How much air your engine can use at peak rpm, and which common carburetor sizes fit it. If you've used the displacement calculator, your cubic inches are already filled in.
Vacuum-secondary carbs are generally more forgiving of a little extra size on a street car. Mechanical secondaries reward precise sizing and an engine that can use them.
3,456 converts cubic inches per minute to cubic feet per minute (1,728) and accounts for a four-stroke engine filling each cylinder every other revolution (× 2). Volumetric efficiency is how well the engine fills its cylinders compared with their full volume. Stock engines usually sit around 80%, and well-developed race engines can exceed 100%.
An oversized carburetor lowers air speed through the venturis, which weakens the fuel signal at low rpm and hurts throttle response. Pick the size that matches the rpm the engine actually reaches, not the rpm you wish it did.
Formula as published by Speedway Motors ("What Size Carburetor Do I Need?") and used throughout the industry.