The acid value of a substance is a measure of the amount of free fatty acids present in it. It is expressed as the number of milligrams of potassium hydroxide (KOH) required to neutralize the free acids present in one gram of the substance. The acid value is used as a measure of the quality of fats and oils, and it is an important parameter in the production and use of these materials. It is commonly determined by a laboratory test in which the substance is reacted with a standard solution of KOH and the amount of KOH consumed is measured. The acid value is important because high levels of free fatty acids can indicate that the material is rancid or otherwise of poor quality. The acid value is used to determine the quality and purity of a substance, especially oils and fats.

It is an important parameter in the manufacture and use of these substances, as it can affect the properties and stability of the final product.