Find the right carburetor size for your engine with the Carburetor CFM Calculator. Enter your engine displacement (in cubic inches or liters), maximum RPM, and volumetric efficiency based on engine build — and get back the recommended CFM (cubic feet per minute) rating your carb needs to flow.
Required Carburetor CFM
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Recommended Carb Size
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Displacement Used
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Volumetric Efficiency
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CFM stands for Cubic Feet per Minute — it measures how much air-fuel mixture a carburetor can flow. Choosing the right CFM rating ensures your engine gets adequate fuel delivery at peak RPM without being over-carbureted, which can hurt throttle response and low-end torque.
The standard carburetor CFM formula is: CFM = (Displacement × Max RPM × Volumetric Efficiency) ÷ 3456. The 3456 constant accounts for the two-stroke cycle of a four-stroke engine and unit conversions. Volumetric efficiency varies by engine build level.
Stock engines typically run around 80% VE. Mildly built engines with aftermarket cam and heads reach about 85%. Dedicated race engines with aggressive cams and ported heads can achieve 90–95% VE. When in doubt, use 85% for a street/strip combo engine.
Carburetors come in standard sizes (390, 450, 600, 650, 750, 800, 850, 950 CFM, etc.). It's generally best to select the next available size up from your calculated value to ensure the engine isn't starved for fuel at peak RPM.
Multiply liters by 61.024 to convert to cubic inches. For example, a 5.7L engine equals approximately 348 ci. You can simply enter the liter value directly in the Liters field and the calculator handles the conversion automatically.
Yes — an oversized carb delivers more fuel than the engine can effectively atomize at low speeds, resulting in poor throttle response, rough idle, and reduced fuel economy. Bigger is not always better; match the CFM rating to your actual engine requirements.
This formula is designed for naturally aspirated engines. Forced induction setups require additional factors to account for boost pressure and increased airflow demand. A boosted engine typically needs a significantly larger carb — consult a specialist for forced induction applications.
A 2-barrel carb has two throttle bores and is better suited for smaller, lower-RPM engines (typically up to about 350 CFM). A 4-barrel carb has four bores and can flow much higher CFM, making it the right choice for performance engines that demand 500 CFM or more.