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Carburetor CFM Calculator
Size a carburetor for displacement and maximum RPM
Carburetor CFM sizing calculates the airflow an engine demands, from its displacement, maximum RPM and volumetric efficiency.
Formula: CFM = CID × RPM × VE ÷ 3456
What this number means
This calculates the airflow a carburetor should supply from engine displacement, maximum RPM and volumetric efficiency. It gives both a theoretical maximum and a practical street-strip figure, which are usually different answers.
What to change next
Choose near the practical figure for a car that must leave cleanly and repeat — an oversized carburetor trades throttle response and low-speed signal for top-end flow you may not use. Recheck the size after any change to displacement or shift RPM.
Worked example
A 355 cu in engine turning 6,500 RPM at 85% volumetric efficiency needs ≈ 568 CFM, so a 650 CFM carburetor is a sensible choice.