Calculate construction area

This construction area calculator works out the area of any rectangular space from its length and width. Enter the dimensions in metres, feet, or yards and it returns the area in square metres, square feet, and square yards, plus the perimeter. It is ideal for estimating flooring, tiling, paint, decking, turf, slabs, and other materials sold by area.

How to measure area

For a rectangle, area = length × width. For an L-shaped or irregular space, split it into rectangles, calculate each part, and add them together. Always measure at the widest points and add about 10% extra when ordering materials to allow for cuts, waste, and breakages.

Estimating materials by area

Once you know the area, you can estimate quantities: paint coverage is usually given in m² per litre, tiles and turf are sold per m², and decking per linear or square metre. For depth-based materials like concrete and gravel, use our concrete calculator and gravel calculator, which factor in thickness.

How to work out the area of construction

The area of construction (sometimes called the built-up or covered area) is the total floor area a structure occupies, measured to the outside face of the external walls. For a simple rectangular building it is just length × width. For an L-shaped or irregular plan, split the outline into rectangles, work out each area, then add them together — subtracting any courtyards, lift shafts or open voids that are not built over. Multi-storey buildings multiply the footprint by the number of floors.

Worked example: a house measuring 12 m × 8 m with a 4 m × 3 m rear extension has a footprint of (12 × 8) + (4 × 3) = 96 + 12 = 108 m², which is about 1,163 ft². Over two storeys the total construction area is 216 m². Quantities for materials follow from this figure, so it is worth measuring twice — use the calculator above to convert between square metres and square feet as you go.

Frequently asked questions

How do I calculate square metres?

Multiply the length by the width, both measured in metres. A room 5 m long and 4 m wide is 20 m².

Area formulas for irregular plots and rooms

Most real rooms and plots are not perfect rectangles. The reliable approach is to split the shape into simple parts, find each area, and add them together. These four formulas cover almost every layout you will meet on site:

Shape	Formula	Example
Rectangle	length × width	8 m × 5 m = 40 m²
Triangle	½ × base × height	½ × 6 m × 4 m = 12 m²
Circle	π × radius²	3.1416 × 2² = 12.57 m²
Trapezoid	½ × (a + b) × height	½ × (6 + 4) × 5 = 25 m²

Worked example: an L-shaped extension

An L-shaped floor plan can be split into a 10 m × 6 m main rectangle and a 4 m × 3 m projection. The areas are 60 m² and 12 m², giving a total of 72 m². In imperial terms, 72 × 10.764 = 775 ft². Measure every wall rather than assuming walls are square — on older buildings, opposite walls often differ by several centimetres.

Turning area into material quantities

Once you know the area, most building materials can be estimated with a simple coverage rate. The figures below are typical rates for standard UK product sizes, including normal joints, before any waste allowance:

Material	Typical coverage
Bricks, 215 × 65 mm, single skin	About 60 per m²
Concrete blocks, 440 × 215 mm	About 10 per m²
Plasterboard, 2.4 × 1.2 m sheet	2.88 m² per sheet
Floor tiles, 600 × 600 mm	2.78 per m²
Emulsion paint	Around 10–12 m² per litre per coat
Floor screed at 50 mm	0.05 m³ per m²

For example, a wall 5 m long and 2.4 m high is 12 m². Subtract a 2 m² door opening to get 10 m². At 60 bricks per m², that is 600 bricks, or 630 with a 5% allowance for cuts and breakages.

Useful area conversions

1 m² = 10.764 ft². 1 yd² = 0.8361 m². 1 acre = 4,046.86 m². In US roofing, one “square” means 100 ft² (9.29 m²). Floor plans for property sales may quote gross internal area, which is different from built-up or carpet area — see our explanation of built-up vs carpet vs plot area.