Door Header Size Calculator
A first-pass size for the timber header or lintel over a door or window — the beam depth that bending strength and deflection each demand for the span and the load on it — with the next standard depth and how much of its capacity the load uses. A sizing estimate, not a code check.
A header is a beam: the load above the opening, per metre, over the span between jack studs.
How the door header size calculator works
A header is a beam: the load above the opening, per metre, over the span between jack studs. Bending gives one depth — the moment wL²/8 over the width and an allowable stress — and deflection gives another, from the limit of span over 360 and the timber's stiffness. The header needs the larger, rounded up to a stock size. The load per metre depends on what sits above: little for a non-load-bearing wall, a roof for a single storey, roof and floor for two. Span tables in the building code give the final answer; this shows the arithmetic behind them.
Formula: M = wL²/8; d_bending = √(6M / (b σ)); d_deflection = ∛(5 w L⁴ × 12 / (384 E b × L/360))
Worked examples
| Inputs | Standard depth that works | Note |
|---|---|---|
| A 1.2 m door under a single-storey roof | 90 mm | a 90 or 120 mm header |
| A 2.4 m opening | — | deflection governs |
| A heavier load | 140 mm | a deeper header |
FAQFrequently asked questions
What size header does a door need?
For a standard door under a single-storey roof, two 90 × 45 mm members (a doubled 2 × 4) usually suffice up to about 1.2 m; wider openings and two storeys need 190 to 290 mm depths. Span tables in the building code are the authority — this page shows why they say what they say.
What load should I enter?
The weight above per metre of header: roughly 2 kN/m for a non-load-bearing wall, 5 to 8 for a single-storey roof, 10 to 15 for a floor and roof. A designer takes it from tributary areas.
Why is deflection often the limit?
Because a header that is strong enough can still sag enough to jam a door. The span-over-360 limit keeps movement to a few millimetres over a normal opening.
What stress and modulus should I use?
They depend on the timber grade — roughly 8 to 14 MPa allowable bending and 8,000 to 12,000 MPa stiffness for structural softwood. Engineered lumber is much higher and lets a shallower header work.
Is this a structural design?
No. It is the arithmetic behind a span table, for understanding and checking. The header on a real building must come from the code table or an engineer.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.