What type does the expression you are passing to Math.Ceiling return? You can find out by placing a breakpoint on that line and pasting that expression into the Watch window. Presumably it is not returning either a Double or Decimal, so it needs to be converted to one of those types because they are the only types that Math.Ceiling accepts. Computers can't "guess" which way to go. If there are two equally likely possibilites a computer just says "don't know, give up". Basically, you have to tell it whether it is dealing with a Double or a Decimal.