Calculate the correct carburetor CFM sizing for any engine. Factors in displacement, RPM, volumetric efficiency, and barrel count with common carb reference.
The **Carburetor CFM Calculator** determines the correct airflow capacity (in cubic feet per minute) for a carburetor matched to your engine. Enter the engine displacement, maximum RPM, volumetric efficiency, and number of barrels, and the calculator returns the required CFM, per-barrel flow, estimated air mass flow, fuel consumption, approximate peak horsepower, and throttle bore diameter. That makes it easier to compare a target carb size against the engine's actual airflow need before buying parts. It gives you a clearer starting point than guessing from engine size alone.
Carburetor sizing is critical for engine performance. Too small a carb starves the engine of air at high RPM; too large a carb reduces throttle response and low-speed driveability due to poor signal (vacuum) at the venturi. The classic formula CFM = (CID × RPM × VE) / 3456 has been the go-to for hot-rodders and racers for decades.
Use the engine presets (SBC 350, Ford 302, LS3, and more), explore the RPM-CFM table, and compare against common carburetors in the reference table. That helps turn the raw airflow number into a practical buying check instead of a standalone formula result.
Choosing the right carburetor is one of the most impactful decisions in building a carbureted engine. This calculator applies the classic CFM formula with engine presets, VE correction, altitude adjustment, and common carb reference data. The result is a practical sizing check before you compare specific carburetor models or barrel layouts.
CFM = (CID × RPM × VE) / 3456 Air Mass Flow: CFM × 0.0765 lb/ft³ Fuel Flow: Air / 14.7 (stoichiometric) Est. HP: Fuel (lb/hr) / BSFC (0.5 lb/HP·hr)
Result: 445 CFM required, est. 252 HP
A stock 350 small-block at 5 500 RPM with 80% VE needs about 445 CFM. A 500–600 CFM 4-barrel (like a Holley 4160) is the right match.
Use consistent units, verify assumptions, and document conversion standards for repeatable outcomes.
Most mistakes come from mixed standards, rounding too early, or misread labels. Recheck final values before use. ## Practical Notes
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Track units and conversion paths before applying the result. ## Practical Notes
Use this note as a quick practical validation checkpoint. ## Practical Notes
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Use as a sanity check against edge-case outputs. ## Practical Notes
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Cubic Feet per Minute — a measure of air volume flow. Carburetors are rated by their maximum CFM at a standard pressure drop (typically 1.5" Hg), which lets different carbs be compared on the same basis.
No — an oversized carb hurts throttle response and low-speed running. Match the carb to your engine's actual peak CFM requirement so the airflow matches the engine's real demand.
The percentage of the theoretical cylinder volume actually filled with air on each intake stroke. Higher VE (better heads, cam, intake) means the engine breathes more, which raises the airflow requirement.
Mostly for throttle response and packaging. A 600 CFM 2-barrel flows the same as a 600 CFM 4-barrel at WOT, but the 4-barrel has better part-throttle response because the airflow is split differently.
Air is less dense at altitude. Size the carb 10% larger per 3 000 ft of elevation, and re-jet for the thinner air so the mixture stays close to correct.
EFI meters fuel electronically. CFM is still relevant for throttle body sizing, but injector flow rate (lb/hr) replaces carb sizing because the fueling strategy is different.