Measuring is actually quite easy simply take a tape measure and measure between the bulkhead near the cab inside the truck bed all the way to the inside of the tailgate the pivot point of your tailgate.
How to measure your truck bed for a canopy.
Step 1 place your measuring tape at the bulkhead of your truck bed.
After selecting your vehicle and a tonneau cover you will see a fitment note directing you to the correct model number for your bed size.
Again the tape measure should be in a straight taut line even with the top of the bed.
Hold one end of your tape measure against the inside of your truck bed s left front corner and have someone pull the tape measure until it reaches the inside of the front right corner.