String Format Numbers Thousands 123K, Millions 123M, Billions 123B

There are different ways to achieve this, but for me the easiest and quickest is to use the "," custom specifier

double value = 1234567890;

// Displays 1,234,567,890   
Console.WriteLine(value.ToString("#,#", CultureInfo.InvariantCulture));

// Displays 1,234,568K
Console.WriteLine(value.ToString("#,##0,K", CultureInfo.InvariantCulture));

// Displays 1,235M
Console.WriteLine(value.ToString("#,##0,,M", CultureInfo.InvariantCulture));

// Displays 1B
Console.WriteLine(value.ToString("#,##0,,,B", CultureInfo.InvariantCulture));

I use this mix of formats in an Extension Method (just add it to your project and enjoy) 😎

public static string ToKMB(this decimal num)
{
    if (num > 999999999 || num < -999999999 )
    {
        return num.ToString("0,,,.###B", CultureInfo.InvariantCulture);
    }
    else
    if (num > 999999 || num < -999999 )
    {
        return num.ToString("0,,.##M", CultureInfo.InvariantCulture);
    }
    else
    if (num > 999 || num < -999)
    {
        return num.ToString("0,.#K", CultureInfo.InvariantCulture);
    }
    else
    {
        return num.ToString(CultureInfo.InvariantCulture);
    }
}

Use:

((decimal)235).ToKMB();
// 235

((decimal)1235).ToKMB();
// 1.2K

((decimal)6271235).ToKMB();
// 6.27M

((decimal)93246571235).ToKMB();
// 93.247B

Notes:

• It return more detail for bigger numbers and I like it.

• It support negative numbers too. (Thanks to @Dusty for his note in comments.

• I write a method for decimal numbers in this example, you can write some override methods for it to support int, long and double to use it without any casting such as:

  myIntNumber.ToKMB();

  myLongNumber.ToKMB();

  myDoubleNumber.ToKMB();

  myDecimalNumber.ToKMB();

You can implement a ICustomFormatter that divides the value by thousand, million or billion, and use it like this:

var result = string.Format(new MyCustomFormatter(), "{0:MyFormat}", number);