Simplest DIY Bed Frame (All Mattress Sizes)

On the inside face of both side rails, attach the 2x2 cleats with 2-1/2" screws.

Attach the top and bottom rails to the side rails to complete the perimeter box.

For full size and up, add the center 2x4 rail so its top surface sits flush with the top of the 2x2 cleats — this is what keeps a wide mattress from sagging in the middle over the unsupported span.

For extra strength, add corner braces cut from 2x6 with both ends at 45°, screwed in from both sides.

Lay the slats across the 2x2 cleats and the center 2x4, then screw each one down. Half slats work fine if you're using up scrap material.

Attach the bed legs to the underside of the frame with screws.

For standard king and California king, add the center leg supports called out in the cut list to help carry the extra weight across the wider center span.