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 Dec2Bin function converts a decimal number into a Binary (Base 2) number.
The format of the function is :
Where the number argument is the decimal number that is to be converted to binary, and the places argument is the number of characters that you want the returned binary number to have. If this is greater than the minimum, the binary number will be padded out using leading zeros.
If the places argument is omitted, then by default, the minimum number of places is used in the returned binary number.
The following spreadsheet shows examples of the Excel Dec2Bin 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 information and examples of the Excel Dec2Bin function can be found on the Microsoft Office website.
If you get an error from your Excel Dec2Bin function this is likely to be one of the following :
#VALUE!    Occurs if either the supplied number argument or the supplied [places] argument is not numeric. 
#NUM!    Occurs if the supplied [places] argument ≤ 0. 
#NAME?   
Occurs when Analysis ToolPak addin is not enabled in your Excel. + Show How to Do This in Excel 2003: 