The weight that is listed on the box is generally a ready-to-fly weight, less fuel (for glow-powered planes). But they usually are quite optimistic. For example, if a trainer plane says recommended engine is .25 to .40 and should weigh 5 pounds, the plane would maybe weigh 5 pounds if you put a very lightweight .25 engine on it. But if you put the larger .40 engine on it it will weigh more.