How to use this calculator
Choose cubic inches or liters and enter total engine displacement. Set the engine speed you want to study, then enter a volumetric-efficiency assumption appropriate to that operating point. Read estimated airflow in cubic feet per minute (CFM) and liters per minute. Compare with the 100% VE reference to see the effect of your assumption. The default 85% is an illustrative scenario, not a measured value for your engine.
This is an intake-volume estimate, not a measured mass-airflow reading or a carburetor, throttle-body or turbocharger recommendation. It does not model air density, manifold pressure, boost, temperature, restrictions or fuel demand. It is not a two-stroke or rotary-engine model. Zero RPM gives zero modeled airflow; it does not describe cranking transients.
The formula
A worked example
A hypothetical 350 cu in four-stroke engine at 6,000 rpm and 85% VE gives 350 × 6,000 × 0.85 ÷ 3,456 = 516.49 CFM, or 14,625.45 L/min. At the same speed and displacement, 100% VE gives 607.64 CFM. A separate metric example, 2.0 L at 6,000 rpm and 90% VE, gives 5,400 L/min (190.70 CFM).
Airflow sensitivity to engine speed
Illustrative 350 cu in engine at a fixed 85% VE; real VE can vary with speed.
| Engine speed (rpm) | Estimated CFM |
|---|---|
| 3,000 | 258.25 |
| 4,500 | 387.37 |
| 6,000 | 516.49 |
Common questions
Why divide by two for a four-stroke engine?
Each cylinder has one intake stroke per two crankshaft revolutions. Total displacement already includes all cylinders, so do not multiply it by cylinder count again.
Does VE mean fuel efficiency?
No. Volumetric efficiency describes cylinder filling relative to the displacement reference. It is a different concept from fuel economy and thermal efficiency. An entered VE is an assumption unless supported by measurements at the operating point.
Can VE exceed 100 percent?
Some engines can achieve cylinder filling above the displacement reference under particular conditions. The input permits this for sensitivity analysis; it does not determine what your engine can achieve or calculate boost from VE.
Is the result SCFM or grams per second?
No standard temperature or pressure is specified here, so the output is labeled CFM, not SCFM. Mass flow needs an appropriate density and measurement basis; this calculator does not supply those.
Should I buy a carburetor with this exact CFM rating?
No. A computed intake-volume estimate alone does not select a component. Rated flow test conditions, engine combination and intended use also matter. Consult the component manufacturer for selection.
Sources & calculation method
Holley: How to Choose Your Next Holley Carburetor presents displacement times RPM divided by 3,456 at 100% VE and discusses adjusting for cylinder filling. This tool applies that four-stroke relationship to an entered VE. Its unit conversions, hypothetical examples and sensitivity table are shown above; it does not reproduce Holley’s component recommendations.
References accessed September 13, 2026. Sources provide context, not endorsement or independent professional review of this calculator.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.