What's inside
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Measuring a timber opening from inside corner to inside corner sounds simple, but small errors can leave a door, window, panel, or infill piece either jammed tight or surrounded by an uneven gap. The measurement is taken between the finished internal faces of the opening—not from the outside edges of the studs, frame, trim, or architrave.
What “inside corner to inside corner” means
For a rectangular timber opening, you normally need three dimensions: width, height, and sometimes depth. Width is measured horizontally from the left inside face to the right inside face. Height is measured vertically from the inside face of the top member down to the finished floor or sill.
Use the surfaces that the new timber will actually meet. If you are fitting a shelf between two walls, measure between the walls. If you are replacing a door lining, measure between the inside faces of the existing jambs. Do not measure across the visible trim, because trim often extends beyond the structural opening.
“Inside corner” refers to the point where two surfaces meet. At the top-left and top-right corners, this is the underside of the head and the inside face of each side. If the corners are rounded, chamfered, or filled with old plaster, measure from the nearest reliable straight surface and allow for the obstruction.
Tools that make the measurement reliable
A standard tape measure is enough for most openings, but a stiff, readable blade is easier to control over longer distances. A wide-blade tape measure is useful when working alone because it holds its shape farther before buckling.
For short openings, a folding rule or steel rule can be more precise than a tape. A pencil, sharp knife, or fine marker is useful for recording reference points. Use a 24-inch carpenter’s level to check whether the sides are plumb and the head is level. A straightedge helps identify bows in timber or plaster.
| Tool | Best use | Main limitation |
|---|---|---|
| Tape measure | Most openings and long dimensions | Blade can sag or hook inaccurately |
| Folding rule | Short, controlled measurements | Less convenient across wide openings |
| Steel rule | Small gaps and fine bench work | Usually too short for an opening |
| Level and straightedge | Finding slopes, bows, and twisted surfaces | Do not replace the actual width measurement |
A cheap tape is fine for a one-off measurement if its hook is secure and the graduations are clear. Spend more on a tougher tape if you work on site regularly; a damaged hook or loose blade can introduce several millimetres of error.
How to measure the width
Clear away dust, loose mortar, blobs of paint, and anything that prevents the tape from reaching the true inside faces. Hold the tape horizontally at the top of the opening. Hook or press the blade into the left inside corner, then read the measurement at the right inside corner.
Take the width at three heights: near the top, through the middle, and near the bottom. Keep the blade level rather than following a sloping floor or head. Record all three measurements, including the exact locations.
If the opening is wider at the top than at the bottom, the sides are not parallel. Do not automatically use the smallest number. The correct choice depends on the item being made. A rigid panel usually needs to be sized from the smallest clear width, with a deliberate fitting gap. A piece that will be scribed to the walls can start larger and be trimmed to suit.
How to measure the height
Measure from the finished lower surface to the underside of the head at the left, centre, and right. If the opening is for a door, measure from the finished floor, not from a subfloor that will later receive tiles, wood flooring, or carpet.
For a shelf or fixed timber panel, also check the depth at several points. Walls may lean or plaster may vary in thickness. Measure the depth at the top, middle, and bottom, then note any pipes, cables, skirting, or projecting screws that will affect the fit.
Write measurements as width first, then height, then depth. For example: “812, 809, 815 mm wide; 2,040, 2,037, 2,041 mm high; 118 mm deep.” This prevents a single optimistic measurement being mistaken for the size of the whole opening.
Check whether the opening is square
Measure both diagonals, from the top-left inside corner to the bottom-right, and from the top-right to the bottom-left. If the diagonals differ, the opening is out of square. A difference of 2 or 3 mm may be manageable for a trim-covered opening, but it matters for a close-fitting door or accurately fitted panel.
Check each side with a level. A side can be plumb while the opening is still too narrow in the middle because the timber is bowed. Run a straightedge down each face and look for daylight or rocking points.
Allow for clearance and fitting
Never make a replacement piece exactly the same size as the largest measurement unless you are certain the opening is perfectly square and stable. Timber moves with moisture, and existing buildings rarely remain perfectly consistent.
| Application | Typical starting clearance | Why it is needed |
|---|---|---|
| Loose shelf between walls | 3–6 mm total | Allows fitting and minor wall variation |
| Trimmed timber panel | 3–5 mm total | Leaves room for scribing and movement |
| Door-related work | Follow the door and frame specification | Clearances affect operation and safety |
These are starting points, not universal rules. A large solid-wood panel may need more movement allowance across its grain. A painted piece may need enough clearance to avoid rubbing after another coat of paint.
Record and transfer the measurement
Write the number immediately and label it “top,” “middle,” or “bottom.” When making the timber, mark the face side and the reference end. A carpenter’s pencil set is inexpensive, but any sharp pencil that makes a fine, visible line will work.
For a tight fit, make a template from cardboard or thin hardboard first. Test the template in the opening, trim it where it binds, and transfer its final shape to the timber. This is often quicker and safer than repeatedly cutting an expensive board.
Common mistakes to avoid
Do not measure from the outer edge of the trim. Do not rely on one width measurement. Do not read the tape at an angle, and do not assume the floor is level. Also check the tape’s hook: if it is bent or loose, the zero point may shift enough to spoil a close fit.
Finally, measure after demolition if old lining, plaster, or trim is being removed. The opening behind the finish may be substantially different from the opening you can see. When accuracy matters, measure twice, confirm the smallest relevant dimension, and leave a planned clearance rather than hoping the timber can be forced into place.