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How do I measure a window — and what is sill height?

Measure the opening, not the glass: width, height, distance from the nearest corner, and the sill height from finished floor to the top of the window frame — the fourth measurement is the one people forget.

The opening, the frame and the glass are three different measurements

A window has three measurements that are all correct, and each is used for its own purpose. Mix them up and everything goes wrong:

• The rough opening — the gap in the wall. This is the measurement a floor plan, an area statement and a contractor's takeoff need.
• The outer frame size — the window's physical size. This is what a glazier or window supplier asks for when a new one is being ordered. The outer frame size is typically 1-2 cm smaller than the rough opening, because there has to be room for sealant all the way round.
• The glass measurement (the daylight opening) — what you can actually see through. Frame and sash typically take up 8-12 cm on each side, so a 120 × 120 cm opening might have only 100 × 100 cm of glass. The glass measurement is used for daylight calculations and for ordering blinds and curtains that fit inside the recess.

Always measure the opening first — that is what the drawing needs. If you happen to be standing there considering new windows anyway, take the outer frame size at the same time.

Sill height: the fourth measurement

Sill height is the distance from finished floor up to the top of the window frame — in other words, how high up the window starts. It is the measurement most often missing from a takeoff, and it settles more practical questions than any other:

• Kitchen: a standard worktop sits at around 90 cm. If the sill height is 85 cm, the worktop clashes with the window, and either the window has to change or the kitchen layout has to work around it.
• Radiator: a radiator under a window typically needs 20-30 cm of clear height plus clearance to the floor. A low sill means the radiator has to go elsewhere.
• Furniture: a sofa back typically sits at 75-90 cm, a bed at 50-60 cm. A sill at 110 cm leaves clear wall for furniture; one at 40 cm makes the wall unusable for tall furniture, but lets light reach right down to the floor.
• Light in the room: a window's top edge decides how far daylight reaches into the room. High-set windows light a room more deeply than low-set ones of the same size.
• The view, both sitting and standing. A sill above 120 cm gives no view from a chair.

Two windows with exactly the same opening size, one with a sill at 40 cm and the other at 110 cm, produce very different rooms. The opening size alone is therefore an incomplete answer.

How to measure a window step by step

1. Measure the width of the opening between the two sides of the rough opening, at roughly mid-height. If there is an internal reveal lining or a splay, measure to the surface that marks the edge of the rough opening — not to the edge of the frame.
2. Measure the height of the opening from the bottom of the rough opening (the level of the sill, or the top of the window frame if there is one) to the top.
3. Measure the sill height from finished floor up to the top of the window frame. If there is no frame — a French door, for instance — measure to the bottom of the opening.
4. Measure the distance from the nearest room corner to the nearest edge of the opening, along the wall at floor level. Note which corner you measured from.
5. Measure the reveal depth from the wall surface in the room to the front edge of the frame. It tells you where the window sits within the wall thickness and is used to draw the reveal.
6. Note how the window opens — side-hung, top-hung, fixed, sliding — and which way. A fixed pane cannot be used as an escape opening or for ventilation.

Always measure at the same point on both sides if the opening looks out of true. The smallest number is the one that governs when ordering.

The maths: glass area and daylight

To find out how much glass a room has, calculate the glass area, not the opening. Example: an opening of 130 × 140 cm with a frame and sash that take up 10 cm all the way round gives a glass measurement of (130 − 20) × (140 − 20) = 110 × 120 cm = 1.32 m². If the room has two such windows, the glass area is 2.64 m².

Set that against the floor area: if the room is 18 m², the glass ratio is 2.64 ÷ 18 ≈ 15%. That figure is used as a common rule of thumb for whether a living space has reasonable daylight — but it is only a rule of thumb. The real daylight also depends on which way the window faces, the sill height, how deep the room is, and what stands outside. Shade from a tree, a neighbouring house, or a balcony above the window can eat up a large share of it.

Window measurements in a drawing and a window schedule

On a floor plan, a window is drawn as a break in the wall with the opening's width and its distance from the corner. Height and sill height can't be read off the plan — they belong in a window schedule, a list with one window per line: number, room, opening width, opening height, sill height, opening type and orientation.

The window schedule is what a contractor wants when quoting on new windows, and it's what makes a takeoff useful to anyone other than yourself.

HouseSense builds a window and door schedule from the home's openings, so each window is listed with its measurements, and the measurements travel with it into the report. Window position and height also feed the 3D model, where the sun's path over the day and the year is calculated from the home's location and orientation — so sill height isn't just a number on a list, it's something you can see in the room. Scan with a LiDAR-equipped iPhone, and the openings are found during the scan, so you can correct each measurement to whatever you've taken with a tape measure yourself.

Four mistakes that cost money when ordering windows

• Measured the glass instead of the opening. Comes up 16-24 cm short in each direction — a window that falls out of its opening.
• Forgot the sealant gap. The outer frame size has to be smaller than the rough opening, typically 1-2 cm all the way round. Order a frame exactly equal to the opening, and the window can't be fitted.
• Measured from the old floor when new flooring is going in. Sill height changes with the thickness of the floor, typically 1.4-2.2 cm for a wood floor on an underlay.
• Forgot the distance from the corner. Without it the window can't be placed on the drawing, and the whole takeoff has to be done again.