Converting binary value from BitArray to an int and back in C#
You've made it much more complicated than necessary.

The conversion to a BitArray needlessly copies the values to the bool array bits.  You could instead use that on the conversion back to int.

public static class BinaryConverter
{
    public static BitArray ToBinary(this int numeral)
    {
        return new BitArray(new[] { numeral });
    }

    public static int ToNumeral(this BitArray binary)
    {
        if (binary == null)
            throw new ArgumentNullException("binary");
        if (binary.Length &gt; 32)
            throw new ArgumentException("must be at most 32 bits long");

        var result = new int[1];
        binary.CopyTo(result, 0);
        return result[0];
    }
}