Format Number like Stack Overflow (rounded to thousands with K suffix)

Like this: (EDIT: Tested)

static string FormatNumber(int num) {
    if (num >= 100000)
        return FormatNumber(num / 1000) + "K";

    if (num >= 10000)
        return (num / 1000D).ToString("0.#") + "K";

    return num.ToString("#,0");
}

Examples:

• 1 => 1
• 23 => 23
• 136 => 136
• 6968 => 6,968
• 23067 => 23.1K
• 133031 => 133K

Note that this will give strange values for numbers >= 10⁸.
For example, 12345678 becomes 12.3KK.

You can crate a CustomFormater like this:

public class KiloFormatter: ICustomFormatter, IFormatProvider
{
    public object GetFormat(Type formatType)
    {
        return (formatType == typeof(ICustomFormatter)) ? this : null;
    }

    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        if (format == null || !format.Trim().StartsWith("K")) {
            if (arg is IFormattable) {
                return ((IFormattable)arg).ToString(format, formatProvider);
            }
            return arg.ToString();
        }

        decimal value = Convert.ToDecimal(arg);

        //  Here's is where you format your number

        if (value > 1000) {
            return (value / 1000).ToString() + "k";
        }

        return value.ToString();
    }
}

And use it like this:

String.Format(new KiloFormatter(), "{0:K}", 15600);

edit: Renamed CurrencyFormatter to KiloFormatter

The code below is tested up to int.MaxValue This is not the most beautiful code but is most efficient. But you can use it as:

123.KiloFormat(); 4332.KiloFormat(); 2332124.KiloFormat(); int.MaxValue.KiloFormat(); (int1 - int2 * int3).KiloFormat();

etc...

public static class Extensions
{
    public static string KiloFormat(this int num)
    {
        if (num >= 100000000)
            return (num / 1000000).ToString("#,0M");

        if (num >= 10000000)
            return (num / 1000000).ToString("0.#") + "M";

        if (num >= 100000)
            return (num / 1000).ToString("#,0K");

        if (num >= 10000)
            return (num / 1000).ToString("0.#") + "K";

        return num.ToString("#,0");
    } 
}