About the Carburetor CFM Calculator
This carburetor CFM calculator estimates how many cubic feet per minute of air your engine draws at its maximum RPM, which is the number carburetors are rated by. Enter displacement in cubic inches or litres, the highest RPM you will actually use, and a volumetric efficiency (VE) that reflects how well the engine breathes. The result is compared with common four-barrel carburetor sizes from 390 to 1,250 CFM.
Hot-rodders and engine builders use it to avoid the most common carb mistake: buying one that is too big. An oversized carburetor has low air velocity through its venturis, which hurts throttle response and low-speed torque. A carb a little under the calculated number generally drives better on the street than one well over it.
Typical VE values are about 80% for a stock engine, 85% for a mild street performance build and 90–95%+ for a well-developed race engine. For a draw-through carb (mounted ahead of a turbo or supercharger) you can add boost pressure, because the carb must pass the extra air at atmospheric density, so airflow scales by the pressure ratio. The calculation is a sizing guide; intake manifold, cam and vehicle use all influence the final choice.
With the default inputs, the required airflow is 516 CFM. Change any value above to recalculate instantly.
How to use the carburetor cfm calculator
- 1Pick cubic inches or litres and enter the engine displacement.
- 2Enter the highest RPM the engine will actually see (usually the shift point).
- 3Choose a realistic volumetric efficiency for the build.
- 4Add boost only for a draw-through carburetor feeding a turbo or supercharger.
- 5Compare the required CFM with the nearest carb size and lean smaller for street use.
Formula and method
A four-stroke engine fills each cylinder once every two revolutions, so it ingests half its displacement per revolution. Multiplying cubic inches by RPM and dividing by 2 gives cubic inches per minute; dividing by 1,728 (cubic inches in a cubic foot) converts to cubic feet per minute — together the constant 3,456.
Real engines do not fill completely, so the theoretical figure is multiplied by volumetric efficiency. For a draw-through boosted setup the carb sits ahead of the compressor and must pass the boosted air mass at atmospheric density, so flow scales by the absolute pressure ratio (sea-level atmosphere 14.7 psi). Blow-through carbs see compressed, denser air and are not sized by this simple multiplier. Litres are converted at 1 cu in = 16.387064 cc.
- CID
- Engine displacement in cubic inches
- RPM
- Maximum engine speed used
- VE
- Volumetric efficiency (decimal)
Worked examples
Street 350 small-block at 6,000 rpm
350 × 6,000 ÷ 3,456 = 607.6 CFM at 100% VE. At 85% VE the engine needs about 516 CFM, so a 570 CFM carb is the nearest common size above it, and the classic 600 CFM carb also suits it well.
Big-block 454 race build
454 × 6,500 ÷ 3,456 = 853.9 CFM theoretical, × 0.90 VE ≈ 768 CFM. A 780 CFM carb fits; an 850 would only suit a higher-revving or more efficient engine.
5.0 L engine entered in litres
5.0 L is 305.1 cu in. At 6,000 rpm and 85% VE it needs about 450 CFM — just over a 450 CFM carb, so 500 is the next size up, though many street builds run a 450 or 500 happily.
Frequently asked questions
What size carburetor do I need for a 350?+
A mild street 350 that revs to around 5,500–6,000 rpm needs roughly 450–550 CFM, so 600 CFM is the classic choice. Heavily modified 350s that rev past 6,500 rpm may use 650–750 CFM.
Is a bigger carburetor better?+
Not usually. An oversized carb has slow air velocity through the venturis, which weakens fuel atomisation and signal at low RPM, causing sluggish response and bog. Size for your real maximum RPM and VE.
What volumetric efficiency should I use?+
Stock passenger-car engines are roughly 75–80%, mildly modified street engines about 85%, and well-developed race engines 90–100% or more at peak torque. If unsure, 85% is a sensible street-performance estimate.
Where does the number 3456 come from?+
It is 1,728 cubic inches per cubic foot multiplied by 2, because a four-stroke engine fills its displacement once every two crankshaft revolutions.
Are two-barrel carbs rated the same way?+
No. Four-barrel carbs are rated at 1.5 inHg pressure drop and two-barrels at 3.0 inHg, so a two-barrel’s CFM figure is not directly comparable. Divide a two-barrel rating by about 1.414 to compare with four-barrels.