The chamber count is one division and one subtraction
Every chamber costs one gap. Every chamber after the first also costs one partition. So a cavity of depth C, split at gap G with partitions of thickness T, holds n chambers where n times G plus n minus one times T is no more than C. Rearranged, n is the whole number below C plus T, divided by G plus T.
At the defaults that is 4 plus 0.5, over 0.75 plus 0.5, which is 4.5 over 1.25, which is 3.6. Three chambers. They use 3 times 0.75 plus 2 times 0.5, which is 3.25 inches, and 0.75 inches of the cavity is left doing nothing. A fourth chamber would want 4 times 0.75 plus 3 times 0.5, which is 4.5 inches — half an inch more cavity than you have.
The leftover is the number to look at when you are deciding the frame depth. Going from a 4 in cavity to a 4.5 in cavity is a half inch of material on two sides and it buys a whole chamber. Going from 4 to 4.25 buys nothing at all.
| Cavity depth | Chambers at 0.75 in gap, 0.5 in partition | Depth used | Left over |
|---|---|---|---|
| 2 in | 2 | 2.00 in | 0.00 in |
| 3 in | 2 | 2.00 in | 1.00 in |
| 4 in | 3 | 3.25 in | 0.75 in |
| 5 in | 4 | 4.50 in | 0.50 in |
| 6 in | 4 | 4.50 in | 1.50 in |
| 7 in | 6 | 7.00 in | 0.00 in |
Notice that 2 in and 3 in both give two chambers, and 5 in and 6 in both give four. The count only moves when the cavity crosses a multiple of 1.25 inches, and between those points extra depth is just extra depth. The 2 in and 7 in rows land exactly on a threshold and waste nothing at all; the 3 in and 6 in rows sit just past one and waste the most.
Panels, and the two that are not partitions
Three chambers need two partitions between them, and then the back panel and the front panel close the stack at each end. That is four panels for three chambers, not three and not six. Leave the front open and it is three panels and the outer chamber has one wall instead of two.
The roost surface figure is just geometry: every chamber is bounded by two panel faces, so a closed three chamber stack has six faces. At 24 by 32 inches that is 5.33 square feet a face and 32 square feet of surface. What that surface amounts to is not something this page has an opinion about — it is reported because it is the number people ask for when comparing one build to another, and it is a multiplication, nothing more.
The spacer strip thickness is the chamber spacing
This is the part that catches people ripping stock. The strips that hold two panels apart are not sized by some separate rule — their thickness is exactly the gap you want. Want a 0.75 in chamber, rip the strips 0.75 in thick. The strip width, the 1.5 inches in the default, only decides how much glue and screw area you have and how much stock you burn; it does not touch the spacing.
Three joints at two side strips of 32 inches and one top strip of 24 is 88 inches a joint, 264 inches in total, 22 feet. Cut from 96 inch stock with a 0.125 in kerf that is three sticks with 7.75 inches left on the last one, because each stick takes 32, 32 and 24 with two kerfs and comes to 88.25 of the 96.
Screws go in from both sides
Each strip has a panel on either side of it, and each of those panels gets screwed to the strip. A 32 inch strip at 6 inch spacing takes 6 spaces and 7 screws counting both ends; a 24 inch top strip takes 5. That is 19 per joint per face, 38 per joint through both faces, and 114 for the three joints. Count it as one face and you will be short by half at the counter.
Everything this page refuses to tell you
Chamber spacing, panel size, height off the ground, which way it faces, how far from what — none of them are here as anything except a field you fill in. Those are species and site questions and they belong to your state wildlife agency, your extension service and the published plans of the conservation organisations that produce them. There is no default in this form that is a recommendation.
Two other things sit outside the arithmetic and neither gets softened. Wildlife can carry disease, and material that accumulates under a used roost is not something an untrained person should be cleaning up; the health department and the wildlife agency are who decide what that job involves. And bats are protected. Whether a house that has been used may be taken down, moved, opened or repaired, and when in the year, is a question for the state wildlife agency before it is a question about a screwdriver.
Where this hands off
Getting the panels out of a sheet with an actual layout rather than an area plus waste is the sheet nesting yield calculator. Board feet of the spacer stock on its own is the board foot calculator. For a box built out of dimensional boards instead of panels, the nest box cut list calculator derives the six pieces and packs them, and putting either one up on posts is the nest box mounting run calculator.
Questions people ask
What chamber spacing should I use?
Not a question this page answers. Chamber spacing is a species and region question, published plans differ on it, and a calculator that handed you a number would be doing something it has no business doing. Your state wildlife agency, your cooperative extension service and the conservation organisations that publish free bat house plans are where that figure comes from. Bring it here and the page will tell you how many chambers your frame depth holds at it, and what a quarter inch either way does to the count.
Why does a deeper cavity sometimes give the same number of chambers?
Because the count only moves when the cavity crosses a multiple of the gap plus the partition. At a 0.75 in gap and a 0.5 in partition that step is 1.25 inches. A 5 in cavity and a 6 in cavity both hold four chambers; the 6 in one just has an inch and a half of slack. If you are choosing a frame depth, work out the next threshold first — the calculator prints exactly what the next chamber would need.
Do I count the front and back as chamber walls?
Yes, and that is why four panels give three chambers. Each chamber is bounded by two faces, and the outermost faces belong to the back panel and the front panel. If you build it with an open front the outer chamber only has one wall, and the calculator drops the roost surface accordingly. It is a bookkeeping distinction, not a design opinion.
How thick should the spacer strips be?
Exactly the chamber spacing you entered. That is the whole job of the strip. The width, which is a separate field, decides how much stock you use and how much surface there is to fasten into, but a wider strip does not change the gap by a thousandth. If you rip strips to a nominal size and the gap comes out different, it is the strip thickness that drifted, not the arithmetic.
Can I clean or move the house once bats have used it?
Ask your state wildlife agency, and ask before you touch it. Bats are protected, an occupied roost carries protections separate from the structure, and both the answer and the time of year it applies to differ by state. Separately, material that builds up under a used roost can carry disease and is not a cleaning job for someone without training and equipment. This page gives you a material list and stops well short of both of those.