Carburetor CFM Calculator: RPM & Engine Size

Carburetor CFM Calculator

Estimate naturally aspirated four-stroke engine airflow from displacement, peak RPM and volumetric efficiency. You can also solve the supported displacement or required volumetric efficiency for a known carburetor rating.

Calculate

Engine displacement unit

Number of carburetors

Engine and airflow inputs

Total engine displacement (CID)

Peak engine speed (RPM)

Volumetric efficiency (%)Use a measured or justified project value; this is not selected automatically.

User-selected airflow allowance (%)Optional arithmetic allowance, not a recommendation to oversize.

Carburetor rating basis

Quoted rating depression

A CFM number is comparable only when the rating depression is known. The calculator normalizes ratings with the square-root pressure relationship.

Carburetor airflow result

Engine airflow at 1.5 inHg basis—

Equivalent quoted rating—

Rating per carburetor—

Engine displacement—

Peak engine speed—

Volumetric efficiency—

Airflow at 100% VE—

Selected rating depression—

User-selected allowance—

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Carburetor airflow formulas

For a naturally aspirated four-stroke engine, airflow at peak RPM is estimated as CFM = displacement in cubic inches × RPM × volumetric efficiency ÷ 3,456. Volumetric efficiency is entered as a decimal in the formula, so 85% becomes 0.85.

To compare ratings quoted at different test depressions, the calculator uses CFM₂ = CFM₁ × √(depression₂ ÷ depression₁). A 2-barrel rating commonly quoted at 3.0 inHg therefore cannot be compared directly with a 4-barrel rating quoted at 1.5 inHg.

Important: calculated engine airflow is a sizing estimate, not an instruction to select the next larger catalog carburetor. Throttle response, booster signal, intake restriction and calibration can make an oversized carburetor perform worse.

Checked example

A 350 CID engine at 6,000 RPM and 85% volumetric efficiency requires 516.5 CFM at the 1.5 inHg basis. With one carburetor and no added allowance, the per-carb rating is also 516.5 CFM. The same equivalent rating expressed at 3.0 inHg is 730.4 CFM.

What each mode solves

Mode	Known inputs	Solved value
Required airflow	Displacement, RPM and VE	Total and per-carb CFM
Supported displacement	Carb rating, RPM and VE	Engine CID, cc and liters
Required VE	Carb rating, displacement and RPM	Volumetric efficiency

Limits and checks

The displacement must be total engine displacement, not displacement per cylinder.
The formula models a naturally aspirated four-stroke engine. It does not model boost, intercooling or mass airflow for supercharged or turbocharged engines.
Peak RPM should be the engine speed where the airflow target matters, not necessarily the rev limiter.
Volumetric efficiency must come from measured data, engine simulation or a justified design assumption.
Catalog ratings may use different standards, temperatures, corrections and tolerances; confirm the manufacturer’s test basis.
This tool does not select venturi or throttle-bore size, manifold type, boosters, jets, air bleeds, fuel pressure, ignition timing or fuel-system capacity.
Multiple carburetors are divided equally; unequal linkage or distribution requires separate analysis and tuning.

Carburetor CFM Calculator: Engine Size, RPM and VE