Air–fuel ratio (AFR) is the mass ratio of air to fuel present in a combustion process such as in an internal combustion engine or industrial furnace. The AFR is an important measure for anti-pollution and performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich. AFR numbers lower than stoichiometric are considered "rich". Rich mixtures are less efficient, but may produce more power and burn cooler, which is kinder on the engine. AFR numbers higher than stoichiometric are considered "lean." Lean mixtures are more efficient but may cause engine damage or premature wear and produce higher levels of nitrogen oxides. For precise AFR calculations, the oxygen content of combustion air should be specified because of possible dilution by ambient water vapor, or enrichment by oxygen additions.

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