-fold combines with numbers to form adverbs which say how much an amount has increased by. For example, if an amount increases fourfold, it is four times greater than it was originally.
By the late eighties their number had grown fourfold...
Pretax profit surged almost twelvefold.ADJ: ADJ n-fold also combines with numbers to form adjectives.
One survey revealed a threefold increase in breast cancer.
English dictionary. 2008.