The two most common ASTM test methods for testing base number - ASTM D2896 and ASTM D4739. ASTM D2896 measures both detergents and dispersants and is the preferred base number test method for new (unused) engine oils. ASTM D4739 measures detergents and is the preferred base number test method for in-service engine oil. It is important to note the difference as it is possible for the detergents in an in-service to be depleted but the dispersants be intact. If this is the case, ASTM D2896 results would not indicate that an oil change was necessary.

The base number test is performed by diluting the sample with a mixed solvent in a reaction beaker. While mixing, hydrochloric acid is titrated to chemically react with alkaline components (detergents) in the sample. When the end point of the chemical reaction is reached, the amount of titrant used is converted to the result and reported in milligrams of potassium hydroxide equivalent per gram of sample (mg of KOH/g).