To take truck measurements:
Measure dimension A, the width of the truck bed. Measure between the top of the bed sidewalls.
Measure the depth of the truck from the top of the truck rail bed. Most standard trucks have a depth of nine inches. A deep model usually has a depth of 12 inches.
To find measurements for the area for truck boxes, imagine a horizontal line at the required depth of the truck box. Measure to find a point 20 1/4 inches away from the front truck bed rail. If this point hits the wheel well, truck boxes will not fit in the truck. Measure the distance between points X on either side of the car. This determines the maximum length of the lower half of any truck boxes.
Measure from the inside of the tailgate to the end of the bulkhead, says AutoAccesoriesGarage.com. If using this measurement to look for accessories bear in mind that most companies round off their measurement to the nearest foot, so accessory sizes may not be listed as an exact match.