Area calculator

Floor area and how many tiles to buy

Say a bedroom floor is 4.2 m long and 3.6 m wide. The area is 4.2 × 3.6 = 15.12 m² and the perimeter 2 × (4.2 + 3.6) = 15.6 m. The perimeter is handy when buying skirting boards: 15.6 m minus the width of the doorways.

For tiles or parquet you usually add about 10% for cutting waste: 15.12 × 1.1 ≈ 16.63 m². If one pack covers 1.44 m², you need 16.63 ÷ 1.44 ≈ 11.55, so 12 packs. Packs are always rounded up, never to the nearest whole number.

You can double-check by counting pieces. A 30 × 60 cm tile is 0.3 × 0.6 = 0.18 m², so 15.12 ÷ 0.18 = 84 tiles without any spare. Laying tiles diagonally produces more offcuts, so the margin should be larger.

Painting walls: area minus openings

To get the wall area, multiply the room's perimeter by the ceiling height. The same room with a 2.6 m ceiling: 15.6 × 2.6 = 40.56 m². Then subtract the openings, since they are not painted.

• Door 0.8 × 2.0 m = 1.6 m²
• Window 1.2 × 1.4 m = 1.68 m²
• Paintable area: 40.56 - 1.6 - 1.68 = 37.28 m²
• Two coats: 37.28 × 2 = 74.56 m²
• With a coverage of 10 m²/l on the tin, you need 74.56 ÷ 10 ≈ 7.5 litres

Awkward shapes: split them into pieces

An L-shaped room does not need a single formula. Split it into two rectangles, calculate each and add them. For example, a 5 × 4 m part gives 20 m² and the 2 × 3 m part around the corner gives 6 m², 26 m² in total. Make sure the pieces do not overlap: the same square metre must not be counted twice.

If a chimney or pillar sticks into the floor, calculate its area separately and subtract it. For a sloped wall or gable use the triangle formula: a base of 8 m and a height of 3 m give 0.5 × 8 × 3 = 12 m². The height must be measured at right angles to the base, not along the sloping edge.

Circles: radius or diameter?

The most common mistake with circle area is entering the diameter instead of the radius. A round flower bed 3 m across has a radius of 1.5 m, an area of π × 1.5² ≈ 7.07 m² and an edging length of 2 × π × 1.5 ≈ 9.42 m.

Enter r = 3 by accident and you get 28.27 m², exactly four times too much. The radius is squared in the formula, so doubling it quadruples the area. The same rule holds for any shape when all of its dimensions grow in the same proportion.

Units: cm², m² and the vanishing zeros

Length units differ by a factor of 100 (1 m = 100 cm), but area units differ by 100 × 100 = 10,000. So 1 m² = 10,000 cm², not 100 cm². Measure the room in centimetres, 420 × 360 = 151,200 cm², divide by 10,000 and you are back at 15.12 m².

The calculator does not convert units for you: enter both sides in centimetres and the result is in cm², even though the label says m². Never mix units within one calculation; multiplying 4.2 m by 360 cm gives 1512, a meaningless number.

• 1 cm² = 100 mm²
• 1 m² = 10,000 cm²
• 1 are = 100 m²
• 1 hectare = 10,000 m²