Inputs
Results
CFM = Displacement × RPM × VE × overhead / 3456 (for ci). 1 ci ≈ 16.39 cc.
What this calculator actually solves
The classic carb-sizing rule of thumb is "1 CFM per cubic inch at redline." That rule assumes 100% VE and zero safety overhead. It undersizes carburetors on street engines (which spend 99% of their time at low load) and oversizes them on race engines (which need every last CFM at redline). This calculator lets you specify the actual VE target for your engine and apply a tunable overhead so the recommendation matches the build.
We also flag the single-feed versus dual-feed question. A single-feed carb is fine for engines up to about 600 CFM; above that, fuel slosh and float-bowl dynamics become the limit.
Who this is for
Restoration shops and hot-rodders picking a carb for a mild street engine and trying not to over-carb (over-carbing kills throttle response and fuel economy).
Drag racers sizing a Dominator for a high-RPM combination who need to know whether 1050 or 1250 is the right call.
How to read the results
The required CFM number is the airflow the engine will demand. Pick the next standard carb CFM above it — never below. If your build recommends 700 CFM, a 750 CFM carb is fine; a 600 will be on the edge.
If you're below 600 CFM, single-feed carbs are acceptable. Above that, go dual-feed.
Limitations
VE varies with RPM. A peak VE of 85% doesn't mean 85% everywhere — it means the engine can hit 85% at its tuned RPM. Sizing the carb for peak VE gives correct airflow at peak but may over-size at low load.
This calculator doesn't model two-quad setups, tunnel rams, or individual throttle bodies. It's intended for single or dual-quad carb applications on V8s and 4/6-cylinder engines.
Frequently Asked Questions
Should I round up or down to a standard CFM?
Round up to the next standard size — common carb CFM values are 350, 390, 415, 450, 500, 600, 650, 700, 750, 800, 850, 950, 1050, 1150, 1250. Going one size larger is fine for street builds; on a race engine it costs you nothing at the carb and saves you if conditions change.
What VE should I use for a stock 350?
For a stock, low-compression small-block with stock intake and a mild cam, 0.78–0.82 is realistic. For a stock engine with aftermarket intake and a moderate cam, 0.85–0.90.
Does this work for fuel injection?
The airflow formula is the same. For fuel injection, use the CFM output here and feed it to the fuel injector sizing calculator.
About this tool
What this tool is for: Carburetor CFM Sizing Calculator is a workbench reference for working mechanics, machinists, restoration shops, and serious hobbyists. It is built because the usual online calculators skip the friction loss, the sign convention, the unit conversion, or the engineering code that actually matters on the job.
Purpose and scope
Carburetor CFM from CID, target RPM, and VE. It rounds to common off-the-shelf carb sizes and flags when you need a custom taper.
How to use the body of this page
The sections above this footer — “What this calculator solves,” “Who this is for,” “How to read the results,” and “Limitations” — describe in detail what the formula does, why a tech would use it, how to interpret the number, and where the model is wrong. The FAQ block answers the three or four questions most asked about this specific tool. Together they make up the “About” content for this page; you do not need to look elsewhere.
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Mechanical, hydraulic, and welding work can cause serious injury, death, or property damage if performed incorrectly. Numbers produced by this calculator reflect simplified textbook models; they do not capture manufacturing tolerances, material defects, environmental conditions, wear, fatigue, regulatory codes (such as ASME, DOT, or AWS), or the specific service conditions of your job. Always cross-check critical values against the original equipment manufacturer's published specifications and a qualified engineer's review before committing to the job.
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