The automated readability index (ARI) uses the below formula to assess the readability of a given text. In the formula, characters is the total number of letters and numbers, words is the number of strings in the text that are separated by spaces from other strings, and sentences is the number of strings separated by sentence-ending punctuation marks.

$$4.71\left(\frac{\text{characters}}{\text{words}}\right) + 0.5\left(\frac{\text{words}}{\text{sentences}}\right) - 21.43$$

The formula produces a decimal value, which traditionally is rounded up to the next integer. This integer represents the approximate U.S. grade level that is necessary for a reader to understand the text. For example, an ARI score of 8.6 should be rounded up to 9, meaning that a reader will need at least a U.S. ninth grade education to fully understand the text.

The ARI scores correspond to U.S. grade levels as shown in the table below.