Holes on a face is a fencepost count, not a division
The number of hole centres across a face is the usable run divided by the spacing, rounded down, plus one. The plus one is the first hole, and forgetting it is the single most common error in this arithmetic. A 5.5 inch face with 0.75 inches of margin each side leaves 4 inches for centres; at 0.75 inch spacing that is 5.33, which rounds down to 5 spaces, which is 6 holes. Not 5.
Square grid on a square face, that is 6 by 6, so 36 holes. Both dimensions use the same arithmetic and there is nothing else to it.
Staggering changes the row pitch, and that is where the extra holes come from
Offset every other row by half a space and the rows can move closer together without any two holes getting nearer than the spacing. The row pitch drops to the spacing times the square root of three over two, about 0.866 of it. At 0.75 inch spacing that is 0.6495 inches a row.
Four inches of run at 0.6495 gives 6.15, so 6 pitches and 7 rows instead of 6. But the offset rows lose a column: their first centre sits half a space in, so the last one would fall past the margin line, and they carry 5 holes rather than 6. Four full rows at 6 and three offset rows at 5 is 24 plus 15, which is 39.
Thirty-nine against thirty-six is not dramatic on one block. Across 200 holes it is the difference between six blocks and six blocks — the same, because 200 over 36 is 5.6 and 200 over 39 is 5.1, and both round up to 6. Across 500 it is 14 blocks against 13. The calculator prints both counts side by side precisely so you can see when the extra rows actually buy you a block and when they do not.
| Face | Square grid | Staggered | Blocks for 500, square | Blocks for 500, staggered |
|---|---|---|---|---|
| 5.5 x 5.5 in | 36 | 39 | 14 | 13 |
| 5.5 x 7.25 in | 48 | 50 | 11 | 10 |
| 7.25 x 7.25 in | 64 | 72 | 8 | 7 |
| 3.5 x 3.5 in | 9 | 12 | 56 | 42 |
Staggering is not always the winner
It would be tidy if the offset layout always came out ahead, and it does not. Sweeping the shipped function across face sizes from 3.5 to 11.25 inches, spacings from half an inch to an inch and margins from three eighths to a full inch, the two layouts differ in most combinations and the staggered one comes out lower in about one in seven of them.
The reason is the column the offset rows lose. Staggering buys you extra rows, because the pitch drops to 0.866 of the spacing, but every offset row starts half a space in and so may hold one fewer hole. When the face is short relative to the spacing you gain no extra row at all and still pay the column, and you end up behind. On a 5.5 inch face at 0.75 inch spacing with a half inch margin, the square grid gives 49 and the staggered one gives 46 — the same face and spacing that produced 36 against 39 at a three quarter inch margin. A quarter inch of margin flipped the answer.
That is why the page runs both every time rather than recommending one. There is no rule of thumb here that survives contact with a range of inputs.
The drilling run is the number that surprises people
Six blocks at 36 holes is 216 holes, and at 5 inches deep that is 1,080 inches of drilling. Ninety feet. Through wood, with a small diameter bit that wants to wander and heats up if it is not cleared. That is a different afternoon from the one people picture when they think of a block with some holes in it.
Bit life is a genuinely personal number and the field is there because of it. A brad point in soft pine may go a long way; the same bit in dry hardwood will not. At 150 holes a bit those 216 holes take two, and the second one is doing 66 holes. Buying a spare is cheaper than stopping.
What the hole takes out of the block
Thirty-six holes at 0.3125 inches across and 5 inches deep remove pi over four times 0.3125 squared times 5, times 36 — 13.81 cubic inches. The blank at 5.5 by 5.5 by 6 is 181.5 cubic inches, so 7.6 percent of it comes out as shavings. That is small enough to reassure anyone worried about the block falling apart and large enough to explain why the chip pile is bigger than expected.
The other check the page does is the one that ruins blocks: hole depth plus the wood you want left behind has to be no more than the block thickness. Five inches deep, half an inch behind, on a six inch block is fine with half an inch to spare. Five inches deep on a five inch block is a hole straight through and the calculator says so.
The wall between two holes
Spacing less diameter is the wood between two neighbours in a row: 0.75 less 0.3125 is 0.4375 inches, seven sixteenths. On a staggered layout the wall between rows is tighter, because the pitch is smaller — 0.6495 less 0.3125 is 0.337 inches, a bit over five sixteenths. That is the number that decides whether the block splits when you drill it, and it is why staggering is not free even though it costs nothing in spacing.
None of the dimensions on this page are recommendations
Hole diameter, hole depth, spacing and margin are all fields you fill in, and the defaults exist so the arithmetic has something to work on. What any species needs is not a question a calculator gets to answer. Your state wildlife agency, your county extension office and the conservation organisations that publish plans are where those figures come from, and they differ by region.
Two things after the drilling that are not arithmetic. Wildlife can carry disease and material that accumulates in a used structure is not a cleaning job for an untrained person. And whether a block that has been used may be opened, cleaned, moved or destroyed, and when, is a question for the state wildlife agency first.
Where this hands off
Cutting the blanks out of a board is the board foot calculator for the stock and the cut list optimizer for the packing. For parts laid out on a sheet rather than holes on a face, the sheet nesting yield calculator handles the grid with kerf and margins. Mounting a set of blocks across a property is the nest box mounting run calculator, and a panel built structure rather than a solid block is the bat house chamber calculator.
Questions people ask
What hole diameter and depth should I drill?
Not something this page will tell you. Diameter, depth and spacing are species and region questions and a calculator handing you a number for them would be doing real harm. Your state wildlife agency, your cooperative extension service and the conservation organisations that publish free plans are the sources. Bring those figures here and the page will tell you how many fit a face, how many blocks a target takes, how far the bit travels and how much wood comes out.
Why is the hole count spacing plus one rather than just a division?
Because you are counting fence posts, not fence panels. Four inches of usable run at 0.75 inch spacing holds 5 whole spaces, and 5 spaces have 6 endpoints. Dividing and rounding down gives you the spaces and misses the first hole. On a 6 by 6 layout that error is 36 holes against 25 — nearly a third of the block.
Is a staggered layout always worth it?
No, and that is worth knowing before you commit a drilling jig to it. Staggering buys extra rows because the pitch drops to about 0.866 of the spacing, but every offset row starts half a space in and may hold one fewer hole, so it can come out behind. Swept across a range of face sizes, spacings and margins, the staggered layout loses in roughly one case in seven. At the defaults it is 39 against 36; move the margin from 0.75 in to 0.5 in on the same face and it is 46 against 49 the other way. The calculator prints both counts every time. Staggering also leaves a tighter wall between rows, which is what splits a block.
How do I know how many holes a bit will last for?
You do not, and neither does this page, which is why it is a field rather than a constant. It depends on the bit, the wood, the speed, whether you are clearing chips and whether the block is dry. The useful thing the field does is show you the shape of the answer: 216 holes at 150 a bit is two bits, and at 100 a bit it is three. If you have drilled this stock before, your own number is worth more than anybody else has.
The calculator says my hole goes through the back. What now?
Either shallower holes or thicker stock, and it is only arithmetic: the depth plus the wood you want behind the hole has to be no more than the block thickness. The default check is half an inch behind a five inch hole in six inch stock, which leaves half an inch of slack. If the plan you are working from calls for a deeper hole than your stock allows, that is a stock decision and it belongs before the drill press, not during.