Carburetor CFM Calculator

Estimate required carb CFM from engine displacement (CID), peak RPM, and volumetric efficiency (VE).

Result:
Enter values and click Calculate.
Notes
  • Street VE often ranges 0.80–0.90; performance builds may be higher.
  • CFM is a guideline—multi-carb setups, altitude, and cam profile affect choice.

How to use the carburetor CFM calculator

  1. Enter actual engine displacement in cubic inches, or load a preset and adjust it for your build.
  2. Enter the highest engine speed you want to evaluate. Use a realistic operating RPM, not an assumed safe redline.
  3. Enter volumetric efficiency as a decimal: 0.85 means 85 percent. The preset VE and RPM values are planning assumptions, not measured factory specifications.
  4. Calculate airflow demand, then compare available carburetors using their manufacturer's specifications and rating conditions.

Carburetor airflow formula

CFM = cubic inches × RPM × volumetric efficiency ÷ 3456

This estimates the intake volume per minute for a four-stroke engine. The divisor combines 1,728 cubic inches per cubic foot with one intake event per cylinder every two crankshaft revolutions. VE describes how effectively the cylinders fill relative to displacement.

Examples for traditional engines

A 239-cubic-inch engine at 4,500 RPM and 0.85 VE calculates to approximately 265 CFM. A 350-cubic-inch engine at 5,500 RPM and the same VE calculates to approximately 473 CFM. These are airflow estimates, not specific carburetor recommendations.

Choosing a carburetor or multiple-carb setup

Airflow demand is one part of selection. Throttle response, the intake manifold, camshaft, engine speed range and linkage arrangement also matter. A larger advertised flow number does not by itself establish a better match.

For dual Strombergs, a three-carb setup or another multiple-carb intake, consider total capacity when all throttles are open and how the linkage brings the carburetors into operation. Dividing estimated demand by the number of carburetors is only an initial comparison; it does not determine jetting or guarantee drivability.

Compare flow ratings measured at the same test pressure. Ratings from different standards should not simply be added or compared directly. Check the exact carburetor model and the manufacturer's test conditions rather than relying on a broad historical CFM table.

Carb sizing questions

What VE should I enter?

If measured engine data is unavailable, use the calculator's assumptions to compare scenarios. For example, run 0.80, 0.85 and 0.90 to see how sensitive the answer is. Engine modifications and operating RPM can change VE substantially.

Can I use liters instead of cubic inches?

This tool expects cubic inches. Divide liters by 0.016387064 to convert them, or use the engine displacement calculator to get both units from bore and stroke.

Can this size a supercharged engine's carburetor?

Not reliably by itself. Boost, air temperature, fuel delivery and the carburetor's location relative to the blower require additional consideration. Use the induction-system supplier's guidance for a boosted combination.

Technical reference: Holley's carburetor selection and airflow explanation.

Related calculators and workshop references

Return to all automotive calculators, check original specifications in the automotive manual library, or browse hot rod parts and workshop tools.