Birdhouse Entrance Hole Size Guide
Look up the correct entrance hole diameter, floor size, and box height for the species you want to house.
Getting the hole size exactly right matters more than how the box looks. A hole even 1/8 inch too big can let starlings or house sparrows in to evict the species you're housing. Skip the perch below the hole too; it only gives predators a foothold.
| Species | Hole (in) | Hole (mm) | Floor (in) | Height (in) |
|---|---|---|---|---|
| Black-capped Chickadee | 1.125 | 28.6 | 4x4 | 8 |
| House Wren | 1.125 | 28.6 | 4x4 | 8 |
| White-breasted Nuthatch | 1.25 | 31.8 | 4x4 | 9 |
| Downy Woodpecker | 1.25 | 31.8 | 4x4 | 9 |
| Eastern Bluebird | 1.5 | 38.1 | 5x5 | 8 |
| Tree Swallow | 1.5 | 38.1 | 5x5 | 8 |
| Purple Martin | 2.125 | 54 | 6x6 | 6 |
| Screech Owl | 3 | 76.2 | 8x8 | 13 |
How it works
Pick a species from the dropdown and the tool looks up its standard entrance hole diameter, floor dimensions, and the height from the floor to the hole, based on the specifications NestWatch and other nest box monitoring programs publish for each cavity nester. The hole size also converts to millimeters, since some drill bits and hole saws are sized metric.
Worked example: pick Eastern Bluebird and you get a 1.5 inch (38.1 mm) entrance hole on a 5 by 5 inch floor, with the hole 8 inches up from the floor. That 1.5 inch opening is small enough to keep out starlings but large enough for a bluebird to enter easily, which is the whole point of matching the hole to the species.
FAQ
Why does hole size matter more than anything else about the box?
A hole that's even an eighth of an inch too large lets house sparrows and European starlings, both non-native and aggressive competitors, into a box built for a smaller native species. Those birds will evict eggs, nestlings, and even adult birds to take over the cavity, so the hole is the one dimension you can't approximate.
Should I add a perch below the entrance hole?
No. Cavity-nesting birds don't need a perch to enter their own nest hole, and a perch mostly gives predators and competing birds an easy foothold to reach in. Leave it off.
What if I want to attract more than one of these species?
Put up separate boxes with the correct hole size for each target species rather than trying to split the difference with one in-between hole size. A box sized for chickadees won't suit a bluebird, and a box sized for bluebirds will let in starlings that outcompete chickadees.
Does the floor size and height actually matter, or just the hole?
Floor size affects how much room nestlings have to grow, and hole height affects how easy it is for a predator to reach in from outside. Both matter, but the hole diameter is the dimension that determines which species can physically get in at all.
For more on building and placing boxes correctly, see nest box hole sizes for different birds, how to build a birdhouse: plans and tips, and where to place a nest box for the best results.