Recognizing that a scale is weighing incorrectly is one thing; correcting it is another. The first step in calibration is the surface the scale sits on: no adjustment will hold on a soft or uneven surface. The second step is weighing a reference item with a known weight, and the third is repeating the same measurement several times. What matters is not the deviation but whether the scale gives the same result each time.
A digital kitchen scale measures how much the weight placed on it bends a sensor inside the housing, then converts that bend into a number. That is why the scale depends directly on the surface beneath it. Soft surfaces such as carpet, a silicone mat, a tablecloth, or a wooden cutting board let one corner of the housing sink; the sensor then registers a bend caused by the surface as well as the weight. That is why the first step in calibration is to place the scale on a hard, level surface.
The second step is testing with an item of known weight. The easiest reference to find at home is an unopened package with its net weight clearly printed on it; a package of flour, sugar, or salt can be used for this. First, tare the scale while it is empty, then place the package in the middle of the platform and read the display. Placing the package near the edge also changes the measurement, because many kitchen scales have a single sensor and expect the load to be centered. The reading is meaningful only after these conditions are met.
The third and most important step is repetition. Remove and replace the same package, taking a few readings in a row; check that the scale returns to zero each time, too. The goal is not to see exactly the same number as the weight printed on the package; the package contents themselves can vary somewhat. What you are looking for is repeatability: does the scale show the same number for the same load every time? If so, the device is consistent. If a different number appears each time, the problem is not a deviation but the scale itself.
A consistent but inaccurate scale can be corrected. Many digital kitchen scales have a calibration mode described in the manual; a specific button combination activates it, and the scale then asks for a reference weight. That weight varies from one device to another, so do not guess it—check the manual. If a scale has no calibration mode, the only option is to account for the difference when using it; this works only as long as the deviation remains the same for every measurement.
The cause of inconsistent readings is often simple, and common causes should be ruled out first. A weak battery can shift the reading even if the display looks normal; replace the battery and test again. A crumb or dried liquid trapped under the platform can prevent the housing from moving freely. Storing the scale somewhere damp, striking it, or dropping it can permanently damage the sensor. If all three causes have been ruled out and the scale still gives a different number each time, it is no longer reliable for kitchen tasks.