Some scales, such as bathroom scales, can be calibrated by simply using known weights and adjusting the scale reading to match the amount of the known weight. Digital scales can be calibrated using a weight and the scale's calibration mode.
To calibrate a bathroom scale, Weight Loss Answers Online recommends using a group of known weights, preferably of 30 pounds or more, and putting them on the scale. If the indicated weight differs from the known weight, calibration entails adding or subtracting the weight difference. For example, if a person put two 10-pound dumb bells and a 12-pound bowling ball on the scale, the scale should read 32 pounds. If, however, the scale reads 30 pounds, there is a two-pound discrepancy. Each time that something is weighed on that particular scale, two pounds should be subtracted from the total. Some bathroom scales have a dial or a knob that can be turned to calibrate the scale to the correct reading.
For digital pocket scales, eBay recommends placing the scale on a flat surface, preferably on a mouse pad or something similar. This keeps the scale from vibrating. Power the pocket unit on, and press the "zero" or "tare" button. This clears any previous data from the scale. Set the scale to calibration mode. The way to do this varies among individual scales. Check the manufacturer instructions for specific directions on how to get to calibration mode. Place a weight on the scale, and wait for the display to indicate that it's ready for calibration. Manually enter the weight's mass into the scale, and wait for the readout to indicate that the weight has been accepted and stored. Once the entered weight has been stored, power the unit off.