Spache Readability Formula

The Spache Readability Formula is a readability test for writing in English, designed by George Spache. It works best on texts that are for children up to fourth grade. For older children, the Dale-Chall Readability Formula is more appropriate.

It was introduced in 1952 in Spache's "A new readability formula for primary-grade reading materials," (The Elementary School Journal, 53, 410-413), and has subsequently been revised.

Calculation
The method compares words in a text to a set list of everyday words. The number of words per sentence and the percentage of unfamiliar words determine the reading age.

The original formula was:

\mbox{Grade Level} = \left ( 0.141 \times \mbox{Average sentence length} \right ) + \left ( 0.086 \times \mbox{Number of unique unfamiliar words} \right) + 0.839 $$

The revised formula is:

\mbox{Grade Level} = \left ( 0.121 \times \mbox{Average sentence length} \right ) + \left ( 0.082 \times \mbox{Number of unique unfamiliar words} \right) + 0.659 $$