Birdhouse Size Calculator
Entrance hole, floor size, and mounting height for common backyard birds.
What it does
Hole diameter picks the tenant. A 1.5 in round hole admits bluebirds while a 1 in hole keeps the box to wrens, and stepping past 1.5625 in invites starlings. Floor size and mounting height tune the rest: tight floors suit small clutches, and low mounts suit open field hunters.
The tool above reads the species and returns the entrance hole, the interior floor, and the mounting height in inches and feet or centimeters and meters. The default Eastern bluebird needs a 1.5 in round hole, a 5.5 x 5.5 in floor, and a 4 to 6 ft mount.
Placement data follows Cornell Lab of Ornithology NestWatch tables with Mass Audubon city guidance beside them. NestWatch lists the bluebird at 1.5 in round with a 2.25 by 1.375 in oval as the alternate, and Audubon caps city boxes at 1.25 in where house sparrows press.
Boxes fail more from predators and rivals than from weather. A free standing metal pole or PVC post with a guard beats a tree trunk, a slight forward tilt sheds rain, and the lack of an outside perch denies jays a footing while the tenants cling to the hole edge.
How to use
- Pick inches or centimeters.
- Choose the species you want to host.
- Cut the entrance to the exact diameter shown.
- Build the floor to the listed interior size.
- Mount inside the height band on a guarded pole.
- Face open habitat for bluebirds and swallows, woods edge for chickadees.
Formula
- hole
- Round entrance diameter; oval 2.25 x 1.375 in is the bluebird alternate.
- floor
- Interior width by length of the box floor.
- mount
- Height band above ground on a guarded pole.
Bluebird pages sometimes print 2.25 in, which is the oval height, not a round diameter. A 2.25 in round hole admits starlings, so cut 1.5 in round or the true oval.
Worked example
Example: Eastern bluebird
Imperial defaults: Eastern bluebird.
- Hole: 1.5 in round (oval alternate 2.25 h x 1.375 w).
- Floor: 5.5 x 5.5 in interior.
- Mount: 4 to 6 ft on a pole facing open field.
- Spacing: 300 ft between bluebird boxes.
- Metric check: 3.8 cm hole, 14 x 14 cm floor, 1.2 to 1.8 m mount.
- Tool shows the hole, floor, and mount together.
Reference table
| Species | Hole | Floor and mount |
|---|---|---|
| Eastern bluebird | 1.5 in round | 5.5 x 5.5 in, 4 to 6 ft, 300 ft apart |
| House wren | 1 in round | 4 x 4 in, 5 to 10 ft, 100 ft apart |
| Chickadee | 1.125 in round | 4 x 4 in, 5 to 15 ft, 650 ft apart |
| Tufted titmouse | 1.25 in round | 4 x 4 in, 5 to 15 ft |
| Tree swallow | 1.375 in round | 5 x 5 in, 5 to 6 ft, 35 ft apart |
Limits
- Diameter bands assume standard boxes; deep or shallow boxes shift results.
- City sparrow pressure can force the 1.25 in cap over target preference.
- Mount bands assume poles; tree mounts draw more predators.
- Occupancy is never promised; habitat and food rule.
Common mistakes
- Drilling a 2.25 in round hole for bluebirds and housing starlings.
- Adding an outside perch that helps jays, not tenants.
- Nailing the box to a tree with no predator guard.
- Skipping cleanouts so old nests host parasites.
- Hanging one box where wrens want several.
Try it yourself
Quick checks. Answers sit tucked under each question.
Chickadee hole in both systems?
1.125 in round, which is 2.9 cm.
Tufted titmouse hole?
1.25 in round, which also blocks starlings.
Tree swallow mount and spacing?
5 to 6 ft high, boxes 35 ft apart.
Questions
What hole for bluebirds?
1.5 in round, or a 2.25 h by 1.375 w in oval. Never 2.25 in round.
How do I block sparrows?
Cap city boxes at 1.25 in for chickadees and wrens; sparrows fit 1.25 in and up.
How do I block starlings?
Stay under 1.5625 in; every target here does.
Perch or no perch?
No perch. Tenants cling to the hole; perches aid predators.
Pole or tree?
Free standing pole or PVC with a predator guard beats a tree trunk.
Which way should it face?
Bluebirds east toward open field; chickadees and titmice toward woods edge.