# Door Opening Calculator

Leaf size plus frame plus fixing gap gives the opening and the lintel span.

- Page: https://jianzhujisuan.com/en/door-frame-opening
- Category: Finishes (https://jianzhujisuan.com/en/c/finishes)
- Chinese version: https://jianzhujisuan.com/zh/door-frame-opening.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Leaf width 0.8 m, Leaf height 2.0 m, Frame width 0.05 m, Fixing gap each side 0.02 m, Wall thickness 0.24 m): **Structural opening width 940 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Leaf width | number | 0.8 | m |  |
| Leaf height | number | 2.0 | m |  |
| Frame width | number | 0.05 | m |  |
| Fixing gap each side | number | 0.02 | m |  |
| Wall thickness | number | 0.24 | m |  |

## Formula

- **Opening width**: B = leaf width + 2×frame + 2×gap
- **Opening height**: H = leaf height + frame + 2×gap
- **Lintel span**: L = opening width
- **Reveal plaster area**: A = 2×(B+H) × wall thickness
- **Frame length**: l = 2×(leaf w + leaf h) + 2×frame

## Worked example

800×2000 mm leaf, 50 mm frame, 20 mm gap each side, 240 mm wall: opening 940×2090 mm, lintel span 0.94 m, ~5.70 m of frame, ~1.45 m² reveal plaster.

## Questions

### What if the opening is built too big?

Small oversize within about 20 mm is taken up with expanding foam and plaster. Anything larger needs brick or concrete infill, and the joint must be roughened and meshed or it will crack and de-bond. A bigger opening also means a longer lintel span - if the lintel was detailed for the original span, check it can still carry before infilling. Building too small is worse: the frame will not fit and the opening has to be cut back, which can undercut the lintel bearing or damage the wall. Build to the calculated size rather than adding a safety margin by eye.

### How big should the fixing gap be?

20 mm each side is the norm. That gap has to do three jobs at once: let expanding foam get in (it needs a void to expand into), allow the frame to be plumbed and levelled, and absorb movement between the wall and the frame. Too small and the foam cannot be injected, so the gap gets filled with mortar and cracks later; too large and the foam shrinks back leaving a void. For lightweight walls such as stud partitions and AAC block, the supplier may recommend 25-30 mm.

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