What is the Javascript equivalent for jQuery's width() function?

I want to be able to calculate the width, in pixels, of an element that has the width css property set to 'auto'.

I have tried element.style.width but didn't work because it returned 'auto'. I notices that the jQuery function width() returns the length in px, but I cannot use jQuery to solve this, because it is not available in my page. So, does anybody know an equivalent method for jQuery width()?

Thanks.