Calibrated to +/- 3% in clockwise direction and +/- 6% in counter clockwise direction. Calibration is accomplished by mechanical loading of each wrench in a horizontal position, and performing a three click test at 20%, 60% and 100% of maximum instrument capacity. Certification documents traceable to N.I.S.T. (National Institute of Standards and Technology). Manufactured to meet precision/highly accurate applications such as military and aerospace.