All Jump Ring manufacturers make rings by wrapping wire around a rod or “mandrel”. Because the wire has a stiffness or “temper” to it, when released, the wire springs back and the rings increase in size slightly. The harder the temper, the more spring back. Typically, thicker wire has more springback. Every chainmaille pattern that I have ever seen takes this into account when developing the pattern. Just like all carpenters know that a “2 x 4″ is not really 2″ x 4”, chainmaille pattern writers realize that jumprings will always be slightly larger that the specified size. If you have a critical application, feel free to contact me in advance and we can discuss this further. If necessary, I can always make up a few rings in my shop and determine the actual size. From there we can determine the size that you need to order.