King size has a shorter headboard than the other sizes. (40 Inches). We have a nine inch mattress. That leaves 17.5 inches of headboard visible. We needed to stay with a low profile not to overbear our small mid century home. For us these dimensions worked perfect. 
I also included a picture of the underbed clearance. You get 8”. 
It’s probably hard to see but the tan upholstered bed matched my target curtains (linen look) perfectly! 
It was actually fun putting this together. So easy with the included special tool!!!
It’s the sturdiest bed I’ve ever owned. We had 500 lbs on it. Kid tested. Solid as a rock. 
We love it!