# Carburetor CFM Calculator

URL: https://speedycalculator.com/calculators/physics/carburetor-cfm/
Last updated: 2026-10-30

> The carburetor CFM calculator estimates the airflow a four-stroke engine needs – and so the carburettor size – from its displacement, maximum rpm and volumetric efficiency.

## Inputs

- Engine displacement (in³ (CID), L, cm³ (cc))
- Maximum engine rpm
- Volumetric efficiency (%)

## Results

- Required airflow
- In litres per second

## Formula

```
CFM = CID × RPM × VE / 3456
```

## Explanation

An engine is an air pump: at full throttle it moves half its displacement every revolution (four-stroke). Choose a carburettor that flows about this much and the engine is neither starved nor bogged down.

### How to use the Carburetor CFM Calculator

- **Engine displacement**
- **Maximum engine rpm**
- **Volumetric efficiency (%)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the displacement, the maximum rpm you will use and the volumetric efficiency (≈ 80% stock, 85–90% performance, 95%+ race).

### Inputs explained

FieldWhat to enterEngine displacementUnits: in³ (CID), L, cm³ (cc)Maximum engine rpmA numberVolumetric efficiency (%)A number

### What each result means

- **Required airflow** = [Engine displacement]×[Maximum engine rpm]×[Volumetric efficiency (%)]÷100÷3456
- **In litres per second** = [Required airflow]×0.4719474

### Example values

How the main result changes when **Engine displacement** changes and everything else stays at the example values above:

Engine displacement (in³ (CID))Required airflow (CFM)175236.7262.5355.1350473.5437.5591.8525710.2700946.9

## Worked example

Inputs — Engine displacement: 350 in³ (CID); Maximum engine rpm: 5,500; Volumetric efficiency (%): 85.

Results — **Required airflow = 473.5 CFM**; In litres per second = 223.4 L/s.

## Key terms

- **Volumetric efficiency**: Actual air drawn in versus the swept volume.

## FAQ

**Why divide by 3456?**
1728 in³ per ft³ × 2 revolutions per intake cycle.

**Is a bigger carb better?**
No – too large a carb reduces air velocity and low-rpm response.

**What VE should I use?**
About 80% for stock engines, 85–90% for mild performance.

**Does it work for two-strokes?**
No – they draw a full displacement every revolution (divide by 1728).


## Sources

- [OpenStax – University Physics Volume 1](https://openstax.org/details/books/university-physics-volume-1)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)