Nest Box Dimensions and Hole Sizes for Common Backyard Birds

Every measurement on a nest box exists for a reason — hole size excludes the wrong species, depth below the hole keeps predators from reaching in, and mounting height matches where each species actually feels safe nesting. Here’s the full spec sheet for the cavity-nesters most likely to use a box in a US backyard.

Full Specs by Species

Species	Hole diameter	Floor size	Depth below hole	Mounting height
Chickadees	1⅛ in	4×4 in	6–8 in	5–15 ft
House wrens	1 in	4×4 in	6–8 in	5–10 ft
Nuthatches	1¼ in	4×4 in	6–8 in	12–20 ft
Downy woodpeckers	1¼ in	4×4 in	8–10 in	6–20 ft
Bluebirds	1½ in	5×5 in	6 in	4–6 ft
Tree swallows	1⅜ in	5×5 in	6 in	5–10 ft
Hairy/red-headed woodpeckers	2 in	6×6 in	8–12 in	12–20 ft
Purple martins	2⅛ in	Compartment or gourd	N/A	12–20 ft, near open water
Screech owls/kestrels	3 in	8×8 in	10–12 in	10–30 ft
Wood ducks	4×3 in (oval)	10×18 in	12–16 in	10–20 ft, near water

Recommended gear

Example: an assortment of nest boxes pre-sized for common backyard species

Depth Below the Hole: Why It’s Not Optional

The distance from the entrance hole down to the nest floor isn’t just about box proportions — it’s a predator-safety measure. A shallow depth lets a raccoon, cat, or snake reach an arm or head far enough in to grab eggs or chicks even without fully entering the box:

• Small cavity-nesters (chickadees, wrens, nuthatches) need at least 6–8 inches below the hole
• Larger woodpeckers and owls need 8–12 inches or more, matching their larger predators’ reach
• Never shorten this dimension to save material — it’s one of the few specs where “close enough” genuinely isn’t

Round vs. Oval Holes

Most backyard species use a standard round hole, but a few need something different:

• Wood ducks require a horizontal oval entrance (roughly 4 by 3 inches) — their body shape doesn’t fit through a round hole sized to exclude predators
• Purple martins nesting in gourds often use a slightly different crescent-shaped or larger round opening depending on the gourd style
• Never round up a hole size “to be safe” — a slightly larger hole is exactly what lets in house sparrows and starlings, the two most common nest-box competitors

Using This Table

1. Identify which species are already common in your yard or neighborhood — that’s a better guide than picking whichever species you’d most like to see.
2. Match the hole diameter first; it’s the single spec most responsible for keeping out unwanted competitors.
3. Check the depth-below-hole measurement on any box you’re buying or building — it’s frequently shortchanged on cheap pre-made boxes.
4. Set mounting height based on the table, and see our placement and predator safety guide for direction and predator-guard details once height is sorted.

For the full setup process beyond just dimensions, see our pillar guide to birdhouses, and our DIY birdhouse plans if you’re building rather than buying.