$('#mydiv').css('margin-left') return 0px (must return auto)

$('#mydiv').css('margin-left') return 0px (must return auto)

Hello,

All is on the title, sorry for my english, i'm french :)

I have an html page with style

<style>
#mydiv {
margin-left:auto;
margin-right:auto;
width:250px;
}
</style>

with jquery, i try to get the margin-left ($('#mydiv').css('margin-left'), but the function return 0px, unable to retrieve the good value (auto)

anyone has idea to retrieve the value "auto" when margin-left is "auto" ?