1. Close your tailgate
2. Measure from the inside — place your tape measure flat against the inside of the bulkhead (the front wall of the bed, behind the cab)
3. Run the tape to the inside of the tailgate in the closed/upright position
4. Record the measurement in both inches and feet

You may notice minor differences in bed length listings across the web — this is simply due to different measuring conventions, not actual bed size differences.