TOOL 05 — BIRD BOX

Bird Box Planner

Turn one board into a nest-box cutting list.

Internal width

Inside the box, side to side. Whole millimetres, 80 to 300.

Internal depth

Inside the box, front to back. Whole millimetres, 80 to 300.

Front panel height

Panel height as cut. Whole millimetres, 120 to 400.

Back panel height

Must be greater than the front height, so the roof slopes and sheds water; up to 500 mm.

Board thickness

Whole millimetres, 6 to 30. The RSPB recommends 15–18 mm timber, to insulate the box and stop warping.⁴

Entrance hole diameter

Your own entry — the RSPB publishes hole-size guidance per species; this planner does not choose a species for you.⁴ Enter 0 for no hole; otherwise 20 to 80 mm, and the front needs 10 mm of timber each side of it.

Hole centre height (optional)

Measured up from the bottom edge of the front panel; leave blank to place the hole yourself later. The hole must clear the base board by 20 mm and stop 15 mm short of the panel's top edge.

Saw kerf

The width your saw cut removes, added between panels. Whole millimetres, 1 to 5.

Board width

The width of the board you plan to buy or already have. Whole millimetres, 100 to 600.

Add a mounting batten extension

Extends the back panel by 120 mm so the box can be fixed up through the batten rather than through the box itself.

WORKED EXAMPLE

Cutting list
Panel	Qty	Cut size	Note
Back	1	186 × 250 mm
Side	2	120 × 250 mm	slopes from 250 mm at the back to 200 mm at the front edge
Front	1	186 × 200 mm
Base	1	150 × 120 mm
Roof	1	216 × 185 mm	cut length allows for the 17.8° slope and overhangs

Board strip layout — 225 mm board

1390 mmboard length needed 1256 mm

1390 mm is the suggested purchase length (+10 % for defects and end checks — workshop rule of thumb). Roof slope 17.8°.

Butt-jointed box: front and back overlap the side faces, the base fits inside all four walls, and the front top edge is cut square — the gap that leaves under the roof is deliberate ventilation.
Roof allows 15 mm of overhang each side and 20 mm at the front, cut long enough for the 17.8° slope.
Drainage holes in the base are common practice — a workshop rule of thumb rather than a sourced requirement.
Entrance hole Ø32 mm is your own entry. Add a hole centre height if you want it marked on the layout.

This is the tool's worked example, not your figures: a 150 × 120 mm internal box, 200 mm front, 250 mm back, 18 mm boards, Ø32 mm hole, 3 mm kerf, 225 mm board. Load it into the form with the example button, or enter your own and press the calculate button to replace it.

This planner needs JavaScript to calculate in your browser — nothing is ever sent anywhere. Without it, the method works on paper: size the six panels from the formulas in “How this works”, lay them along the board in the order Back, Side, Side, Front, Base, Roof, and add one kerf between each. The panel above shows the full worked example: the default 150 × 120 mm box in 18 mm boards needs a board length of 1256 mm, with a suggested purchase of1390 mm and a roof slope of 17.8°.

How this works

Inputs

The internal width and depth, the front and back panel heights (back taller, so the roof sheds water), the board thickness, an optional entrance hole (diameter, and a centre height if you want it marked on the layout), the saw kerf, the board width you are cutting from, and the mounting batten toggle. All dimensions are whole millimetres.

Method

The construction convention is a butt-jointed box: front and back overlap the side faces, the base fits inside all four walls, and the front top edge is cut square — the small gap that leaves under the roof at the front is deliberate ventilation. Heights are panel heights as cut. The roof slope follows from the height difference:

θ = atan((Hb − Hf) ÷ (D + 2t))Hb back height, Hf front height, D internal depth, t board thickness

Panel sizes — computed from your inputs
Panel	Qty	Cut size
Back	1	(W + 2t) × Hb, plus 120 mm with the batten on
Side	2	D × Hb, sloping to Hf at the front edge
Front	1	(W + 2t) × Hf
Base	1	W × D
Roof	1	(W + 2t + 2 × 15) × ⌈(D + 2t + 20) ÷ cos θ⌉

The roof allows 15 mm of overhang each side and 20 mm at the front — fixed, documented conventions of this planner. Each panel is then placed on the board: its longer side across the board where that fits, otherwise its shorter side; if neither fits, there is no cutting list and the planner names the board width the panels need. Layout order is Back, Side, Side, Front, Base, Roof.

board length = Σ along + kerf × 5 · suggested = ⌈board length × 1.1 ÷ 10⌉ × 10the +10 % for defects and end checks is a workshop rule of thumb

Rounding

The planner works in floats internally and rounds every panel dimension up to the next whole millimetre at the end, so no panel is ever cut short. The roof slope is displayed to 0.1°. The suggested purchase length rounds up to the next 10 mm after adding 10 %.

Limitations

The cutting list assumes the simple butt-joint convention above, with a square-cut front edge — the front roof gap is deliberate ventilation, not an error.
Drainage holes in the base are common practice — a workshop rule of thumb here, not a sourced requirement.
Siting and species guidance is the RSPB's⁴, not this tool's; a cutting list is not a wildlife-outcome guarantee.
This is a planning aid, not an engineering or safety certification.

Worked example

BB-1: all defaults — 150 × 120 mm internally, 200 mm front, 250 mm back, 18 mm boards, Ø32 mm hole, 3 mm kerf, 225 mm board, batten off. The slope is atan(50 ÷ 156) = 17.8°. Panels: Back 186 × 250, Side ×2 120 × 250 sloping to 200, Front 186 × 200, Base 150 × 120, Roof 216 × 185. Each panel lies with its longer side across the 225 mm board where that fits, so the along-the-board lengths run 250 + 250 + 250 + 186 + 120 + 185 = 1241 mm; with5 kerfs of 3 mm the board length is 1256 mm, and the suggested purchase is 1390 mm. This is the reading shown in the result panel until you build your own.

Sources

The 15–18 mm timber recommendation and the materials panel below are the RSPB's⁴. The overhangs, kerf count and purchase margin are this planner's documented conventions and workshop rules of thumb, and carry no source. The full reference is in the ledger below and on thesources page.

Materials — RSPB-sourced

Materials for a nest box

Use 15–18 mm timber, to insulate the box from cold and heat and to stop it warping.⁴
Exterior-quality plywood makes a lightweight box; hardwood such as oak and beech is sturdier; softwood such as pine works but deteriorates more quickly.⁴
Avoid pressure-treated timber.⁴
Look for the FSC logo for Forest Stewardship Council-approved timber.⁴

References

Sources

Ref	Reference	Used by
4	Build a nestbox — The Royal Society for the Protection of Birds (RSPB). https://www.rspb.org.uk/helping-nature/what-you-can-do/activities/build-a-birdbox Accessed 2026-08-05.	Bird Box Planner (materials panel; 15–18 mm timber guidance); timber selection guide