There are a few different types of oil filters, and the diameter can vary depending on the type. The most common type of oil filter is a spin-on oil filter, which has a threaded base that screws onto the engine. The diameter of this type of filter is usually about 3 inches. Another type of oil filter is a cartridge oil filter, which is more commonly found on older vehicles. These filters have a housing that needs to be unscrewed and then the cartridge inside needs to be replaced. The diameter of these filters is usually about 2-3 inches.