Dear JA:

You wrote:

Which is correct: square-foot or square foot ?

In normal usage, both are correct. The hyphenated form is used for adjectives; the two words are used for nouns (technically, “foot” is the noun modified by the adjective “square.”

Examples:
They marked out in the pattern in square-foot increments. (adjective modifying “increments”)
We had one square foot of fabric left over. (noun, direct object of “had”)