Measure the factory size of car speakers by using a ruler or tape measure to determine the diameter of circle speaker or the length and width of oval speakers. Some speakers or cars may also require you to measure the depth of the speaker hole as well as the diameter of the magnet at the base of the unit.

Car speakers typically come in either a circle or oval shape, each of which imparts its own system of measurement. Circular speakers focus on the diameter of the basket at the rim, which is the largest portion of the unit. Speakers with an oval shape measure the length and width of the basket, also at the largest points. The majority of speakers use inches as the unit of measurement, with most following a standard set of increments ranging from 3 inches to 18 inches. Some speakers made in other countries may measure the values in centimeters and may not be compatible with all car models.

The depth of a speaker is also important to ensure that it fits in the car and sits flush against the surrounding area. This is most important when dealing with door speaker holes, as they have less room for the units compared to rear speakers. The depth should start at the top of the basket and extend down to the bottom of the magnet. The magnet diameter may also matter for certain car models, with these usually appearing in a circular shape.