Error inconsistency is often cited as a characteristic of children with speech disorders, particularly developmental apraxia of speech (DAS); however, few researchers operationally define error inconsistency and the definitions that do exist are not standardized across studies. This study proposes three formulas for measuring various aspects of articulatory error consistency: proportion of errors, consistency of error types, and consistency of the most frequently used error type. Each formula is explained using examples of productions from children with DAS and phonological delay. Clinical implications for the use of error consistency to differentially diagnosis DAS and phonological delay are discussed.