Six pieces, and only two of them are the full board width
Take the box apart on paper and it is always the same six: two sides, a back, a front, a floor and a roof. The construction this page assumes is the common one — the two sides run the full width of the board, so the board width becomes the box depth front to back, and the front and back sit between the sides. That means the interior depth is the board width less two thicknesses, and the front, back and floor all get ripped down to the inside width you typed.
Only the two sides come off the board with a crosscut and nothing else. Everything else needs a rip first. That is worth knowing before you start, because it decides whether this is a circular saw and a straightedge job or a table saw job.
At the defaults — a 5.5 in board, 0.75 in thick, 4 in inside width, 8 in front, 10 in back, 6 in of back extension, 2 in of roof overhang past the front — the six pieces come to 10, 10, 16, 8, 4 and 7.5 inches. That is 55.5 inches of board for one box.
Why four boxes take three boards and not four
An eight foot board is 96 inches, and an inch of trim leaves 95. One box takes 55.5 inches plus five kerfs, so a fresh board holds exactly one box and leaves about 38.9 inches. Nothing about 38.9 inches makes a second box. But it makes a side, a side and a back — three pieces of the next one — and the moment you allow a box to span two boards, four boxes come off three boards instead of four.
That is the entire argument for cutting a run rather than a box. Cut them one at a time and you buy a board per box. Cut all twenty-four pieces as one stream and you buy three boards for four, six for eight, and so on. The calculator packs the whole run, which is why its answer is usually lower than the one you get by multiplying.
| Board length | Usable after 1 in trim | Whole boxes from a fresh board | Boards for 4 boxes | Offcut |
|---|---|---|---|---|
| 6 ft | 71 in | 1 | 4 | 20.7% |
| 8 ft | 95 in | 1 | 3 | 21.0% |
| 10 ft | 119 in | 2 | 2 | 5.5% |
| 12 ft | 143 in | 2 | 2 | 21.3% |
The ten foot board is the row worth noticing. It holds two whole boxes and comes out at 5.5 percent waste, which is a quarter of what any other length in that table throws away. The twelve foot board also gets four boxes out of two boards, so the board count is the same — but it wastes 21.3 percent, because the third box needs 55.5 inches and only about 31 are left after two. Longer stock is not automatically less waste. It is less waste only when the leftover happens to be a multiple of something you still need, and at 12 ft it is not.
The six foot board is the other surprise. It wastes 20.7 percent, less than the twelve, and it still needs four boards for four boxes. Waste percentage and board count are two different questions and they do not move together.
Cut order changes the offcut, and sometimes the count
Packing a run of pieces onto boards in the order you cut them is a real constraint, not a bookkeeping detail. Cut in assembly order — side, side, back, front, floor, roof — and the pieces arrive at the end of a board in a particular order, and whether the next piece fits depends on which one it happens to be. Cut longest first across the whole run and the long pieces get placed while boards are still empty, and the short ones fill the tails.
The calculator runs both and shows both numbers, because neither is reliably better. Run the shipped function over a grid of board lengths, board widths, box sizes and run sizes and the two orders give a different board count in about one case in thirteen; in the other twelve they agree and only the distribution of the offcut moves. At the defaults they agree. Rather than assert a rule of thumb that would be wrong most of the time in both directions, the page just prints what each order costs at your numbers.
The roof is the piece that fights the board
The roof blank has to be as wide as the box is across plus whatever overhang you want on each side: inside width, plus two board thicknesses, plus twice the side overhang. At the defaults that is 4 plus 1.5 plus nothing, which is exactly 5.5 — the roof lands flush with the sides and comes off the same 1x6 as everything else, with nothing to spare.
Ask for any side overhang at all and it stops fitting. Half an inch each side makes the blank 6.5 inches against a 5.5 inch board, and the calculator says so and tells you the largest side overhang the board can still carry, which at these dimensions is zero. This is the one piece where the box width and the board width fight each other, and it is worth settling before you buy, because the overhang past the front — the 2 inches in the default — costs you nothing at all, while the overhang at the sides costs you a wider board for the whole run.
What this page will not tell you
It will not tell you what size to make anything. Every interior dimension on the form is a number you bring, and the places to get it are your state wildlife agency, your county extension office, and the free published plans that conservation organisations produce. There is no default here that is a recommendation — the defaults are there so the arithmetic has something to chew on.
It also will not tell you whether you may open, clean, move or remove a box once it is up. Most native birds are legally protected, an occupied nest carries its own protections, and the rules differ by state and by time of year. That question goes to the wildlife agency before it goes to a screwdriver.
Where this hands off
For an arbitrary parts list rather than a box, the cut list optimizer packs whatever you type into it with the kerf accounted for. For the board feet of a stack of stock on its own, the board foot calculator does that and handles quarter thicknesses. Putting the finished boxes up on posts across a property is the nest box mounting run calculator, and if the structure you are building is a bat house instead the bat house chamber calculator works the same way from panels rather than boards.
Questions people ask
What dimensions should I use for the box?
This page does not say, and no calculator should. Interior floor size, box height, entrance size and every other dimension are species questions, they differ by region, and getting them from a calculator is exactly the wrong way round. Your state wildlife agency publishes plans, most cooperative extension services do too, and several conservation organisations put full dimensioned drawings out for free. Take the dimensions from one of those and type them in here, and this page will tell you what they cost in board.
Why does the floor piece come out shorter than the board is wide?
Because the floor sits between the two sides and between the front and the back. The sides run the full board width front to back, so the interior depth is the board width less two board thicknesses. At a 5.5 in board and 0.75 in stock that is 4 inches, which is the floor length the calculator reports. If your plan has the floor sitting under the walls rather than inside them, the floor is the full outside footprint instead and you should cut to the plan, not to this.
Does buying longer boards always waste less?
No, and that is the useful part of the table in the guide. A ten foot board at these dimensions holds two whole boxes and wastes 5.5 percent. A twelve foot board also holds two, because the third would need 55.5 inches and only about 31 remain, and it wastes 21.3 percent — more than the six foot board does. Waste depends on whether the leftover is a multiple of something you still need, not on length. Run your own dimensions through several lengths before you buy the longer stock.
How much does the kerf actually cost me?
Less than people expect on a job this size, and it is still worth counting. Four boxes is 24 pieces across three boards, which is 21 cuts between pieces at 0.125 in, so 2.6 inches — under one percent of the 288 inches you bought. The trim costs more than the kerf does. Where kerf matters is at the margin: if a piece needs 16 inches and the tail of the board is 16.05, the kerf is what decides it does not fit.
Can I get two narrow pieces from one crosscut?
Often yes, and the calculator deliberately does not assume it. If the inside width is less than half the board width minus a kerf, you can rip a crosscut blank down the middle and get two fronts or two floors from one length of board. At a 4 in inside width on a 5.5 in board that does not work, because two fours plus a kerf is more than 5.5. At a 2.5 in inside width it would. The page counts one piece per crosscut position because that is the arrangement that is always available, and any gang ripping you manage on top is a bonus rather than something the material list depended on.