What is a window casing?

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.

How much architrave do I need per opening?

Each window or door needs architrave on both sides of the wall. Per side: 1 head piece + 2 legs. So per opening: 2 heads + 4 legs. Allow extra for mitre cuts — typically add 100mm per head piece and 50mm per leg for the mitre. Total per opening ≈ 2 × (Width + 2 × Height + 0.3m).

What size casing timber should I use?

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.

How many packers do I need for a window casing?

Pack the casing at each fixing point — typically 3 points per jamb and 2 points on the head = 8 packing points total. Use 2-3 packers per point to achieve plumb and level. Plastic packers are preferred as they don't rot or compress like timber offcuts. Always check plumb and level before final fixing.

Should I fit casings before or after plastering?

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.

TradeCalculator

Window Casing Calculator

Calculate casing sets per opening (head + 2 jambs), architrave metres both sides, fixings and packers for window and door casings.

Number of Openings

Opening Width

Clear opening width

Opening Height

Clear opening height

Price Per Metre Casing (optional)

Price Per Metre Architrave (optional)

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 (width + margins) + 2 × Jamb height
Architrave = 2 sides × (Head + 2 × Legs + mitre allowance)
Packers = Fixing points × Packers per point
Fixings = Fixing points × 2 screws per point

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

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.