Generic File Size Calculator

Question 1

As an extension

Imports System.Runtime.CompilerServices

Module UnitExtension

    Public Enum Units
        B = 0
        KB = 1 'kilo
        MB = 2 'mega
        GB = 3 'giga
        TB = 4 'tera
        PB = 5 'peta
        EB = 6 'exa
        ZB = 7 'zetta
        YB = 8 'yotta
        'add new values as needed
        Auto = -1
    End Enum
    'compute max value of enum
    Private ReadOnly maxU As Integer = [Enum].GetValues(GetType(Units)).Cast(Of Integer)().Max

    'kFactor should be set according to your use
    'see
    'http://physics.nist.gov/cuu/Units/prefixes.html
    'and
    'http://physics.nist.gov/cuu/Units/binary.html
    Private ReadOnly Kfactor As Integer = 1024 'or 1000

    <Extension()>
    Public Function ToUnit(ByVal theNumberToConvert As Object, _
                           Optional ByVal precision As Integer = 0, _
                           Optional ByVal whichUnit As Units = Units.Auto) As String

        Dim _aNumber As Double = CType(theNumberToConvert, Double) 'the number being converted
        Dim _fmt As String = "n" & precision.ToString 'format string

        Dim _unit As Integer
        If whichUnit = Units.Auto Then 'auto unit
            _unit = CInt(Math.Floor(Math.Log(_aNumber, Kfactor))) 'yes
            If _unit > maxU Then '> max unit
                _unit = maxU 'use max unit
            End If
        Else
            _unit = whichUnit 'no, force unit
        End If

        Dim _numberOfUnits As Double = _aNumber / (Kfactor ^ _unit) 'calculate number of units

        Return String.Format("{0} {1}", _numberOfUnits.ToString(_fmt), CType(_unit, Units))
    End Function

End Module

edit:

An overload for unit to unit conversion

<Extension()>
Public Function ToUnit(ByVal theNumberToConvert As Object, _
                       fromUnit As Units, _
                       Optional ByVal precision As Integer = 0, _
                       Optional ByVal whichUnit As Units = Units.Auto) As String

    Dim _aNumber As Double = CType(theNumberToConvert, Double) * (Kfactor ^ fromUnit) 'the number being converted
    Dim _fmt As String = "n" & precision.ToString 'format string

    Dim _unit As Integer
    If whichUnit = Units.Auto Then 'auto unit
        _unit = CInt(Math.Floor(Math.Log(_aNumber, Kfactor))) 'yes
        If _unit > maxU Then '> max unit
            _unit = maxU 'use max unit
        End If
    Else
        _unit = whichUnit 'no, force unit
    End If

    Dim _numberOfUnits As Double = _aNumber / (Kfactor ^ _unit) 'calculate number of units

    Return String.Format("{0} {1}", _numberOfUnits.ToString(_fmt), CType(_unit, Units))
End Function

Question 2

You've just mixed up your math logic. To fix it, change this:

Dim Kbs As Double = bytes * FromSize.kilobyte
Dim mbs As Double = bytes * FromSize.megabyte
Dim gbs As Double = bytes * FromSize.gigabyte
Dim tbs As Double = bytes * FromSize.terabyte
Dim pbs As Double = bytes * FromSize.petabyte

To this:

Dim Kbs As Double = bytes / FromSize.kilobyte
Dim mbs As Double = bytes / FromSize.megabyte
Dim gbs As Double = bytes / FromSize.gigabyte
Dim tbs As Double = bytes / FromSize.terabyte
Dim pbs As Double = bytes / FromSize.petabyte

Question 3

Here is the solution:

Enum Units As Long
    bytes = 1L
    kilobyte = 1024L
    megabyte = 1048576L
    gigabyte = 1073741824L
    terabyte = 1099511627776L
    petabyte = 1125899906842624L
End Enum

Private Function Size_To_Size(ByVal Size As Long, _
                              ByVal FromSize As Units, _
                              ByVal ToSize As Units, _
                              Optional ByVal decimals As Integer = 2 _
                              ) As Double

    Dim bytes As Double = Convert.ToDouble(Size * FromSize)
    Dim result As Double = 0

    If ToSize < FromSize Then

        Select Case ToSize
            Case Units.bytes : result = bytes
            Case Units.kilobyte : result = bytes / Units.kilobyte
            Case Units.megabyte : result = bytes / Units.megabyte
            Case Units.gigabyte : result = bytes / Units.gigabyte
            Case Units.terabyte : result = bytes / Units.terabyte
            Case Units.petabyte : result = bytes / Units.petabyte
            Case Else : Return -1
        End Select

    ElseIf ToSize > FromSize Then

        Select Case ToSize
            Case Units.bytes : result = bytes
            Case Units.kilobyte : result = bytes * Units.kilobyte / Units.kilobyte ^ 2
            Case Units.megabyte : result = bytes * Units.megabyte / Units.megabyte ^ 2
            Case Units.gigabyte : result = bytes * Units.gigabyte / Units.gigabyte ^ 2
            Case Units.terabyte : result = bytes * Units.terabyte / Units.terabyte ^ 2
            Case Units.petabyte : result = bytes * Units.petabyte / Units.petabyte ^ 2
            Case Else : Return -1
        End Select

    ElseIf ToSize = FromSize Then

        Return Size.ToString("n" & decimals)

    End If

    Return result.ToString("n" & decimals)

End Function

Question 4

Below is a VBA function that I use in Microsoft Access that can easily be adapted to VB.

Public Function FormatFileSize(ByVal lngFileSize As Long) As String

  Dim x      As Integer:      x = 0
  Dim Suffix As String:  Suffix = ""
  Dim Result As Single:  Result = lngFileSize

  Do Until Int(Result) < 1000
     x = x + 1
     Result = Result / 1024
  Loop

  Result = Round(Result, 2)

  Select Case x
         Case 0
              Suffix = "Bytes"
         Case 1 'KiloBytes
              Suffix = "KB"
         Case 2 'MegaBytes
              Suffix = "MB"
         Case 3 'GigaBytes
              Suffix = "GB"
         Case 4 'TeraBytes
              Suffix = "TB"
         Case 5 'PetaBytes
              Suffix = "PB"
         Case 6 'ExaBytes
              Suffix = "EB"
         Case 7 'ZettaBytes
              Suffix = "ZB"
         Case 8 'YottaBytes
              Suffix = "YB"
         Case Else
              Suffix = "Too big to compute :)"
  End Select

  FormatFileSize = Format(Result, "#,##0.00") & " " & Suffix

End Function 'FormatFileSize