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
The Excel Bin2Dec function converts a Binary (a base2 number) into a decimal number.
The syntax of the function is:
Where the number argument is the binary number that is to be converted to a 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 spreadsheets show five examples of the Excel Bin2Dec function.
Formulas:

Results:

Note that, in the above example spreadsheet, the negative binary number in cell A4 is represented by two's complement notation.
Further details and examples of the Excel Bin2Dec function are provided on the Microsoft Office website.
If you get an error from the Excel Bin2Dec function this is likely to be the #NUM! error:
#NUM!    Occurs if the supplied number argument is not recognised as a binary number, or contains more than 10 characters. 