Quick answer
Required airflow (CFM) = engine displacement (cubic inches) × peak rpm ÷ 3,456 × volumetric efficiency. A 350 cubic inch engine turning 6,000 rpm at 85% efficiency needs about 516 CFM.
Enter displacement, peak engine speed and volumetric efficiency. The calculator returns the airflow a carburetor must supply. Use this carburetor size calculator to check your own figures.
Related tools: Engine Displacement Calculator: Displacement from bore, stroke and number of cylinders. Compression Ratio Calculator: Static compression ratio from bore, stroke and clearance volume. Horsepower Calculator: Power from torque and engine speed.
How to use the Carburetor Size Calculator
- Enter displacement (ci) (above 0).
- Enter peak engine speed (rpm) (above 0).
- Enter volumetric efficiency (%) (between 0 and 150). It starts at 85.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Displacement | A number, in ci | > 0 |
| Peak engine speed | A number, in rpm | > 0 |
| Volumetric efficiency | A number, in % | > 0, ≤ 150, starts at 85 |
Formula and method
Formula
CFM = displacement × rpm ÷ 3,456 × VE
IEEE-754 double precision; shown to the decimals indicated.
3,456 = 1,728 in³ per ft³ × 2 (four-stroke engines draw air every second revolution).
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Displacement = 350; Peak engine speed = 6000; Volumetric efficiency = 85.
| Result | Value |
|---|---|
| Required airflow | 516 CFM |
Example 2
Inputs: Displacement = 302; Peak engine speed = 5500; Volumetric efficiency = 90.
| Result | Value |
|---|---|
| Required airflow | 433 CFM |
Example 3
Inputs: Displacement = 454; Peak engine speed = 5800; Volumetric efficiency = 80.
| Result | Value |
|---|---|
| Required airflow | 610 CFM |
The first line is the headline result (required airflow). IEEE-754 double precision; shown to the decimals indicated.
Where 3,456 comes from
A four-stroke engine draws its displacement of air once every two revolutions. Converting cubic inches per minute to cubic feet per minute adds a factor of 1,728, so 1,728 × 2 = 3,456. Multiply by the volumetric efficiency, which is how much of that theoretical air the cylinders actually take in.
Picking a standard size
Carburetors come in set sizes such as 450, 500, 600 and 650 CFM. Choose the nearest size at or slightly above the result; a much larger carburetor can hurt low-speed response. Street engines often run 75 to 85% efficiency, while engines with tuned intakes can exceed 100%.
How the result changes with displacement
Holding every other input at the first worked example, this table changes only displacement. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Displacement | Required airflow |
|---|---|
| 175 | 258 CFM |
| 350 | 516 CFM |
| 700 | 1,033 CFM |
| 1400 | 2,066 CFM |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Displacement: entering "0" shows "Displacement must be greater than 0." beside that field and no result.
- Volumetric efficiency: entering "0" shows "Volumetric efficiency must be greater than 0." beside that field and no result.
Choosing between the Carburetor Size Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Engine Displacement Calculator](/automotive/engine-displacement-calculator/) | bore, stroke and cylinders | Displacement |
| [Compression Ratio Calculator](/automotive/compression-ratio-calculator/) | bore, stroke and clearance volume per cylinder | Compression ratio |
| [Horsepower Calculator](/automotive/horsepower-calculator/) | torque, torque unit and engine speed | Power |
| [Tire Size Calculator](/automotive/tire-size-calculator/) | section width, aspect ratio and rim diameter | Overall diameter |
| [Tire Size Comparison Calculator](/automotive/tire-comparison-calculator/) | old tire: Section width, old tire: Aspect ratio and old tire: Rim diameter | Diameter difference |
Background: vehicle sizing and speed
A taller tyre travels farther per revolution, so the speedometer reads low and the effective gear ratio becomes numerically lower; a shorter tyre does the opposite. True speed is roughly the speedometer reading times the new tyre diameter divided by the old one. Verify with GPS over a known distance before changing parts.
Assumptions and limitations
- Choose the nearest standard carburetor size
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for displacement and volumetric efficiency to confirm this.
Frequently asked questions
What is volumetric efficiency?
The ratio of the air an engine actually draws in to its displacement; 80 to 90% is typical for a street engine.
Is bigger always better?
No. An oversized carburetor reduces air speed and can make low-rpm driving rough.
Does it apply to fuel injection?
Throttle-body sizing uses the same airflow idea, but injected engines are sized differently.
What does the Carburetor Size Calculator calculate?
Carburetor airflow needed from displacement, peak rpm and volumetric efficiency. It returns Required airflow.
What formula does the Carburetor Size Calculator use?
CFM = displacement × rpm ÷ 3,456 × VE. 3,456 = 1,728 in³ per ft³ × 2 (four-stroke engines draw air every second revolution).
What do I need to enter in the carburetor Size Calculator?
You enter Displacement (ci), Peak engine speed (rpm) and Volumetric efficiency (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Choose the nearest standard carburetor size.
What is an example of a carburetor Size result?
With displacement 350, peak engine speed 6000 and volumetric efficiency 85, the required airflow is 516 CFM.
Where does the method come from?
The method follows: Four-stroke airflow relation (e.g. Mondello, Carburetor tuning references).
Related calculators
- Engine Displacement Calculator — Displacement from bore, stroke and number of cylinders.
- Compression Ratio Calculator — Static compression ratio from bore, stroke and clearance volume.
- Horsepower Calculator — Power from torque and engine speed.
- Tire Size Calculator — Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17.
- Tire Size Comparison Calculator — Compare two tire sizes and the resulting speedometer error.
- Sample Size Calculator — Sample size for estimating a mean or a proportion.
- Wheel Offset Calculator — Wheel offset from width and backspacing.
Sources
- Four-stroke airflow relation (e.g. Mondello, Carburetor tuning references)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.