Round to increments of 2.5?

Question

I need to round a value up to the nearest multiple of 2.5.

For example:
6 --> 7.5
7.6 --> 10
etc.

This seems like the best way to do this?

   Function RoundToIncrement(ByVal originalNumber As Decimal, ByVal increment As Decimal) As Decimal

        Dim num = Math.Round(originalNumber / increment, MidpointRounding.AwayFromZero) * increment
        If originalNumber Mod increment <> 0 And num < originalNumber Then
            num += increment
        End If
        Return num

    End Function

Answer 1

Divide the number by 2.5, round up to the nearest integer, then multiply the result by 2.5.

You're close.

Function RoundToIncrement(ByVal orignialNumber As Decimal, ByVal increment As Decimal) As Decimal
    Return Math.Ceiling( orignialNumber / increment ) * increment
End Function

Math.Ceiling will always round non-integers up, so you don't need the post-adjustment.

Answer 2

        /*
               This will round up (Math.Ceiling) or down (Math.Floor) based on the midpoint of the increment.  
               The other examples use Math.Ceiling and therefore always round up.
               Assume the increment is 2.5 in this example and the number is 6.13
            */
            var halfOfIncrement = Increment / 2;                                    // 2.5 / 2 = 1.25
            var floorResult = Math.Floor(originalNumber / Increment);               //Math.Floor(6.13 / 2.5) = Math.Floor(2.452) = 2
            var roundingThreshold = (floorResult * Increment) + halfOfIncrement;    //(2 * 2.5) = 5 + 1.25 = 6.25

            if (originalNumber >= roundingThreshold)                                //6.13 >= 6.25 == false therefore take Math.Floor(6.13/2.5) = Math.Floor(2.452) = 2 * 2.5 = 5
                result = Math.Ceiling(originalNumber / Increment) * Increment;
            else
                result = Math.Floor(originalNumber / Increment) * Increment;