Calibration weights should be manufactured to international standards (e.g., ASTM, OIML) and traceable to a national standards agency (e.g., the U.S. National Institute of Standards and Technology [NIST] or the U.K. National Weights and Measures Laboratory [NWML]). If using weights not specifically sold for measurement (e.g., exercise weights), weigh each weight using a calibrated scale and label it with the accurate value.