An oven that isn't displaying the proper temperature can throw off many good cooks. If your brownies seem runny, despite baking them for the right amount of time, or your roast chicken is dry and overcooked, even when you followed the directions, then your oven temperature may be off.
Buy an Oven Thermometer
An oven monitoring thermometer is a valuable tool for cooks to have. You can buy one of these in most supermarkets, as well as kitchen stores. You can hang an oven monitoring thermometer from your oven rack or stand it on a base. It's best to position the oven monitoring thermometer in the center of the oven to get the most accurate reading. Many oven racks come with a clip for easy hanging. You want an oven monitoring thermometer to be easy to read so you can read it when you open your oven door. Ideally, you'll be able to read the oven thermometer through the glass front on the oven door when you turn the oven light on.
If you are living with an old oven, especially if you're renting, you may have to rely on the oven monitoring thermometer for cooking. There are ways to calibrate the old knob thermometers, however. If you own a newer model oven, you shouldn't have much trouble calibrating it yourself, unless the temperature is considerably off.
Measure Your Oven Temperature
Using the oven monitoring thermometer, turn your oven temperature on to 350 degrees Fahrenheit. Most ovens have hot spots caused by the heating elements, which are the coils in the oven that generate heat. Record the temperature every 30 to 40 minutes for three or four cycles. Then, add the numbers together and divide by four. If your average is around 350 degrees, your oven is calibrated correctly. There isn't much you can do about these hot spots, but knowing where they are will help you when positioning food to cook properly.
You need to check the temperature multiple times because the oven will heat to a slightly higher temperature, then drop to a slightly cooler temperature before the elements start to heat it again. The temperature range on electric ovens is typically a little closer to the temperature you set in an electric model than a gas model.
Calibrate the Thermostat
If the temperature is off, but only about 35 degrees or so, you can calibrate it yourself. Older model ovens have a set screw inside the temperature knob. To calibrate, pull the knob off the thermostat, and there should be a small set screw in the middle of the shaft. Turn it clockwise to lower the temperature and counterclockwise to raise the temperature. You may need to use needle nose pliers to hold the shaft in place.
For digital ovens, most thermostats are adjusted using keypads. Press and hold the bake and broil buttons to access the calibration setting, then use the up and down controls on the bake button to adjust the temperature. Most ovens will allow the temperature to be adjusted up or down by 35 degrees using the up/down arrows. If the temperature is off by more than 35 degrees, manufacturers of many oven models suggest you call a professional. Once you've made the adjustment, press the start pad to return to the time of day display. Then, your oven is ready to be used normally.
You should probably check your oven's temperature every few months. That way, if your oven temperature is starting to alter, you'll catch the problem before you burn that pizza.
You may have a tear in the gasket, which could allow heat to escape and cause your oven's temperature to be off.