The Binary (Base 2) Numeral System uses the digits 0 & 1.
The following table shows the first 8 binary values, along with the equivalent decimal values:
Binary  1  10  11  100  101  110  111  1000 
Decimal  1  2  3  4  5  6  7  8 
For further information on the binary numeral system, see the Wikipedia Binary Page
Hexadecimal (base 16), decimal (base 10), octal (base 8), and binary (base 2) are the most commonly used numeral systems in engineering and computing. Therefore, Excel has provided functions to convert numeric values to and from each of these systems.
The Excel Bin2Dec function converts a Binary (a base2 number) into a decimal number.
The format of the function is :
Where the number argument is the binary number that is to be converted to decimal.
Note that the supplied number argument must not be more than 10 characters (40 bits) long. The most significant bit of this value denotes the sign of the number and the remaining 39 bits denote the magnitude. Negative numbers are represented using two's complement notation.
The following spreadsheet shows examples of the Excel Bin2Dec function. The format of the function is shown in the spreadsheet on the left and the result is shown in the spreadsheet on the right.
Formulas:

Results:

Note that, in the above example spreadsheet, the negative binary number in cell A4 is represented by two's complement notation.
Further information and examples of the Excel Bin2Dec function can be found on the Microsoft Office website.
If you get an error from your Excel Bin2Dec function this is likely to be one of the following :
#NUM!    Occurs if the supplied number argument is not recognised as a binary number, or contains more than 10 characters 
#NAME?   
Occurs when Analysis ToolPak addin is not enabled in your Excel. + Show How to Do This in Excel 2003: 