Spache readability formula

It was introduced in 1953 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:

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

The revised formula is:

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

Further reading