To accurately calibrate a digital bathroom scale, set it on a flat surface and use one foot to press on the top of it. Remove your foot when zeros or dashes appear, and once it turns off, the scale is calibrated. The procedure varies depending on the scale.
Some digital scales have a button titled "Cal," "Function" or "Mode" that you must press to enter calibration mode. Some have an automatic calibration feature that sets the scale to zero each time you step off or remove all weight from the scale. Analog scales are manually calibrated. Some of these scales require you to remove the main housing and calibrate them by adjusting a spring, while others use a dial or a knob.
To check the accuracy of a scale, find an object that you already know the weight of and weigh it with the scale. Common household objects with the weight listed include dumbbells, bowling balls and some packages of food, such as bags of flour.
Scales work most effectively when you place them on a hard surface. Don't shift your weight while standing on the scale, as this leads to inaccurate readings. Scales that aren't accurately calibrated can be off by five to 10 pounds. Scales tend to become less accurate with time.