powershell convert text 20MB to bytes

$str = '3GB'

$int64 = [int64]$str.Replace('GB','') * 1GBfunction Convert-Size {            
[cmdletbinding()]            
param(            
    [validateset("Bytes","KB","MB","GB","TB")]            
    [string]$From,            
    [validateset("Bytes","KB","MB","GB","TB")]            
    [string]$To,            
    [Parameter(Mandatory=$true)]            
    [double]$Value,            
    [int]$Precision = 4            
)            
switch($From) {            
    "Bytes" {$value = $Value }            
    "KB" {$value = $Value * 1024 }            
    "MB" {$value = $Value * 1024 * 1024}            
    "GB" {$value = $Value * 1024 * 1024 * 1024}            
    "TB" {$value = $Value * 1024 * 1024 * 1024 * 1024}            
}            
            
switch ($To) {            
    "Bytes" {return $value}            
    "KB" {$Value = $Value/1KB}            
    "MB" {$Value = $Value/1MB}            
    "GB" {$Value = $Value/1GB}            
    "TB" {$Value = $Value/1TB}            
            
}            
            
return [Math]::Round($value,$Precision,[MidPointRounding]::AwayFromZero)            
            
}