Why Measure

Why measure twice? Well, the jump ring is (ideally) a perfect circle before it is cut. After it is cut, the ring has a gap, also known as a kerf. When you close the jump ring, it is actually an oval shape, because of the missing metal where the kerf is. So, you need to measure up and down from the kerf (photo 2a); this is the long side. Additionally, you must the shorter side, which is perpendicular to the kerf (photo 2b).

Measuring the OD is better than measuring the ID, because it is easier to keep the ring straight in the calipers. If you're measuring the ID and the ring is slanted, the calipers will not give an accurate measurement. However, when you're measuring the OD, the ring can only slant ever so slightly before it falls out of the caliper jaws. So, you're much more likely to wind up with accurate readings by measuring the OD. (The widget then subtracts the WD twice, once on each side, to determine the ID used for the AR formula.)