[Cariño]

Как вариант:

block=document.getElementById("main");

blockHeight=block.offsetHeight; 

var padding = 0;  
                           
if (block.currentStyle)                    
    padding = parseInt(block.currentStyle.paddingTop) + parseInt(block.currentStyle.paddingBottom);
else if (window.getComputedStyle)             
    padding = parseInt(window.getComputedStyle(block, null).paddingTop) + parseInt(window.getComputedStyle(block, null).paddingBottom);

blockHeight = blockHeight-padding;