Carburetor Size Calculator: CFM & mm Guide

This carburetor size calculator works out the recommended airflow capacity in CFM (cubic feet per minute) for a four-stroke engine, based on engine displacement, maximum RPM, and volumetric efficiency. You enter those three values, and the calculator returns the theoretical CFM needed, plus the equivalent throttle bore diameter in millimeters for a single-barrel carburetor. The result is a starting point for selecting a carburetor, not a final specification, since real-world tuning depends on intake design, cam timing, and application.

Formula

The standard formula for carburetor sizing is:

CFM = (Displacement in cubic inches × Maximum RPM × Volumetric Efficiency) ÷ 3456

Where:

• Displacement – total swept volume of all cylinders, in cubic inches (1 liter = 61.02 ci)
• Maximum RPM – the highest engine speed you plan to use, not the redline if you rarely go there
• Volumetric Efficiency (VE) – a percentage (0–100%) representing how completely the cylinders fill with air. A stock street engine is typically 75–85%, a mild performance build 85–95%, and a highly tuned race engine can approach 100%.
• 3456 – a constant that converts cubic inches per revolution to cubic feet per minute (1728 in³/ft³ × 2 revolutions per intake cycle for a four-stroke).

The second output, throttle bore diameter, is an approximation for a single-barrel carburetor. It uses the airflow area needed at roughly 90% of the speed of sound to avoid excessive restriction. For multi-barrel carbs, divide the total CFM by the number of barrels to get the per-barrel flow, then compare with manufacturer flow ratings.

Worked Example

Using the default values from the calculator: a 350 cubic inch engine, 6000 RPM, and 85% volumetric efficiency.

1. Multiply displacement by RPM: 350 × 6000 = 2,100,000
2. Multiply by VE (as a decimal): 2,100,000 × 0.85 = 1,785,000
3. Divide by 3456: 1,785,000 ÷ 3456 ≈ 516.5 CFM

The calculator shows 516.5 CFM. For a single-barrel carb, the bore diameter would be approximately 48 mm (the calculator gives 47.8 mm). In practice, you would choose a carburetor rated around 500–550 CFM, such as a 4-barrel rated at 525 CFM, because the rating is measured at a specific pressure drop (usually 1.5 inHg for 4-barrels, 3.0 inHg for 2-barrels).

How to Choose the Right Carburetor Size

Start with the calculated CFM, then adjust based on your engine’s characteristics:

• Street driving – choose a carburetor slightly smaller than the calculated CFM (e.g., 500 CFM for a 516 CFM result) to improve low-speed throttle response and fuel atomization. A smaller venturi increases air velocity, which helps atomize fuel at low RPM.
• Performance or racing – choose a carburetor at or slightly above the calculated CFM, especially if the engine has a high-performance camshaft, ported heads, or a free-flowing exhaust. These parts increase VE beyond the stock estimate.
• Multi-barrel carburetors – a 4-barrel carburetor only uses the primary barrels at light throttle, so the total rating can be larger than the calculated CFM without hurting drivability. For example, a 650 CFM 4-barrel on a 350 ci engine is common and works well.
• Altitude – at higher altitudes, air density is lower, so the engine needs less CFM. For every 1000 feet above sea level, airflow demand drops roughly 3%. If you live at 5000 feet, reduce the calculated CFM by about 15%.

Common Mistakes and Misconceptions

A frequent error is using the engine’s redline instead of the RPM you actually use. If you rarely exceed 4500 RPM, sizing for 7000 RPM gives you a carburetor that is too large and sluggish. Another mistake is using metric displacement without converting to cubic inches. The formula requires cubic inches; if your engine is in liters, multiply by 61.02 first.

Many people think a bigger carburetor always makes more power. That is false. An oversized carburetor reduces air velocity at low RPM, causing poor fuel atomization, bogging, and worse fuel economy. The goal is to match the air demand, not exceed it.

Also note that carburetor CFM ratings are not directly comparable across brands because they are measured at different pressure drops. A 4-barrel rated at 750 CFM at 1.5 inHg flows about 600 CFM at 3.0 inHg. Always check the rating standard.

When to Use This Calculator

Use this calculator when you are:

• Selecting a new carburetor for a rebuild or engine swap
• Checking if your current carburetor is too large or too small
• Planning a performance build and want a baseline
• Converting between CFM and throttle bore size for custom fabrication

It is not a substitute for a dyno test or a professional tuner’s advice. The formula assumes a standard four-stroke cycle, so it does not apply to two-stroke engines (which have different intake timing) or rotary engines. For forced induction (turbo or supercharged), the engine consumes more air than the displacement suggests; multiply the calculated CFM by the pressure ratio (absolute boost pressure ÷ atmospheric pressure) to get a rough estimate.

Limitations of the Formula

The formula is a theoretical approximation. Real airflow demand depends on valve timing, intake manifold design, exhaust scavenging, and air temperature. A camshaft with more overlap can increase VE at high RPM but reduce it at low RPM. The calculator gives a single number, but a real engine has a VE curve that varies with RPM. For best results, use the VE value that matches your engine’s peak power RPM.

The bore diameter output is a rough guide for a single-barrel carburetor. Most automotive carburetors have multiple barrels, and the relationship between bore size and CFM is not linear. For a 4-barrel, the primary and secondary bores are different sizes, so the total CFM is the sum of all four. Use the CFM number to select a carburetor from a catalog, not the bore diameter.

Frequently Asked Questions

What is a good volumetric efficiency to use?

For a stock street engine, use 80–85%. For a mild performance engine with headers and a performance intake, use 85–90%. For a race engine with ported heads and a big cam, use 90–100%. If you are unsure, 85% is a safe default for a typical V8.

Can I use this calculator for a 2-stroke engine?

No. Two-stroke engines fire every revolution, so they need approximately double the airflow of a four-stroke of the same displacement. The formula’s constant of 3456 assumes a four-stroke cycle. For a two-stroke, use a different constant (1728) or multiply the result by 2.

What does CFM stand for?

CFM stands for cubic feet per minute, a measure of volumetric flow rate. In carburetor sizing, it refers to the volume of air the carburetor can flow at a given pressure drop. Higher CFM means more airflow capacity.

Is a bigger carburetor better for fuel economy?

No. A larger carburetor typically has larger venturis, which reduce air velocity at low throttle. This leads to poor fuel atomization and can actually hurt fuel economy. A properly sized carburetor that matches your engine’s airflow demand will give the best balance of power and efficiency.

How do I convert engine displacement from liters to cubic inches?

Multiply the displacement in liters by 61.02. For example, a 5.0 liter engine is 5.0 × 61.02 = 305.1 cubic inches. The calculator expects cubic inches, so do this conversion before entering the value.

What if my calculated CFM is between two available carburetor sizes?

Choose the smaller one for street driving and the larger one for performance use. If you are unsure, err on the smaller side for better drivability. You can always upgrade later, but a too-large carburetor is hard to tune for daily driving.

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in disp | Engine displacement | ci | 350 | 1 | 10000
in rpm | Max RPM | rpm | 6000 | 100 | 20000
in ve | Volumetric efficiency | % | 85 | 1 | 100
out cfm | Carburetor CFM | CFM | disp * rpm * ve / 3456 / 100 | 1
out mm | Single bore diameter | mm | sqrt(cfm * 0.7854 / 0.9) * 25.4 | 1