The plurals of nouns ending in -ful are usually formed by adding -s to the suffix: two cupfuls; two scant teaspoonfuls. Perhaps influenced by the phrase in which a noun is followed by the adjective full ( both arms full of packages ), some speakers and writers pluralize such nouns by adding -s before the suffix: two cupsful.
(forming adjectives)full of or characterized bypainful; spiteful; restful
(forming adjectives)able or tending tohelpful; useful
(forming nouns)indicating as much as will fill the thing specifiedmouthful; spoonful
Word Origin for -ful
Old English -ful, -full, from full1
Where the amount held by a spoon, etc, is used as a rough unit of measurement, the correct form is spoonful, etc: take a spoonful of this medicine every day . Spoon full is used in a sentence such as he held out a spoon full of dark liquid, where full of describes the spoon. A plural form such as spoonfuls is preferred by many speakers and writers to spoonsful