This is the consistent cross-browser way I found:
var heightComponents = ['height', 'paddingTop', 'paddingBottom', 'borderTopWidth', 'borderBottomWidth'],
widthComponents = ['width', 'paddingLeft', 'paddingRight', 'borderLeftWidth', 'borderRightWidth'];
var svgCalculateSize = function (el) {
var gCS = window.getComputedStyle(el), // using gCS because IE8- has no support for svg anyway
bounds = {
width: 0,
height: 0
};
heightComponents.forEach(function (css) {
bounds.height += parseFloat(gCS[css]);
});
widthComponents.forEach(function (css) {
bounds.width += parseFloat(gCS[css]);
});
return bounds;
};