CFM Calculator

Enter your engine size in cubic inches and its maximum RPM to see how many cubic feet per minute of air it needs, as a starting point for sizing a carburetor. Results are shown at three levels of volumetric efficiency, from a modern stock engine to a highly modified one.

How to calculate carburetor CFM

CFM = cubic inches × max RPM × volumetric efficiency ÷ 3,456

A four-stroke engine fills each cylinder once every two revolutions, so it pulls in its full displacement every two turns of the crank. The 3,456 is 1,728 cubic inches in a cubic foot times those 2 revolutions. Volumetric efficiency is entered as a decimal, so 85% is 0.85.

Volumetric efficiency

Volumetric efficiency is the actual amount of air flowing through the engine compared with its theoretical maximum. An engine at 100% is using all of its volumetric capacity. The calculator uses the three levels from Summit Racing:

Engine	Typical modifications	Volumetric efficiency
Modern stock engine	None	85%
Slightly modified engine	Performance cam, performance intake manifold, headers, ignition upgrade	100%
Highly modified engine	Aggressive cam, performance intake manifold, headers, ignition upgrade, aluminum or ported iron cylinder heads, 11:1 or higher compression	115%

Pick the line that best matches your engine.

Example: a 350 turning 6,000 RPM

350 × 6,000 × 0.85 ÷ 3,456 = 516 cfm

If the engine is slightly modified, it needs 608 cfm at 100% volumetric efficiency. Highly modified, it needs 699 cfm at 115%.

How the formula compares with manufacturer recommendations

Chevrolet Performance engine	Max engine speed	Calculator at 85% VE (stock)	Recommended carburetor
350/290 HP	5,100 RPM	439 cfm	670 cfm
350 HO	5,100 RPM	439 cfm	670 cfm
SP350/357	5,600 RPM	482 cfm	670 cfm
ZZ5 350	5,800 RPM	499 cfm	750 cfm

Maximum engine speed and recommended carburetor from the Chevrolet Performance spec sheets for the 350/290 HP, 350 HO, SP350/357 and ZZ5. Volumetric efficiency levels from Summit Racing.

For each of these engines, Chevrolet Performance recommends a larger carburetor than the formula gives at the engine's maximum speed. Holley's guide, How to Select the Right Carburetor Size, also notes that engines from 300 to 400 cubic inches usually end up with a 650 to 700 cfm carburetor. Use the calculator as a starting point, and go with the engine maker's recommendation when there is one.

Choosing between sizes

Holley's carburetor sizing guide says to use a carburetor rated at or slightly below your engine's calculated airflow, and to pick the smaller size when in doubt because it usually accelerates better. That matters most with mechanical secondaries: on a carburetor that's too big, the accelerator pump shot runs out before the main system starts to flow, and the engine bogs.

The same Holley guide says supercharged engines generally need 40% to 50% more cfm than the same engine without a supercharger.

To size the rest of the fuel system, use the fuel pump flow calculator. If you're converting to fuel injection, the fuel injector size calculator takes the place of this one.

Frequently Asked Questions

How do you calculate CFM for a carburetor?

Multiply the engine size in cubic inches by the maximum RPM and the volumetric efficiency as a decimal, then divide by 3,456.

What size carburetor do I need for a 350?

At 6,000 RPM the formula gives 516 cfm for a stock engine at 85% volumetric efficiency and 608 cfm for a slightly modified one at 100%. Chevrolet Performance recommends a 670 cfm carburetor for its 350/290 HP, 350 HO and SP350/357 crate engines and a 750 cfm carburetor for the ZZ5.

What volumetric efficiency should I use?

Use about 85% for a modern stock engine, 100% for a slightly modified engine with a performance cam, intake, headers and ignition at 100%, and a highly modified engine with an aggressive cam, better heads and 11:1 or higher compression at 115%.

Is it better to go bigger or smaller on carburetor size?

Choose a carburetor rated at or slightly below the engine's calculated airflow, and the smaller size when in doubt. An oversized carburetor with mechanical secondaries can bog.