It’s good practice to make the width of your website less than your users screen resolution. You need this cushion to display the sides of the web browser which includes things like scroll bars. This part of the browser (that’s not part of your webpage) is known as “chrome”.
Ex. if you expect your user’s screen resolution to be 800×600 pixels, then you should set the width of your website layout to be 760 pixels. This leaves 40 pixels of cushion for the “chrome” of the browser.