Carburetor CFM Calculator (engine airflow from displacement, rpm & VE)

Understanding the carburetor CFM calculator — concept and principle

Carburettors are rated by how much air they flow, in cubic feet per minute (CFM). This calculator estimates the airflow an engine needs at its maximum speed, so you can choose a carburettor of about the right size.

Each cylinder fills every second revolution, so 350 in³ at 6,000 rpm and 85% fill moves about 516 cubic feet a minute.

How it is calculated

A four-stroke engine fills each cylinder once every two revolutions, so ideal airflow = displacement × rpm ÷ 2.
With displacement in cubic inches and 1,728 in³ per ft³: CFM = CID × rpm × VE ÷ 3,456.
VE (volumetric efficiency) is the share of the ideal air actually drawn in.
Conversions: 1 CFM = 0.0283168 m³/min = 0.471947 L/s.

The principle

Each intake stroke can at most draw in one cylinder volume of air. Multiplying the swept volume by the number of intake strokes per minute gives the ideal flow. Real engines fill their cylinders less than completely at high speed because of restrictions in the intake, valves and ports; VE captures that. A carburettor much larger than this flow reduces air velocity through the venturi, which weakens fuel metering and throttle response at low speed.

Worked example

Default inputs: 350 cubic inches, 6,000 rpm, VE 85%.

Ideal flow = 350 × 6,000 ÷ 3,456 ≈ 607.6 CFM.
With VE: 607.6 × 0.85 ≈ 516 CFM.
In metric: 516 × 0.0283168 ≈ 14.63 m³/min, or about 243.8 L/s.
Displacement used = 350 cubic inches.

Each point is the calculator re-run with only “Engine displacement” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Engine displacement	Airflow (CFM)
87.5	129
175	258
262	387
350	516
438	646
525	775
700	1,033

How to read the result

Choose a carburettor rated near this figure; for a typical street engine, being slightly under is often better for drivability than being far over. Note that four-barrel carburettors are rated at 1.5 inches of mercury pressure drop and two-barrels at 3.0 inches, so their CFM numbers are not directly comparable.

Common mistakes

Using a racing VE for a mild street engine.
Using a redline rpm the engine never actually reaches.
Comparing two-barrel and four-barrel ratings directly.
Entering displacement in litres while the unit is set to cubic inches.

Keep in mind: Carburettor selection also depends on cam, manifold and use. Follow manufacturer guidance and have engine work done by qualified people.

Frequently asked questions

What CFM carburettor does a 350 need?

At 6,000 rpm and 85% VE, about 516 CFM, so roughly a 500–600 CFM carburettor.

What is volumetric efficiency?

The actual air drawn in as a percentage of the cylinder's swept volume.

Is a bigger carburettor always better?

No. Too large a carburettor lowers air velocity and hurts low-speed response.

Sources: Heywood, J. B., Internal Combustion Engine Fundamentals, 2nd ed. McGraw-Hill (2018) (volumetric efficiency) · SAE J1349, Engine Power Test Code (air standard conditions)

Checked 9 Nov 2026. C. Shih, “Internal Combustion Engines” lecture notes, FAMU–FSU College of Engineering: volumetric efficiency η_v = 2·(ṁ_a/ρ_a)/(N·V_d), i.e. air volume = VE × V_d × N/2. 1 ft³ = 1,728 in³ = 28.316846592 L; 1 L = 61.0237 in³.

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