Door and Window Casing Calculator
Casing metres, pieces and board area from opening size and quantity.
Quick answer
With the default inputs: Opening width 1.5 · Opening height 1.5 · Quantity 4 pcs etc.Total casing length 24.00 m
Edit the inputs below to recalculate.Inputs
Result
Total casing length
24.00m
| Perimeter per opening | 6.00m |
| With waste | 25.92m |
| Pieces to buy | 11bars |
| Length bought | 26.40m |
| Casing area | 1.44m² |
| Backing board (3 sides) | 18.00m² |
| External corners per opening | 2pcs |
| Mitre joints | 8joints |
Assumes casing on three sides (two verticals plus head); sills are extra. Pieces are usually 2.4 or 3 m, so openings taller than one piece need a joint placed away from eye level.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Perimeter | P = 2 × (width + height) |
| Total length | L = P × quantity |
| With waste | L = L × (1 + waste) |
| Pieces | n = ceil(L ÷ piece length) |
| Casing area | A = L × casing width |
Four 1.5 × 1.5 m windows with 60 mm casing, 2.4 m pieces and 8% waste: 6.00 m per opening, 24.00 m total, 25.92 m with waste, so 11 pieces and 1.44 m² of casing.
How to work it out
- An architrave set is a three-sided U: two legs and a head, no bottom. So metres per opening = opening height × 2 + opening width.
- Backing board area = (opening width + 2 × architrave width) × (opening height + architrave width), covering the exposed face only.
- Total metres = per opening × quantity, times (1 + waste), divided by stock length and rounded up.
- Allow 8-12% waste: every corner is mitred at 45°, at least two per opening, and each cut loses usable length.
- Count the ironmongery separately: hinges, locks, door stops and seals per leaf; fixings or adhesive by metre.
Where it goes wrong
- Using the full opening perimeter (height × 2 + width × 2) — that adds a bottom leg; four-sided sets exist for windows, not doors.
- Using the profile width to compute board area — the part that laps onto the wall and the part that shows are different; only the showing part is the architrave width.
- Allowing only 5% waste — every 45° corner consumes a length, and wider architraves consume more; 8% is the floor.
- Fitting architraves before the wall finish — they must sit on the finished surface; fitting first and sealing the gap leaves a dark line that never closes.
Material and cost
Veneered MDF 15-30 CNY/m, solid timber 40-90, stainless or aluminium 30-80. Installation 50-120 CNY per opening. A complete residential door set with backing, architrave and ironmongery runs 400-1200 CNY per leaf.
On site
- Install only after plastering, tiling or painting is complete — otherwise the architrave gets soiled and still leaves a gap.
- Use moisture-resistant backing or stainless in bathrooms; MDF swells and skins within two years in a wet room.
- Seal the wall junction with neutral silicone or a trim sealant, never with skim — skim shrinks and cracks.
Basis
- GB 50210-2018 Code for acceptance of building decoration, Chapters 6 and 12
- GB 50327-2001 Code for construction of residential decoration, Chapter 9
FAQ
Are door and window casings measured the same?
No. Doors are cased on three sides with nothing at the floor; windows need a sill board in addition, usually projecting 50-100 mm each side with returns. A window casing therefore uses more length and board than a door of the same size — do not price it at the door rate.
How is casing width chosen?
It must relate to skirting height and door proportions. The usual rule is casing equal to or slightly under the skirting: 60 mm casing with 80 mm skirting is commonest, 100 mm casing with 100-120 mm skirting. Too narrow looks flimsy; too wide crowds a small room and eats wall area.
Why do mitres crack?
Timber and MDF move with humidity; a mitre has a large contact area but the two pieces expand in different directions, so the joint opens. Better practice is a scribed or dowelled joint, proper wood glue rather than white glue, and sealing the end grain with finish promptly. Commercial work often uses square corner blocks instead.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Door and Window Casing Calculator. 2026-10-25. https://jianzhujisuan.com/en/door-window-frame
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.