These were the best value that I could find online. So I bought two weights to calibrate a postage scale that required 50 Kg's. They arrived, much postman's chagrin. Next, I was able to measure each one on a calibrated scale. One measured 50.002 Kg and the other measured 50.003 Kg. Even the 500g and 20g verification weights measured exactly. Bottom line, I am very satisfied with my purchase. However, it turns out the postage scale was already very accurate. So, were they really necessary and how often will I need to use them? Maybe I can use them to build some muscles?