How to make an element width: 100% minus padding?

Question

I have an html input.

The input has padding: 5px 10px; I want it to be 100% of the parent div\'s width(which is fluid).

However using widt

Answer 1

Assuming i'm in a container with 15px padding, this is what i always use for the inner part:

width:auto;
right:15px;
left:15px;

That will stretch the inner part to whatever width it should be less the 15px either side.