Converting a binary number to decimal is much simpler than the other way around. You dont need a pencil and paper for this. Lets say you have the number (110001)2 and you wish to convert it.
So start from the right. Keep ticking off powers of two in you head. Every time you encounter a 1 add the power of two to the current answer. If you encounter a 0 just move on to the next place.
Here doing this we would get:
If the binary numer has a decimal point(?) then its a little more tricky(Maybe you should get your pencil now). Anyway lets say you have (.101)2. The process is the same except that you start from the left and must use inverse powers of two. So here we have (.101)2 = (.5+.125)10 = (.625)10 .