The four numbers
- Length and width
- The floor footprint. Measure at floor level, against the skirting, not at waist height where furniture gets in the way. These two give you floor area directly and wall area via the perimeter.
- Height
- Floor to ceiling. Measure it — do not assume 2.4 m or 2.7 m. Older buildings vary enormously and the height multiplies the entire wall calculation.
- Openings
- Doors and windows, measured as rectangles, added up into one total. This is the only subtraction in the whole process and the one people skip.
walls = 2 × (length + width) × height
net wall area = walls − openings
floor or ceiling = length × width
Measuring without introducing error
- Measure each wall separately rather than assuming opposite walls match. In a room built before 1980 they usually do not, and a 10 cm difference over a 4 m wall is 0.27 m² of paint you did not account for.
- Measure height in at least two corners. Floors slope, ceilings sag, and the difference decides whether the last row of tiles fits.
- Use the longest measurement of each dimension, not the average. Materials are cut to fit the widest point; the surplus can be trimmed, the shortfall cannot.
- Keep everything in one unit. Millimetres for tiling and flooring, metres for paint. Mixing centimetres and metres in the same list is the origin of most factor-of-ten disasters.
- Write measurements down as you take them, with a sketch. Remeasuring is quick; discovering at the shop that you have three numbers and needed four is not.
- A laser measure costs less than one wasted tin of paint and removes the two-person problem entirely.
Openings: when to subtract
Openings are where reasonable people disagree, so here is a defensible rule rather than a tradition.
| Opening | Typical size | Subtract? |
|---|---|---|
| Internal door | 0.8 × 2.0 m = 1.6 m² | Yes |
| External door | 0.9 × 2.1 m = 1.9 m² | Yes |
| Standard window | 1.2 × 1.2 m = 1.44 m² | Yes |
| Patio or French doors | 1.8 × 2.1 m = 3.8 m² | Yes, always |
| Small window under 0.5 m² | — | No, it is inside the waste |
| Fitted wardrobe front | varies | Yes if not being painted |
| Radiator | — | No, you paint behind it badly but you paint it |
The principle: subtract anything over roughly half a square metre, ignore anything under. Below that threshold the opening is smaller than your waste allowance, and subtracting it makes the estimate falsely precise rather than more accurate.
Subtracting openings makes you buy less. If you are between two tin sizes, do not subtract — the cost of one extra litre is far below the cost of stopping mid-wall.
Rooms that are not boxes
- L-shaped rooms
- Split into two rectangles, calculate each, add them. For wall area, measure the actual perimeter by walking it with the tape — an L-shape has more wall than its bounding box suggests, not less.
- Sloped or vaulted ceilings
- Treat the sloped section as a rectangle using the slope length, not the horizontal distance. A roof pitch adds 15-40% to the surface compared with its plan area, and the steeper it is the more it hides.
- Gable end walls
- The triangle at the top is base × height ÷ 2. Measure the height of the triangle alone, not the total wall height.
- Alcoves and chimney breasts
- They add wall area, never subtract it. A chimney breast projecting 30 cm adds 0.6 m of wall run per storey height. Walk the perimeter with the tape rather than calculating it from the footprint.
- Stairwells
- Measure the tallest point and treat the wall as a full rectangle. The triangle you gain is smaller than the access difficulty you are about to discover.
- Curved walls
- Run the tape along the curve at skirting level. Do not measure the chord — for a shallow bay the difference is a few percent, for a real curve it is much more.
Where precision actually pays
Not every measurement deserves the same care, and knowing which ones matter saves an hour on your knees with a tape.
| Input | Effect of a 5% error | Worth remeasuring? |
|---|---|---|
| Room length or width | 5% on the total | Once, carefully |
| Ceiling height | 5% on wall area only | Yes, in two places |
| Openings | under 2% typically | Rough is fine |
| Coats of paint | 50-100% on the total | This is the one that matters |
| Coverage rate claimed on tin | 10-20% real-world shortfall | Assume the pessimistic figure |
| Tile or flooring waste % | 5-20% on the total | Depends on pattern, see the waste guide |
The pattern is consistent across every material: the multipliers — coats, waste, depth — dominate the dimensions. People spend twenty minutes measuring a window to the centimetre and then guess the number of coats. Reverse that.
A worked example
An L-shaped living room. The main part is 4.0 × 3.5 m; the return is 2.0 × 1.5 m. Height 2.55 m, measured. One door, two windows.
floor = (4.0 × 3.5) + (2.0 × 1.5) = 14 + 3 = 17 m²
perimeter, walked with tape = 17.0 m
walls = 17.0 × 2.55 = 43.4 m²
openings = 1.6 + 1.44 + 1.44 = 4.5 m²
net wall area = 43.4 − 4.5 = 38.9 m²
Note the perimeter was walked, not calculated. The bounding box of this room is 4.0 × 5.0 m, which would give a perimeter of 18 m — but the actual walls total 17 m because the L cuts a corner off. Calculating from the footprint would have over-ordered by 6%. In the other direction, if the L had been an alcove rather than a missing corner, the same shortcut would have under-ordered.
Before you leave the room
- Do you have every wall length, or did you assume two of them?
- Did you measure the height, or did you assume it?
- Is every number in the same unit?
- Did you sketch the room, including which wall the window is on?
- For floors: did you measure the diagonals? If they differ by more than 2 cm the room is out of square and your waste allowance needs 5% more.
- Did you write down what you are estimating for? Paint needs wall area, tiles need floor area, and skirting needs perimeter — the same room produces three different answers.