Personally, I am familiar with adidas sizes and prefer them as they are quite slim line. They fit us wonderfully as they come in very well in terms of dimensions. However, if the child for whom it is intended weighs more than 27 kilos, you may need the larger size (we ordered it for a 9-year-old weighing 23 kilos, it would have fit us if we had two extra kilos as it has elastic)