TradeCalculator.co.uk
Window Casing Calculator
Calculate casing sets per opening (head + 2 jambs), architrave metres both sides, fixings and packers for window and door casings.
Clear opening width (between reveals)
Clear opening height (sill to head)
Adds 10% extra for cutting waste and mitre offcuts
10% is standard for casing and architrave; 15% for many mitres
25mm or 32mm typical; sets the head over-length
69mm standard; 57mm minimal; 96mm period
2-3 packers per point
3 per jamb standard
Next up: related pages
How We Calculate This
This calculator determines casing timber, architrave and fixings for window and door openings. Each opening needs a casing set (head + 2 jambs) and architrave on both sides.
The formula
Casing head = Opening width + 2 × casing thickness
Casing jambs = 2 × opening height
Architrave leg = Opening height + architrave width + 10mm
Architrave head = Opening width + 2 × reveal + 2 × architrave width
Architrave = 2 sides × (Head + 2 × Legs)
Packers = Fixing points × packers per point
The architrave width is added because each 45° mitre runs the outer edge longer than the inner edge by one architrave width. A wastage allowance (10% by default) is then applied to every length to cover mitre offcuts and cutting errors. Fixing and packer counts are a trade rule of thumb rather than a published standard.
Installation sequence
- Measure reveal width, height and depth accurately
- Cut casing head and jambs, allowing for joints
- Pack and fix jambs plumb, then head level
- Fit architrave after plastering is complete
Frequently Asked Questions
A window casing (also called a window lining or sub-frame) is the timber frame fitted inside a window reveal to form a neat opening for the window. It consists of a head (top piece) and two jambs (side pieces). The casing is plumbed and squared in the opening, then the architrave trim is fitted over the joint between casing and wall.
Each window or door needs architrave on both sides of the wall. Per side: 1 head piece + 2 legs, so per opening that is 2 heads + 4 legs. Because the corners are 45° mitres, each leg is cut longer than the opening — leg length = Opening Height + Architrave Width + 10mm cutting margin, and head long-point = Opening Width + reveal each side + one Architrave Width at each end. With standard 69mm architrave on a 1200mm × 1200mm opening that is about 1.35m per head and 1.28m per leg, roughly 3.9m per side before wastage. Add 10% for mitre offcuts (15% if you have many corners).
Window casing is typically 25mm or 32mm thick, with the width matching the wall thickness (reveal depth). For a standard masonry wall: 100-125mm wide. For stud walls: 89mm or 100mm wide. Use PAR (planed all round) softwood for paint finishes or hardwood for stained finishes.
Pack the casing at each fixing point — typically 3 points per jamb plus points along the head (the calculator scales head points with width, roughly one every 600mm, minimum 2). Use 2-3 packers per point to achieve plumb and level, and about 1-2 screws per packed point. Plastic packers are preferred as they don't rot or compress like timber offcuts. Always check plumb and level before final fixing. Fixing quantities here are a trade rule of thumb, not a published standard — buy screws and packers by the box.
Casings are fitted before plastering (first fix). The plaster is then finished up to the edge of the casing, creating a neat junction. Architrave is fitted after plastering (second fix), covering the joint between the plaster and casing. This sequence ensures a clean, professional finish with no gaps or rough edges.
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.