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AFR Calculator

Ready to calculate
AFR ↔ Lambda.
6 Fuel Types.
Tuning Targets.
100% Free.
No Data Stored.

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How it Works

01Choose Fuel

Gasoline, diesel, ethanol, methanol, LPG, CNG.

02Enter AFR or Lambda

Convert between AFR, lambda, and equivalence ratio.

03Rich/Lean %

Reports % rich or lean from stoichiometric.

04Tuning Reference

Targets for cruise, WOT, idle, and boost.

What is an Air-Fuel Ratio Calculator?

The Air-Fuel Ratio (AFR) Calculator computes the mass ratio of air to fuel for combustion engines, comparing actual mixture against the stoichiometric AFR (the chemically perfect ratio for complete combustion). Outputs include AFR, Lambda (λ), and equivalence ratio (φ) — the three interchangeable expressions of mixture strength used by engine tuners, EFI calibrators, and emissions engineers.


Pick fuel type (gasoline, diesel, E10, E85, methanol, propane, hydrogen) and enter air mass + fuel mass. Calculator returns AFR plus λ (1.0 = stoich, <1 rich, >1 lean) and φ (inverse of λ). Standard reference: gasoline AFR_stoich = 14.7:1; E85 = 9.8:1; diesel = 14.5:1; methanol = 6.4:1; hydrogen = 34.3:1.

How to Use the Calculator

Pick fuel type: determines stoichiometric AFR.
Enter air mass and fuel mass in same units (g, kg, lb).
Calculate: Returns AFR, λ, φ, and rich/stoich/lean classification.

The Math Behind It

AFR = mass of air / mass of fuel


Lambda λ = AFR_actual / AFR_stoich


Equivalence ratio φ = 1 / λ = AFR_stoich / AFR_actual


λ < 1 = rich (excess fuel); λ = 1 = stoichiometric; λ > 1 = lean (excess air). Best power: λ ≈ 0.85–0.90 (rich); best fuel economy: λ ≈ 1.05 (slightly lean); catalytic converter operation: λ = 1.00 ±0.01.

Real-World Example

Worked Example

Gasoline engine, measured 50 g air per 4.0 g fuel:

  • AFR = 50 / 4.0 = 12.5:1
  • λ = 12.5 / 14.7 = 0.85 (rich)
  • φ = 1 / 0.85 = 1.18
  • Classification: rich-of-stoich (full-power region; high HC + CO emissions)

Who Uses It

1
🏎 Engine Tuners: Dyno-tune for power (rich) or economy (slightly lean).
2
🔧 EFI Calibrators: Set base fuel maps and target λ tables.
3
🌱 Alt-Fuel Conversions: Adjust calibration for E85, methanol, propane.
4
🏭 Emissions Engineers: Optimize catalytic converter operating window.
5
🎓 Combustion Students: Solve stoichiometry homework with verification.
6
🚜 Diesel Specialists: Manage smoke (low λ) vs NOx (high λ) tradeoffs.

Technical Reference

Stoichiometric AFR by fuel:

  • Gasoline (RON 95): 14.7:1
  • Diesel #2: 14.5:1
  • E10 (10% ethanol): 14.1:1
  • E85 (85% ethanol): 9.8:1
  • Methanol (M100): 6.4:1
  • Ethanol (E100): 9.0:1
  • Propane (LPG): 15.7:1
  • Methane / CNG: 17.2:1
  • Hydrogen: 34.3:1

Operating targets (gasoline λ): Idle 1.00 · Cruise 0.97–1.00 · Acceleration 0.85–0.90 · Wide-open throttle 0.85 · Catalyst lightoff 1.00.

Key Takeaways

AFR, λ, and φ are three views of the same mixture-strength reality. Stoichiometric values vary by fuel — 14.7 for gasoline, 9.8 for E85, 6.4 for methanol — and matching the right value is essential when running alternative fuels. Catalytic converters demand λ = 1.00 ±0.01 for full-three-way operation; deviating costs emissions compliance.

Frequently Asked Questions

Why does E85 have a much lower stoichiometric AFR?
Ethanol contains oxygen in its molecule (C₂H₅OH). The combustion reaction needs less external air per gram fuel because some oxygen is already there. E85 also has lower energy density, so EFI systems inject ~30% more fuel by mass.
What's the difference between AFR and λ?
AFR is the absolute mass ratio (12.5:1, 14.7:1). λ is the fuel-agnostic ratio of actual to stoich (0.85, 1.00, 1.10). λ lets you talk about mixture strength without specifying the fuel — useful when running flex-fuel or alt-fuel.
Best AFR for power?
Naturally aspirated gasoline: λ ≈ 0.86–0.90 (AFR ~12.5–13.2). Forced induction: λ ≈ 0.78–0.82 (AFR ~11.5–12.0) for cooling and detonation margin. Going richer than λ 0.78 wastes fuel; leaner risks knock and overheating exhaust valves.
How do wideband O₂ sensors work?
Wideband sensors directly measure exhaust gas O₂ and report λ across 0.7–1.5 (some 0.5–32). Narrowband sensors only resolve λ near 1.00 (binary rich/lean). Tuning requires wideband; closed-loop catalyst control uses narrowband.
What about diesel?
Diesel runs lean overall (λ > 1.5 typical) — fuel is injected directly and mixes locally. Local stoich combustion produces NOx; soot forms in fuel-rich zones. Modern diesels balance EGR, injection timing, and aftertreatment (DPF, SCR) to manage both.
Why does λ = 1.00 matter for catalytic converters?
Three-way catalysts oxidize HC + CO and reduce NOx simultaneously. The reduction step needs reductant (CO, H₂); the oxidation step needs O₂. Both work in a narrow ±1% window around λ 1.00. EFI cycles between slightly rich and slightly lean to keep average at stoich.

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Disclaimer

Stoichiometric AFR values are theoretical complete-combustion ratios. Real engine operation deviates due to incomplete combustion, charge inhomogeneity, and aftertreatment requirements. For tuning, always use a calibrated wideband O₂ sensor and follow manufacturer/dyno operator safety procedures.