Air-Fuel Ratio Calculator
Calculate air-fuel ratio (AFR) for any fuel type and compare against the stoichiometric ideal for complete combustion.
Reviewed by the ToolNestr Editorial Team — July 2026
How air-fuel ratio calculation works
The air-fuel ratio (AFR) is the mass ratio of air to fuel in an internal combustion engine. It is one of the most critical parameters for engine performance, efficiency, and emissions. The stoichiometric AFR is the chemically ideal ratio where all fuel is completely burned using all available oxygen. For petrol, this is 14.7:1 — meaning 14.7 grams of air for every 1 gram of fuel.
Lambda (λ) is the normalised AFR: actual AFR divided by the stoichiometric AFR for the given fuel. Lambda is a universal tuning metric because it abstracts away the fuel type. A lambda of 1 means ideal combustion regardless of whether the fuel is petrol, diesel, or ethanol. Tuning for maximum power typically targets λ ≈ 0.85–0.90 (rich), while maximum efficiency targets λ ≈ 1.05–1.10 (lean).
The formula explained
Worked example
Air: 14.7 g · Fuel: 1 g · Petrol (stoich 14.7:1)
Engine Tuning
Tune your carburettor or fuel injection system using lambda as a universal target. Adjust jetting until λ reads correctly.
Dyno Testing
During dynamometer testing, monitor AFR across the RPM range. Adjust fuel maps to keep lambda in the target window for power or economy.
Diagnostics
A faulty oxygen sensor or fuel pressure regulator causes abnormal AFR. Compare measured AFR to the ideal to identify system problems.
Fuel Economy Optimisation
For maximum fuel economy, engines run slightly lean (λ ≈ 1.05–1.10). Calculate your target AFR for optimal cruising efficiency.
How to use the air-fuel ratio calculator
Enter air and fuel mass
Type the mass of air and fuel in grams. These can come from a wideband AFR meter or flow measurement.
Select fuel type
Choose the fuel from the list. Each fuel has a different stoichiometric ratio used for lambda calculation.
Read AFR, lambda and status
See the AFR ratio, lambda value, and a text description of whether the mixture is rich, lean, or ideal.
Tips for air-fuel ratio tuning
Use lambda as the universal metric
Lambda normalises AFR across fuel types. A lambda of 0.85 means 15% rich regardless of whether you run petrol, E85, or methanol. This makes it the standard for tuning.
Power vs economy targets
Maximum power: λ ≈ 0.85–0.90 (rich). Maximum efficiency: λ ≈ 1.05–1.10 (lean). Idle: λ ≈ 1.00. Full throttle: λ ≈ 0.85–0.90 for safety (rich mixture cools the combustion chamber).
Be careful with lean mixtures
Running too lean (λ > 1.15) causes high combustion temperatures that can damage pistons, valves, and spark plugs. Always err on the rich side for safety.
E85 requires different tuning
E85 has a stoichiometric ratio of 9.8:1, much richer than petrol. This means you need approximately 30% more fuel flow when switching from petrol to E85.
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Frequently asked questions
What is the ideal air-fuel ratio?
For petrol (gasoline), the stoichiometric (ideal) ratio is 14.7:1 — 14.7 parts air to 1 part fuel by mass. This gives complete combustion.
What is lambda?
Lambda (λ) is the ratio of actual AFR to stoichiometric AFR. λ = 1 is ideal, λ < 1 is rich, λ > 1 is lean.
What happens with a rich mixture?
A rich mixture (AFR below 14.7:1) has excess fuel. It produces more power but wastes fuel and increases emissions.
What happens with a lean mixture?
A lean mixture (AFR above 14.7:1) has excess air. It is more fuel-efficient but can cause overheating and engine damage if too lean.
What fuels have different stoichiometric ratios?
Diesel: ~14.5:1, E85: ~9.8:1, Methanol: ~6.4:1, LPG: ~15.5:1. Each fuel has its own ideal ratio.
Sources & references
This tool uses standard formulas and reference values from:
- • SAE International standards (e.g. J1349 engine power, J1634 EV range). sae.org
- • U.S. DOE / EPA — fueleconomy.gov, official efficiency, MPGe and charging figures. fueleconomy.gov
- • Vehicle manufacturer service specifications — always defer to the OEM figures for your specific vehicle.
General estimates. Follow your manufacturer’s published specifications and a qualified mechanic for safety-critical work.