The viewable rectangular screen dimensions for an exactly 18 inch diagonal 16:9 aspect ratio TV would be 15.69 inches wide by 8.82 inches high. In reality, 18 inch TV screen dimensions will likely vary slightly by a fraction of an inch either more or less, as television screen size classes are rounded to the nearest whole inch. Aspect ratios other than 16:9 will also alter the screen dimensions. Keep in mind, a TV’s enclosure (and pedestal stand, if applicable) adds to its actual overall dimensions.