Carburetor CFM Calculator — Engine Airflow Sizing
Enter engine displacement, peak RPM and volumetric efficiency to calculate the required airflow in CFM and find the closest standard carburetor size for street or race use.
Displacement unit
CID
RPM
%
Cubic feet per minute of air at peak RPM — round up to the next standard carb size
- 1
VE as decimal
85 ÷ 100 = 0.85 - 2
Required airflow
350 × 5,000 × 0.85 ÷ 3456 = 430÷ 3456 = ÷ (2 strokes/rev × 1,728 in³/ft³) converts to cubic feet per minute
How does this calculator work?
CFM = (CID × RPM × VE%) / 3,456 for a 4-stroke engine. A 350 CID engine at 5,000 RPM with 85% VE needs ~430 CFM; round up to the nearest standard size (typically 450 or 600 CFM). For street use, size down 10–15% to preserve low-RPM throttle response and fuel atomisation.
Formula
How this is calculated
A four-stroke engine draws in fresh charge once every two crankshaft revolutions. The theoretical volume of air consumed per minute equals the engine displacement (CID) multiplied by half the RPM. Dividing by 1,728 converts cubic inches to cubic feet, giving a theoretical CFM in an ideal engine. The volumetric efficiency (VE) factor corrects for real-world losses: friction, valve timing overlap, port restrictions and charge heating all prevent complete cylinder filling. Combining these steps gives the industry-standard formula: CFM = (CID × RPM × VE%) / 3,456.
Volumetric efficiency spans a wide range. Stock passenger engines typically achieve 75–85%; performance engines with ported heads and longer-duration camshafts can reach 90–98%; theoretical 100% means the cylinder fills exactly to displacement volume. Forced-induction engines (supercharged or turbocharged) can exceed 100% VE because the boost pressure packs more charge than the displacement alone could draw. This calculator is for naturally aspirated engines only.
Sizing strategy differs by use: for a dedicated race engine, size the carb directly to the calculated peak-RPM CFM. For a street or street-strip engine, a carb sized 10–15% below peak CFM improves low-RPM throttle response, fuel atomisation and idle quality, because larger venturis move air more slowly at part-throttle. Standard carb sizes (390, 600, 650, 750, 850, 950, 1,050 CFM) are shown for both scenarios.
Frequently asked questions
Stock passenger car engine: 75–85%. Mild performance street build (cam, heads, headers): 85–92%. Full race engine (aggressive cam, CNC-ported heads, tuned exhaust): 95–105%. Supercharged/turbocharged: can exceed 100% (use actual measured VE if known). When unsure, 85% is a safe starting point for a street/strip engine.
For street or dual-purpose use, size down 10–15% from the calculated peak CFM. An oversized carb has large venturis that move air slowly at part-throttle, degrading fuel atomisation, throttle response and fuel economy. A race-only engine running near wide-open throttle benefits from matching or slightly exceeding the calculated CFM.
Yes — throttle bodies and carburetors are both air-metering devices governed by the same engine-displacement physics. The CFM formula gives the required airflow through the throttle body; fuel is metered separately by the injectors. Multi-point injection systems use this calculation to size the throttle body, not the injectors.
TG we-Calculate Editorial Team. (2026). Carburetor CFM Calculator — Engine Airflow Sizing [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/carburetor-cfm-calculator
TG we-Calculate Editorial Team. "Carburetor CFM Calculator — Engine Airflow Sizing." TG we-Calculate. 2026. https://we-calculate.com/calculator/carburetor-cfm-calculator.
TG we-Calculate Editorial Team, "Carburetor CFM Calculator — Engine Airflow Sizing," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/carburetor-cfm-calculator
@misc{wecalculate_carburetor_cfm_calculator, title = {Carburetor CFM Calculator — Engine Airflow Sizing}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/carburetor-cfm-calculator}}, year = {2026}, note = {TG we-Calculate} }
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