Carburetor CFM Calculator

Enter your engine's displacement, maximum RPM, and volumetric efficiency to calculate the carburetor CFM (airflow) it needs, plus the nearest standard carburetor size to buy.

Quick Facts

CFM formula
CFM = (CID × RPM × VE) / 3456
3456 = 1728 in³/ft³ × 2, since a 4-stroke engine draws air in on only one of every two revolutions.
Typical volumetric efficiency
75% - 110%+ VE
Stock engines run about 75-85% VE; well-tuned performance engines can reach 95-110%+.
Carburetors come in fixed sizes
350 - 1050+ CFM
Carbs are manufactured in standard ratings, so round your result up to the nearest available size.
Forced induction adjustment
× (boost psi + 14.7) / 14.7
Boosted engines need proportionally more CFM because the compressed intake charge is denser.

Your Results

Calculated
Required Carburetor CFM
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CFM = (CID × RPM × VE) / 3456
Engine Displacement
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Converted to cubic inches (CID)
Recommended Carburetor Size
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Nearest standard size at or above calculated CFM
Forced Induction Adjustment
-
Multiplier applied for boost pressure

Ready

Enter your engine specs, then press Calculate.

How to Calculate Carburetor CFM (Airflow Requirement)

A carburetor's CFM (cubic feet per minute) rating tells you the maximum volume of air it can flow. The goal when sizing one is to match that rating to what your engine can actually consume at peak RPM — a carburetor that's too small restricts airflow and chokes power at high RPM, while one that's far too large hurts idle quality, throttle response, and low-speed drivability. Engine builders and carburetor manufacturers (Holley, Edelbrock, and others) all size carburetors with the same standard formula, which this calculator applies to your engine's displacement, redline RPM, and volumetric efficiency.

Where the formula comes from

A four-stroke engine completes one intake stroke every two crankshaft revolutions, so at a given RPM it "breathes" its full displacement (CID, in cubic inches) only RPM/2 times per minute. Dividing by 1728 (the number of cubic inches in a cubic foot) converts that to cubic feet per minute, giving CID × RPM / 3456 — the theoretical maximum airflow. Real engines never fill a cylinder completely at speed, so the result is multiplied by volumetric efficiency (VE), the ratio of the air an engine actually ingests to that theoretical maximum: CFM = (CID × RPM × VE) / 3456. For example, a 350 ci engine at a 6,000 RPM redline with 85% VE needs (350 × 6000 × 0.85) / 3456 ≈ 516 CFM.

Picking volumetric efficiency and accounting for boost

VE depends on how well the engine breathes: a stock engine with a mild cam typically runs 75-85% VE, a performance engine with a bigger cam, headers, and a matched intake reaches roughly 85-95%, and a well-tuned high-performance or race engine with ported heads can reach 95-110% or more. For turbocharged or supercharged engines, forced induction compresses the intake charge so it is denser than atmospheric air, meaning the engine consumes more air mass per revolution than the naturally aspirated formula alone predicts. Approximate the increase by multiplying the naturally aspirated CFM by (boost pressure in psi + 14.7) / 14.7, using standard atmospheric pressure of 14.7 psi as the baseline.

Frequently Asked Questions

What is the formula for calculating carburetor CFM?
CFM = (Engine Displacement in cubic inches × Maximum RPM × Volumetric Efficiency) / 3456. For example, a 350 ci engine revving to 6,000 RPM at 85% VE needs about (350 × 6000 × 0.85) / 3456 ≈ 516 CFM.
What volumetric efficiency should I use?
Use 75-85% for a stock or mildly modified engine, 85-95% for a performance engine with a bigger cam and headers, and 95-110%+ for a well-tuned high-performance or race engine with matched heads, cam, and intake. If unsure, 80-85% is a safe starting estimate for a street engine.
Should I round the calculated CFM up or down when buying a carburetor?
Round up to the nearest standard carburetor size (e.g., 600, 650, 750, or 850 CFM). An undersized carburetor chokes airflow at high RPM and costs power, while a carburetor that is modestly larger than calculated is usually the smaller compromise — a slightly softer idle and part-throttle response.
How does a turbocharger or supercharger change the CFM I need?
Forced induction compresses the intake charge, so it is denser and the engine consumes more air mass per revolution than a naturally aspirated engine of the same displacement. Multiply the naturally aspirated CFM figure by (boost psi + 14.7) / 14.7 to approximate the increased airflow demand at your target boost level.

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