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Door Opening Calculator

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

Quick answer

With the default inputs: Leaf width 0.8 m · Leaf height 2.0 m · Frame width 0.05 m etc.Structural opening width 940 mm

Use your own dimensions ↓

Inputs

Result

Structural opening width
940mm
Opening height2090mm
Lintel clear span0.940m
Frame length (est.)5.70m
Reveal plaster area1.45m²
Head height2.090m
How this is worked out

An opening is not the leaf size. Opening width = leaf width + frame width both sides + fixing gap both sides; opening height = leaf height + head frame + gaps top and bottom (normally no frame at the threshold, just a gap). Frame widths and recommended gaps vary between systems, so confirm the frame dimensions and fixing gap with the supplier before building the opening - too small and the frame will not go in, too large and the gap cannot be sealed. A 20 mm gap each side is typical, enough for expanding foam and for plumbing the frame plumb. Check the head height together with the finished ceiling and the lintel depth so the opening is right but the headroom is not lost. Reveal plaster area is estimated as opening perimeter x wall thickness, excluding the base, for door casing takeoff.

Next up: Door and Window Casing Calculator · Door and Window Schedule Calculator

Formula

Opening widthB = leaf width + 2×frame + 2×gap
Opening heightH = leaf height + frame + 2×gap
Lintel spanL = opening width
Reveal plaster areaA = 2×(B+H) × wall thickness
Frame lengthl = 2×(leaf w + leaf h) + 2×frame
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.

Basis

FAQ

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Door Opening Calculator. 2026-10-25. https://jianzhujisuan.com/en/door-frame-opening

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.