What is the time complexity of **Binary to Decimal Conversion** ?

I think If there are K bits in a binary number, then TC would be **O(K)**, but, as we always tend to go for N bits in a binary number, then TC would be **O(N)**.

**How i got this** ,as i think like how many digits a decimal number would include to represent **N bit binary numbers.** 

Then, it is 10^k - 1 = 2^N - 1 => K = N Log2 base10 => which gives **TC = O(N)**.

Can someone clarify this ? 

Also, Is there is any chance to reduce this Time complexity ?