How to measure a room: the floor is one number, the walls another

Two questions that are easy to confuse

floor = length × width · perimeter = 2 × (length + width) · gross wall = perimeter × height · net wall = gross wall − openings

The floor comes from two measurements, the wall from three. So the floor grows when a room gets wider, while the wall grows when the room gets longer around its perimeter or taller — which is why two rooms with the same floor area can have noticeably different wall areas.

The ratio is not a constant, and that is what surprises people. The table below shows five rooms with the same single door: in the 6 × 4 m room the net wall is 2.26 times the floor, while in the 2.4 × 2 m storeroom it is 4.29 times. Small rooms have proportionally the most wall — which is why a bathroom or a storeroom eats unexpectedly much paint or tile per square metre of floor.

Practically: decide what you are buying first, then measure. The floor needs length and width; the walls need the height and a list of openings as well.

Openings come off the wall, never off the floor

Doors and windows are holes in the wall, not in the floor. Flooring is laid up to the threshold and under the window, so an opening takes not one square centimetre off the floor — while it takes its full area off the wall.

In our room the gross wall is 44.88 m². A 0.9 × 2.05 m door brings it down to 43.04 m², and two 1.4 × 1.4 m windows take it further to 39.12 m². That is 5.77 m² of openings, or 12.85 % of the gross wall — enough to decide whether you buy one more can of paint or not.

The ceiling is a third number, and it equals the floor: 18.72 m². Paint it too and you have 57.84 m² to cover. In the calculator that is a separate checkbox, because a ceiling is left out about as often as it is included.

Alcoves, window reveals, a chimney breast projecting into the room and a column in the middle of it are not deductions but additions — each has sides of its own, and those sides are wall like any other.

What deducting the door and windows is actually worth in cans of paint →

A room that is not a rectangle

Most rooms are not rectangles. The honest method is not a polygon formula but a split: sketch the plan, divide it into rectangles, work each one out separately and add them up. An L-shape needs two.

Our L-shaped room is a 5.2 × 3.6 m rectangle with a 2.4 × 2 m part attached. The floor is 18.72 + 4.8 = 23.52 m². So far so simple.

The wall is not. Add the two perimeters and you get 26.4 m — but the real outline of the L is 21.6 m. The difference is exactly twice the length of the wall where the two parts meet, and that wall does not exist. At 2.55 m high that is 67.32 m² against 55.08 m², so 12.24 m² of wall you would fill, paint and pay for although it is not there.

The good news is that an L-shape has a shortcut: its outline equals the perimeter of the rectangle that bounds it. Our L fits into 5.2 × 5.6 m, and that is exactly 21.6 m. So enter that bounding rectangle in Walls mode and read only the wall lines — the floor line then means nothing, because the bounding rectangle has 29.12 m² of floor while the room has 23.52 m².

The shortcut fails when the recess is in the middle of a wall rather than at a corner. A niche 1 m wide and 0.8 m deep does not shorten the outline, it lengthens it by twice its depth — 1.6 m — because its two sides are added.

What you do with the number next

Area is an input, not an answer. Every material adds its own cutting percentage and its own packaging on top, so the same square metres turn into different quantities — boxes, cans, packs and rolls, always rounded up.

A floor of 18.72 m² with 10 % waste means buying 20.59 m² of laminate or tile, i.e. 1.87 m² of surplus. Those percentages are a planning convention you choose and edit, not a norm or a regulation: 10 % is a common choice for straight laying, while diagonal laying and a room with many corners need more. The table runs the same floor through five of them.

A wall of 39.12 m² becomes paint, wallpaper or plasterboard. There you do not buy square metres but cans, rolls and sheets, so every rounding up costs a whole package — which is exactly why entering the real openings beats skipping them “to be safe”.

Paint calculator →

Tile calculator →

Laminate calculator →

How to measure so the number is right

Measure at floor level, not at waist height: walls are less parallel than they look. Measure each side at both ends and, when the two readings differ, take the larger one for materials and the smaller one for furniture.

Measure openings as the builder does — wall to wall, not across the door leaf. The leaf is narrower than the opening, so measuring the leaf understates the deduction.

A fitted wardrobe running from floor to ceiling is the opposite case to an alcove: if you are not painting or tiling behind it, that wall does not exist for your calculation, so deduct it like any other opening.

And finally, write down the measurements, not just the result. A square-metre figure with no recorded lengths cannot be checked or corrected when it turns out one side was 20 cm longer than you thought.

Check your own numbers

Open the calculator in Walls mode with this article's room — 5.2 × 3.6 m, 2.55 m high, one door and two windows, ceiling included. The result is 39.12 m² of net wall, 44.88 m² gross and 5.77 m² of openings.

For the floor, Floor / area mode is enough: the same length and width with 10 % waste give 18.72 m² of floor and 20.59 m² to buy.

Open the calculator with this article's room (walls) →

The same example in Floor / area mode →