To keep a scale in top performing condition, you must calibrate it on a regular basis. Most scales come from the manufacturer with a default calibration weight, but these weights usually wind up in the junk drawer or an infrequently used tool box that may or may not produce the weight when you really need it. Luckily, it's easy to calibrate a scale even without these special weights.
Determine the capacity of your scale. Scales are classified by the maximum amount they can weigh, and these specifications can be found in your owner's manual. If you have misplaced your owner's manual along with your calibration weight, you can probably find the maximum weight capacity listed somewhere on the machine itself. Check the manufacturer's information tag on the bottom of the scale; if it doesn't list the maximum weight, it will at least have the manufacturer's name and the scale's model number, which you can then plug into an Internet search engine to find the information.
Locate an object that weighs exactly the same as the maximum capacity of your scale. Coins are especially good to use for calibration of smaller scales, since their weights are generally exact. A penny weighs 1.5g and a nickel weighs 5g. Stack them to add up to the weight you need. Flour and sugar come in 2.27kg. bags, so these can be used to calibrate larger scales.
Place the scale on a level, stable surface.
Turn on the scale with the weighing tray empty, allowing the measurements to stabilise. This will take about 30 seconds to 1 minute.
Put the scale into calibration mode.
Place the weight on the centre of the weighing tray, allowing a few seconds for the measurement to stabilise.
Press the calibration button again. The scale will now register that this is the weight of its maximum capacity and automatically adjust itself to this measurement. When it has completed the process, the screen will display "End" or all zeros.
Remove the weight from the weighing tray.
Turn off the scale for 30 seconds to 1 minute.
Turn the scale back on. It should now be properly calibrated and ready to use.
Locate any object whose precise weight you know. As mentioned above, coins, flour and sugar are all good choices.
Put the scale on a stable, level surface.
Place the object on the scale.
Allow the reading to stabilise.
Find the adjustment dial. It is usually located on the back or side of the scale, and can be moved either from side to side or up and down, depending on its orientation on the scale.
Turn the dial to move the weight indicator to point to the weight of the object.
Remove the object. The dial should now return to 0, and the scale should be properly calibrated.
Remember that, although food packaging lists the precise weight of the contents, this figure does not include the weight of the packaging materials. Therefore, if you use a food product for your calibration weight, you will need to remove it from the packaging if it is a particularly bulky package. Sugar and flour should be fine in their packaging, since the paper weighs very little in comparison to the weight of the product and the capacity of a scale that would take such a large weight.
If you are calibrating scales for businesses that rely on government standards for weight and measures, you may want to consider having a professional come in to do the calibration for you, to avoid any legal issues.