Bat House Calculator
Bat boxes needed from colony size, with mounting rules.
What it does
Bat houses fail on size and sun before bats ever arrive. A 100 bat colony fits one 24 inch wide, 12 inch tall multi-chamber box, while 300 bats need two tall boxes side by side. Small single-chamber boxes sit empty for the same reason small ponds sit fishless: nursery colonies want volume.
The tool above sizes boxes to colonies. Enter the bat count and it prints box count with size, capacity, and the mounting band. Rules ride along: 12 to 20 ft high, 7 or more sun hours facing southeast or southwest, within a quarter mile of water.
Two authorities agree. Bat Conservation International favors tall four-chamber houses with roost gaps capped at 1 inch, pole or building mounts, and open flight space below. Penn State Extension pins capacities: 12 inch tall boxes to 100 bats, 24 inch to 200, with crevices at 3/4 to 1 inch.
Trees and shade doom most boxes. Trunks invite predators and shade starves warmth, while metal siding cooks occupants. Mount pairs back to back on poles for north-south faces, caulk seams tight, and give new boxes two summers before judging.
How to use
- Enter the colony size to house.
- Read the box count and size first.
- Read the capacity line for margin.
- Mount at the printed height band.
- Face SE/SW for 7+ hours of sun.
- Keep within a quarter mile of water.
Formula
- colony
- Bats to house, 10 to 1,000.
- boxes
- 24 in wide multi-chamber boxes by height.
- height
- Mount band with open flight space below.
Capacities follow Penn State (12 in tall to 100, 24 in tall to 200); four-chamber preference and 1 in roost gaps follow BCI.
Worked example
Example: 100 bat colony
Defaults: 100 bats.
- Input: 100 bat colony.
- Rule: 24 in wide, 12 in tall box holds up to 100.
- Boxes: 1 multi-chamber box.
- Mount: 12 to 15 ft with 7+ hours SE sun.
- Tool shows 1 x 12 in tall box, holds 100 bats, 12 to 15 ft, SE sun 7+ h.
Reference table
| Colony | Boxes | Mount |
|---|---|---|
| To 100 | 1 x 12 in tall | 12-15 ft, SE sun |
| 101-200 | 1 x 24 in tall | 12-20 ft, SE/SW |
| 201-300 | 1 large or 2 tall | 12-20 ft pairs |
| 300+ | 2+ tall side by side | 12-20 ft, N/S faces |
Limits
- Planning rules only. Occupancy takes up to two summers.
- Rough capacity bands, not per-bat guarantees.
- Climate shifts ideal sun and color choices.
- Local permits and species rules still apply.
Common mistakes
- Hanging small single-chamber boxes.
- Mounting on trees or metal siding.
- Shading boxes under 7 sun hours.
- Hanging under 10 ft in predator reach.
- Judging empty boxes after one season.
Try it yourself
Quick checks. Answers sit tucked under each question.
200 bat colony?
101 to 200 band: 1 x 24 in tall box, holds 200 bats.
300 bat colony?
Over 200: ceil(300/200) = 2 tall boxes, holds 400 bats.
Questions
What houses 100 bats?
One 24 in wide, 12 in tall multi-chamber box, mounted 12 to 15 ft with 7+ hours of sun.
What houses 200 or 300 bats?
Step to the 24 in tall box (holds 200), or pair boxes above 200: 300 bats take two.
How high should boxes hang?
At least 10 ft, better 12 to 20 ft, with 12 ft of open space under exits.
How much sun?
Seven or more hours of direct sun a day, facing southeast or southwest.
Pole, tree, or building?
Poles or buildings only, never trees or metal siding. Back-to-back pairs on poles work best.
How near water?
Within a quarter mile of water, where nursery colonies hunt nightly.
Sources
- Bat Conservation International: Bat House Guidelines
- Penn State Extension: Homeowner's Guide to Northeastern Bats (box sizes, capacities)