How is the getBBox() SVGRect calculated?

People often get confused by the behavioral difference of getBBox and getBoundingClientRect.

getBBox is a SVG Element's native method as equivalent to find the offset/clientwidth of HTML DOM element. The width and height is never going to change even when the element is rotated. It cannot be used for HTML DOM Elements.

getBoundingClientRect is common to both HTML and SVG elements. The bounded rectangle width and height will change when the element is rotated or when more elements are grouped.

Apparently getBBox() doesn't take the transformations into consideration.

I can point you here, unfortunately I wasn't able to make it working: http://tech.groups.yahoo.com/group/svg-developers/message/22891

The behaviour you see is correct, and consistent with the spec. The transform gets applied, then the bbox is calculated in "current user units", i.e. the current user space. So if you want to see the result of a transform on the element you'd need to look at the bbox of a parent node or similar. It's a bit confusing, but explained a lot better in the SVG Tiny 1.2 spec for SVGLocatable That contains a number of examples that clarify what it's supposed to do.

there are at least 2 easy but somewhat hacky ways to do what you ask... if there are nicer (less hacky) ways, i haven't found them yet

EASY HACKy #1:
a) set up a rect that matches the "untransformed" bbox that group.getBBox() is returning
b) apply the group's "unapplied transform" to that rect
c) rect.getBBox() should now return the bbox you're looking for

EASY HACKY #2: (only tested in chrome)
a) use element.getBoundingClientRect(), which returns enough info for you to construct the bbox you're looking for