The average height of an electrical outlet is 12 to 14 inches from the floor. The ADA requirement is anywhere between 15 inches and 54 inches from the floor.
The National Electric Code does not specify an exact height, according to SFGate. Many builders use a drywall square to quickly mark the placement of electrical outlets. To use this technique, builders place the T part of the square on the floor and use the longest part of the square as a straight edge to mark outlet locations. This technique allows builders to quickly and accurately mark all outlets at the exact same height in every room.