How to Calculate Carburetor CFM (Airflow Requirement)
A carburetor's CFM (cubic feet per minute) rating tells you the maximum volume of air it can flow. The goal when sizing one is to match that rating to what your engine can actually consume at peak RPM — a carburetor that's too small restricts airflow and chokes power at high RPM, while one that's far too large hurts idle quality, throttle response, and low-speed drivability. Engine builders and carburetor manufacturers (Holley, Edelbrock, and others) all size carburetors with the same standard formula, which this calculator applies to your engine's displacement, redline RPM, and volumetric efficiency.
Where the formula comes from
A four-stroke engine completes one intake stroke every two crankshaft revolutions, so at a given RPM it "breathes" its full displacement (CID, in cubic inches) only RPM/2 times per minute. Dividing by 1728 (the number of cubic inches in a cubic foot) converts that to cubic feet per minute, giving CID × RPM / 3456 — the theoretical maximum airflow. Real engines never fill a cylinder completely at speed, so the result is multiplied by volumetric efficiency (VE), the ratio of the air an engine actually ingests to that theoretical maximum: CFM = (CID × RPM × VE) / 3456. For example, a 350 ci engine at a 6,000 RPM redline with 85% VE needs (350 × 6000 × 0.85) / 3456 ≈ 516 CFM.
Picking volumetric efficiency and accounting for boost
VE depends on how well the engine breathes: a stock engine with a mild cam typically runs 75-85% VE, a performance engine with a bigger cam, headers, and a matched intake reaches roughly 85-95%, and a well-tuned high-performance or race engine with ported heads can reach 95-110% or more. For turbocharged or supercharged engines, forced induction compresses the intake charge so it is denser than atmospheric air, meaning the engine consumes more air mass per revolution than the naturally aspirated formula alone predicts. Approximate the increase by multiplying the naturally aspirated CFM by (boost pressure in psi + 14.7) / 14.7, using standard atmospheric pressure of 14.7 psi as the baseline.
Frequently Asked Questions
What is the formula for calculating carburetor CFM?
What volumetric efficiency should I use?
Should I round the calculated CFM up or down when buying a carburetor?
How does a turbocharger or supercharger change the CFM I need?
Common Tip Percentages
| Service | Typical Tip Range | Notes |
|---|---|---|
| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
| Food delivery | 10–20% | Consider extra for long distances |
| Taxi / rideshare | 10–20% | More for assistance with luggage |
| Hotel housekeeping | $2–5/night | Leave daily, not just at checkout |
| Barber / hair salon | 15–20% | On pre-discount total |
| Pizza delivery | 10–15% or $3–5 minimum | Higher in bad weather |
General Household Budget Ratios (50/30/20 Rule)
| Category | % of Net Income | Examples |
|---|---|---|
| Needs | 50% | Rent, groceries, utilities, insurance |
| Wants | 30% | Dining out, entertainment, hobbies |
| Savings / Debt | 20% | Emergency fund, retirement, extra payments |
Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |