If you'd rather look at the number on your bathroom scale than that on your household thermostat, it must be time for a calibration. After all, it's no laughing matter when your thermostat seems to be misreading the room's actual temperature. Such a miscalculation could cause your air conditioner or furnace to run more often than it really has to, wasting energy and money. Even if you're new to this adjustment, you can calibrate your thermostat in a few simple steps.
Check the area around the thermostat for elements that might be interfering with an accurate temperature reading, and remove or fix them. For instance, candles placed too close to a thermostat can cause the thermostat to register a higher reading; a cold draft from a nearby window could trigger a lower reading.
Remove the thermostat faceplate, unscrew the thermostat and pull it from the wall. Inspect the thermostat for dust and dirt, the most likely cause of malfunction. Carefully remove these obstructions, as well as any corrosion, with a cotton swab.
Place a bubble level over the top of the thermostat base -- there should be a flat "ledge" here -- to see if it's mounted evenly on the wall. If it's not, it could be affecting the interior mercury switch. Jiggle the base of thermostat until it's level, and tighten the screws firmly to the wall.
Place a thermometer inches away from the thermostat. Let it sit for 20 minutes to register an accurate reading. Compare it to the temperature on the thermostat.
Call a licensed heating and air conditioning professional if the difference in temperature readings is more than 2 degrees, which is generally considered an acceptable deviation. A disparity may signal a problem with your thermostat and the need for a new one.