Even when dictated as two and a half centimeters, it is generally preferred to not use fractions with metric units of measurement. This is a style issue and it would not necessarily be incorrect to edit 2-1/2 if dictated that way. It is generally preferred to spell out fractions when they are less than one and do not precede a noun. What is the preferred way to write 2.5 cm?
1) 2.5 cm
2) 2-1/2 cm
3) 2 and a half cm
4) 2 cm