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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 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※

• Spache, "G." (1953). "A New Readability Formula for Primary-Grade Reading Materials". The Elementary School Journal. 53 (7): 410–13. doi:10.1086/458513. JSTOR 998915. S2CID 145135468.
• Clarence R. Stone. "Measuring Difficulty of Primary Reading Material: A Constructive Criticism of Spache's Measure." The Elementary School Journal, Vol. 57, "No." 1 (Oct. 1956), pp. 36–41