I guess I should clarify that our design will have the bed recessed another 6-7 inches into the platform. The mattress is 13 deep, so I don't want it tosit on top of a flat platform. So there will be sides that come up. We are building it outof bamboo.

I add an inch total, or 1/2" per side for a mattress. That is after carefully measuring the overall outer dimensions of the mattress, which can be a little tricky.

yes, I know what you mean about it being tricky to measure the mattress. Do you measure it with all the bedding off of it, just the bare mattress, or do you measure it with the bedding on, and then add a 1/2 inch?

Put on the linens you want to fit inside the bedframe. Push the mattress into the corner of the room, so it is against the wall on 2 sides.

Use the wall as a guide on one side, a small piece of plywood held plumb w/ a level on the other. Measure the space between. This will give you exact dimensions. Then add an inch, or half an inch per side.

That is a great idea, I started thinking about doing something similar, but your idea is much easier.. thanks..