Place your scale on a flat and level surface. For ultimate accuracy, use a level to make sure the surface area is level.
Zero out your weight scale. Adjust your manual scale with its adjusting wheel or screws so it reads zero. A digital scale will need to be turned on, allowed to warm up for a few seconds, then adjusted with a knob to read zero.
Place a known weighted object onto the scale and read the results. For absolute accuracy, certified calibration weights must be used.
Read the weight that is registered on the scale. Adjust the scale, if needed, by turning a specifically labeled calibration knob or wheel to show the exact weight of the object you are testing. Digital scales may have an electronic calibration feature to calibrate the weight during a test by just a push of a button. Not all scales will have a calibration feature, just having a knob or wheel to zero the scale out when no weight is present. If you need to have absolutely exact weights for any object being weighed, a calibration feature is a must.
Repeat the testing procedure two more times, and after the initial adjustment, each weight that is registered should be exactly the same as the one previous. For ultimate accuracy, each test of the three must register the exact same weight.