Almost every renovation problem starts at measurement. You order 30 m² of tile, it turns out you needed 34. You buy 8 L of paint, you're 2 L short with one wall to go. You quote a client for 42 m² of flooring, it's actually 47 — and you eat the difference.
Measuring a room isn't hard, but it does need a method. Here's the one that works for every shape, every room, and stops those "5% off" errors that cost real money.
Tools you actually need
Skip the fancy stuff. For 95% of interiors, this is enough:
- Tape measure (5 m or 8 m) — a wide 25 mm tape stays rigid over long spans; skinny ones sag.
- Laser distance meter (optional, €25–€60) — pays for itself the first time you measure a ceiling alone.
- Pencil + notebook — or an app that keeps a running sketch.
- Sketch of the floor plan — draw it before you start. Even a rough one saves you from missing walls.
Step-by-step method
Do this in order. Skipping steps is where errors sneak in.
- Sketch the room shape from above. Rough is fine. Number each wall (1, 2, 3, 4).
- Measure length and width at floor level — but always double-check at a second height. Walls are rarely square.
- Measure ceiling height in at least two corners. Old buildings can vary by 2–3 cm across a room.
- Measure every opening — doors, windows, radiators, alcoves, chimney breasts. Write dimensions on the sketch.
- Note anomalies — sloped ceilings, uneven floors, pipes in the corner. These matter later.
📏 Measure everything twice
Always take each measurement twice — once when you start, once when you finish. If they don't match, measure a third time. A 2 cm slip on paper becomes 15 tiles too few on order day.
Rectangular room
The easy case. Multiply length by width for the floor area, and the walls are just perimeter × height.
L-shaped and irregular rooms
Break the shape into rectangles. Draw dashed lines on your sketch to divide it into 2 or 3 simple rectangles, calculate each one, then add them together.
For rooms with a curve or diagonal wall, split into rectangles for the straight sections and add a triangle for the diagonal: ½ × base × height. Don't stress the last centimetre — round up.
| Room shape | Method | Formula |
|---|---|---|
| Rectangle | Single measurement | L × W |
| L-shape | Split into 2 rectangles | (L₁ × W₁) + (L₂ × W₂) |
| U-shape | Split into 3 rectangles | Sum of all three |
| Diagonal wall | Rectangle + triangle | (L × W) + (½ × b × h) |
| Sloped ceiling (attic) | Trapezoid | ½ × (a + b) × h |
Ceilings and walls
For paint, wallpaper, or plaster, you're measuring wall area — not floor area. Take the room perimeter (add all wall lengths), multiply by height, then subtract openings.
Perimeter = 2(4 + 3) = 14 m
Wall area = 14 × 2.6 − 1.8 − 2.4 = 32.2 m²
📐 Skip the sketch — measure it in BuildExact
Enter length, width and height, then tick doors and windows. BuildExact gives you floor area, wall area and material quantities in seconds — and stores every project so you can share it as a PDF.
Download freeWindows and doors
Here's where a lot of people go wrong: whether to subtract openings depends on what you're calculating for.
- Paint, wallpaper, plaster — subtract every opening. You're not painting the glass.
- Flooring, tiles, underfloor heating — don't subtract standard doorways. The floor runs continuously under the doorway threshold and you'll need offcuts anyway.
- Skirting boards / trim — subtract door widths, leave alcoves in.
🚪 Standard opening sizes (NL/EU)
If you don't have exact measurements, use these averages: interior door ±1.8 m² (0.83 × 2.15 m), window ±1.2 m² (1.0 × 1.2 m), French doors ±3.6 m². These are close enough for material orders — but measure exactly when it's for a quote.
5 common mistakes
- Measuring only at floor level. Walls bulge. Measure at floor, mid-height and just under the ceiling — use the smallest number for material orders.
- Forgetting alcoves and chimney breasts. These change the wall area and the material you need. Sketch them in.
- Rounding too early. Round only at the final answer. 3.86 × 4.13 is not the same as 4 × 4.
- Missing doorways and thresholds. Do you need material under it, or does it stop at the threshold? Decide before ordering.
- Only one measurement per wall. Old houses drift 1–3 cm per meter. Take multiple readings and use the largest for cuts, smallest for finish.
Doing it in seconds with BuildExact
The math above is straightforward, but doing it for every room in a house — and re-doing it every time you change a material — is where the time goes. BuildExact takes your dimensions and does the calculation for you, per room, per material, including waste percentages. Change the tile size and every quantity updates. Change ceiling height and paint recalculates. Save the project and it's there next week when the client changes their mind again.
Accurate measurements aren't glamorous. But they're the difference between a job that lands on budget and one that quietly bleeds cash. Do it once, do it right, and the rest of the project gets much easier.