Calculate the required CFM for your engine’s carburetor based on displacement, RPM, and volumetric efficiency.
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Enter engine details and calculate to see breakdown.
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When you’re building an engine or upgrading a carburetor, one of the most critical decisions is choosing the right size. Too small, and you’ll choke the engine, robbing it of power and responsiveness. Too large, and you’ll lose velocity through the venturis, hurting throttle response and fuel atomization. Getting the CFM (Cubic Feet per Minute) right is the sweet spot where performance meets drivability.
This carburetor CFM calculator helps you determine the ideal airflow for your engine based on displacement, maximum RPM, and volumetric efficiency. Enter your engine specs, the tool computes the required CFM at 100% VE and at your actual VE, then recommends a carburetor size. Whether you’re a hot rodder building a street machine, a racer tuning for the track, or a student learning about engine airflow, this calculator delivers instant, accurate results. All calculations run locally in your browser, keeping your data private.
Enter your engine’s displacement using the first input field, choose from cubic inches (in³), liters (L), or cubic centimeters (cc).
Specify the maximum RPM your engine will reach, this is typically the redline or the peak horsepower RPM.
Enter the volumetric efficiency (VE) of your engine, 85% is typical for a street engine, 100% for a race engine.
Click Calculate or simply wait for automatic updates the required CFM appears instantly.
Review the CFM at 100% VE, the actual CFM with your VE, and a recommended carburetor size.
Check the common CFM sizes listed for reference your recommended size will be rounded up to a standard available size.
The calculator applies the standard formula for estimating the airflow required by an internal combustion engine. This formula has been used by engine builders and tuners for decades to select the right carburetor size.
Formula: CFM = (Displacement × RPM) / 3456 × VE
Where displacement is in cubic inches, RPM is the maximum engine speed, and VE is the volumetric efficiency expressed as a decimal (e.g., 85% = 0.85). The constant 3456 comes from converting cubic inches to cubic feet and accounting for the fact that a four-stroke engine only draws air on every other revolution.
For example, a 350 cubic inch engine at 5,500 RPM with 85% VE: CFM = (350 × 5500) / 3456 × 0.85 = 1,925,000 / 3456 × 0.85 = 556.7 × 0.85 = 473.2 CFM.
The calculator also shows the CFM at 100% VE (the theoretical maximum) so you can see how volumetric efficiency affects airflow.
Consider a 350 cubic inch small-block Chevy engine with a maximum RPM of 5,500 and a volumetric efficiency of 85%. What size carburetor is needed?
Step 1: Identify the known values
Displacement = 350 in³
RPM = 5,500
VE = 85% (0.85)
Step 2: Apply the formula
CFM at 100% VE = (350 × 5500) / 3456 = 1,925,000 / 3456 = 556.7 CFM
Step 3: Apply the volumetric efficiency
CFM actual = 556.7 × 0.85 = 473.2 CFM
Step 4: Select a recommended carburetor size
The closest common carburetor size above 473.2 CFM is 500 CFM.
Interpretation: A 350 cubic inch engine with these specifications needs approximately 473 CFM of airflow. A 500 CFM carburetor would be a good match providing adequate airflow without being oversized.
Computes required CFM instantly based on displacement, RPM, and VE.
Handles multiple displacement units, cubic inches, liters, and cubic centimeters.
Shows both the theoretical 100% VE CFM and the actual CFM with your VE.
Recommends a carburetor size rounded up to a common available CFM rating.
Displays common CFM sizes for reference to help you understand the range.
Updates results instantly as you change inputs.
Runs entirely client-side with no server communication, keeping your data private.
Free to use on any device with responsive design.
CFM (Cubic Feet per Minute) is the amount of air a carburetor can flow. It’s a measure of the carburetor’s capacity to deliver air to the engine. A carburetor that flows 600 CFM can deliver 600 cubic feet of air per minute.
The formula is: CFM = (Displacement × RPM) / 3456 × VE. Displacement is in cubic inches, RPM is the maximum engine speed, and VE is the volumetric efficiency expressed as a decimal.
A 350 cubic inch Chevy engine typically needs 400–500 CFM for a street application, depending on RPM and VE. A 500 CFM carburetor is a common choice for mild street builds, while 600–650 CFM is often used for more performance-oriented setups.
The formula for CFM is: CFM = (Displacement × RPM) / 3456 × VE. For a four-stroke engine, the constant is 3456. For a two-stroke engine, the constant is 2592.
Volumetric efficiency (VE) is the ratio of the actual air volume drawn into an engine to the theoretical volume the engine could draw if it were perfectly efficient. Street engines typically have 75–85% VE, while race engines can achieve 95–110% VE.
A carburetor that’s too large will have low air velocity through the venturis at low RPM, which can cause poor fuel atomization, sluggish throttle response, and reduced fuel economy. It can also make tuning more difficult.
A carburetor that’s too small will restrict airflow at high RPM, choking the engine and preventing it from making maximum power. It can also cause a lean condition at wide-open throttle.
Higher altitudes have lower air density, which means the engine needs less air volume (CFM) to make the same power. For every 1,000 feet of elevation, you may need to reduce carburetor size by about 3–4%.
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