How to make my form1 in my masterpage 100%

Hi guys. Currently, i'm trying to ensure that all my header/navbar/homebutton fit to screen with width="100%". Despite putting my header as 100% i can see spaces on the left and right side. I took a closer look at my design of my form1 in my masterpage, there's a small space in between the grey colour background and form1. Hence i would like to ask, how to ensure my form1 enclose the entire webpage? This is what i meant. http://imgur.com/YxTNXdy Regards.  
...