and powering the compressor you pick

A compressor is one of the loads the extension cord gauge guide leaves out of its tool tables, because equipment with its own manual follows its own cord instructions. The garage shop layout guide puts a compressor in the floor zone and advises leaving a dedicated circuit for it before a wall system goes up. The sourcebook also notes that a receiver can "drop out some of the condensate", and that water ends up in the tank, one more reason to keep a big compressor away from tool storage and to manage shop humidity as the rust prevention guide describes.

If the arithmetic says your 1/2 in impact needs a 16 CFM pump you do not have room or power for, a cordless impact sidesteps the whole calculation. Compare packs on watt hours rather than the label voltage, as worked out in whether 20V MAX is the same as 18V.

The thirty-second check

1. Find the tool's average CFM and its continuous or at-load CFM, both at 90 psi.
2. Find the compressor's CFM at 90 psi. If it is below the tool's average, stop: no tank fixes it, buy more pump.
3. Compute usable air: gallons / 7.48 x (max psi - 90) / 14.7. Compare tanks on this, not on gallons.
4. Divide usable air by the shortfall (continuous CFM minus pump CFM) for minutes of continuous run. If that covers your longest burst, the tank is big enough.
5. Pick a bigger tank only for longer bursts or fewer motor starts. It never raises the CFM.