Ovens often run hotter or cooler than displayed due to thermostat drift or age; using an oven thermometer reveals true temperature, enabling compensation through adjusted settings or cooking times. Even small variances (25°F) significantly impact baking success.
Place an oven thermometer in your oven at the set temperature and observe actual temperature multiple times. Document the variance and adjust future recipes accordingly.
You already know from oven preheating that setting a temperature on the dial tells the oven to reach and hold that temperature. What you may not realize is that the thermostat controlling this process is a mechanical or electronic device that drifts over time. The dial reads 350°F, but what the oven actually delivers may be 325°F or 375°F. For stovetop cooking this rarely matters — you can see and adjust. For baking, a 25°F error is the difference between a properly risen cake and a sunken one, because chemical reactions like leavening and protein coagulation happen at specific temperatures.
The fix is an oven thermometer — an inexpensive, standalone probe you place on the rack. Unlike the built-in thermostat, it measures the actual air temperature at that location. To calibrate your oven, set it to a round number (say 350°F), let it fully preheat for 20 minutes, then read the thermometer. If it reads 325°F, your oven runs 25°F cold. Write this down. From that point on, when a recipe calls for 350°F you set your dial to 375°F to compensate.
There are two ways to apply this correction. You can adjust the set temperature (as above: add or subtract the measured offset). Or you can adjust the cooking time: a cooler oven bakes more slowly, so you extend the time and check for doneness earlier than the recipe suggests. Adjusting temperature is cleaner for most baking; adjusting time works better for long braises where a few degrees matter less.
The oven's interior is also not uniform. The back tends to run hotter than the front (closer to the heating element and less heat lost when you open the door). Upper racks are hotter because heat rises. Lower racks get more radiant heat from the bottom element. Most baking recipes assume the middle rack as the default. Rotating a sheet pan halfway through baking compensates for front-to-back variation. Once you've verified your oven's actual temperature and learned its hot spots, you stop guessing and start baking predictably — the same skill that separates consistent home bakers from frustrated ones.