Ideally, a good recipe will specify which position the oven rack should be in. That being said, not all do, and even the instructions you do get will probably not account for the vagaries of ovens and how inconsistent they can be.

Which oven rack you use depends on what you’re making and what you’re trying to achieve, with the aforementioned caveat that you may need to adjust if, say, your oven tends to run hot or cold near one of the heating elements. Try to think about where you want the heat most applied — or where you don’t.
With those factors in mind, here’s a quick guide.

Top oven rack

When grilling, use the top rack. If your griller is in the main oven, rather than a separate drawer, aim to position the rack eight to 15 centimetres from the element, closer for more char, farther away for less. I particularly like using the grill with quick-cooking vegetables, such as thin slices of eggplant or spears of asparagus, and it’s a natural for certain cuts of meat, such as lamb chops, assuming you keep a close eye on them. If you’re looking to recreate restaurant-style pizza, the grill can help with that, too.

Middle oven rack

The default oven rack position is the middle if it’s not specified. For the most even coverage, this is where you want to put pretty much anything that you’re roasting or baking (like Katrina Meynink’s rib-eye roast, below). It’s never a bad idea, though, to rotate the sheet pan or dish from front to back halfway through in case one side runs hotter than the other. (This is less necessary if you are using your convection feature, in which a fan blows to better circulate the hot air.)

If you are baking multiple items at the same time that won’t fit on one rack, such as two trays of cookies or a sheet pan meal with several elements, you can use what might alternatively be called the upper and lower thirds of the oven, or the upper- and lower-middle positions. These would be about halfway between the middle rack and the uppermost and lowermost positions, which keeps the food toward the centre of the oven, away from the extremes; again, in this scenario, it’s best to move the food around, switching the pans between racks as well as rotating from front to back.

If you know your oven skews hot on top or bottom, move a notch above or below the middle as needed. (Example: Your cookies are burning on the bottom.)

Bottom oven rack

If you’re trying to get a crispy, not soggy, bottom, use the bottom rack closest to the lower heating element, unless your recipe tells you otherwise. Especially in conjunction with a pizza stone or steel, or even a preheated sheet pan, the lowest rack can give you the best results for a pizza (like the one pictured above) or pie crust. If you are making something really big, like a huge piece of meat, or even baking bread in a large Dutch oven, you may need to use the lowest rack, or second-lowest, to ensure everything fits.

The Washington Post