You can go to your gym, pick up an olympic plate (eg. 5kg, 10kg etc2) which always very accurate one. Go and weigh it and see how much is the difference. Better yet, if you have olympic weight plate at your house (most people dont have) so you can compare the reading between your house and the gym to know which one is more accurate and how much delta you need to figure out based on the reading for machine in question. You can do this on other object that you know the weigh also... just go and compare... I hope this helps.