Four calipers before you reach the case
Start with a 9.25 in tall book and add an eighth of an inch of clearance, a sixteenth on each side, and the base tray inside is 9.375. Its walls stand on the bottom board, so the bottom is 9.571 — a board caliper on each edge. The lid tray has to go over that, plus the covering on the base tray walls and a clearance of its own, so its inside is 9.657 and its own bottom board is 9.853. Two squares of 0.1875 and the case board is 10.228 in.
The box has grown 0.978 in over the book in that one dimension, and 0.392 in of it — 40 percent — is nothing but board caliper counted four times. Nobody adds that up by eye, which is why a box built by feel is usually wrong by about the thickness of two boards.
The lid tray is bigger than people expect
It is bigger than the base tray inside by 0.282 in in both directions: two board calipers, two covering thicknesses and two clearances. That is more than a quarter of an inch of difference between two trays that look, in the finished box, like the same size. Cutting them the same, which is the mistake, gives a lid that sits on the base tray rim instead of over it.
The wall heights are the other pair that catches people. A 1 in base wall and a 0.75 in lid wall add to 1.75 against a book of 1.5 plus 0.0625 of clearance, so the lid overlaps the base by 0.1875 in. Set them so they add to exactly the book thickness and the trays meet edge to edge with no lip at all, which shuts but does not hold register.
The case is the same sum as a book case
Squares show at head, tail and fore edge but not at the spine, so the case board picks up two squares in height and one in width. The spine inlay spans both tray heights plus a board caliper for each case board it has to bridge — 1.098 plus 0.848 plus 0.196, which is 2.142 in. The covering blank is then two case boards, two hinge gaps, the inlay and a turn-in at each end: 18.099 by 11.728 in, which the page rounds to 1.5 square feet of cloth for one box.
What it does not decide
It has no view on the clearances, and those are the numbers that make a box good or annoying. A book that has to be prised out with two hands is a book that will eventually be dropped; a book that slides in its tray takes the impact on a corner instead. Neither failure is arithmetic and neither is settled here.
It also says nothing about materials or about how long anything will last. The board caliper field asks for a measurement because catalogue names and actual boards disagree often enough to ruin a fitted box, and because nothing on this page is a specification.
Questions people ask
How do I size the trays for a clamshell box?
Work outward. The base tray inside is the book plus clearance on both sides; its bottom board is that plus a board caliper on each edge because the walls stand on it. The lid tray inside is the base tray outside plus the covering on those walls plus its own clearance, and its bottom board adds two more calipers. At the values in the form a 9.25 in book ends up under a 9.853 in lid tray bottom.
How deep should each tray be?
That is yours, but the two together want to add up to more than the book is thick so the lid tray walls overlap the base tray walls rather than meeting them. A 1 in base and a 0.75 in lid against a 1.5 in book gives 0.1875 in of overlap. The page prints the overlap and tells you when it has gone negative, which is when the box would stand open.
Why is the lid tray so much bigger than the base tray?
Because it has to clear the covered outside of the base tray, not the inside. That is two board calipers, two covering thicknesses and two clearances, which at the values in the form is 0.282 in in each direction. Cutting the two trays to the same size is the single most common way a clamshell fails to close.
How much board does one box take?
Add the thirteen pieces. At the values in the form the cut list comes to about 2.45 square feet of board with no waste allowed, spread over two tray bottoms, eight walls, two case boards and a spine inlay. A real layout on a sheet never nests to the area figure, so treat it as a floor and lay the largest pieces out first.
Does the board caliper really matter that much?
It enters the height four times before the case board is cut, so at 0.098 in board it accounts for 0.392 in of the 0.978 in the box grows over the book. Switching to a board a few thousandths different changes all ten tray blanks. That is why the field asks for a caliper reading off the board you have rather than a product name.