how to convert arabic number 2.5 into a roman numeral?

Your are definitely within the wrong category, but anyways:
The Romans didn't have a standard road to write fractions. Instead, they just wrote out the word for the fraction: for example, two-sevenths be "duae septimae" and three-eighths was "tres octavae." The Romans did not own a word for every imaginable fraction: how repeatedly do you need to enunciate thirty-three seventieths? If necessary, they would probably own said something like, "thirty-three seventieth parts," or "triginta tres septuagensimae partes."
The Romans did most of their practical calculation with fractions by using the uncia. The uncia started out as 1/12 of the as, a element of weight (the word uncia is related to our word "ounce"), but it soon come to mean 1/12 of anything. You can append up twelfths to make halves, thirds, or base, so the uncia was truthfully versatile. When they required smaller fractions, the Romans usually cut the uncia into smaller parts. The system is very similar to measure length in inches and fractions of the inch: you might not method an object's length exactly, but you can still come very close.
I could be wrong, but I don't believe the Roman numeral system be ever designed to include decimals.
i dont think that it is a medical grill am i wrong
the roomans would round 2.5 to 3 ( iii, III )