Buying Materials Without Waste: Area, Volume and Allowances

• Rectangle: length × width — a 5 m × 4 m room = 20 m²
• Triangle: base × height ÷ 2 — 10 and 6 = 30
• Trapezoid: (a + b) ÷ 2 × height — 4, 8, height 5 = 30
• Circle: π × r² — radius 3 = 28.27

For an irregular room, split it into rectangles and add them up — more reliable than forcing one formula.

Cuboid length × width × thickness: a 5 × 4 × 0.1 m slab = 2 m³. At 2,400 kg/m³ that is about 4.8 tonnes.

Any prism is base area × height; a cone is one third of the matching cylinder. Height is the perpendicular distance between bases, not the slant edge.

First subtract what will not be covered: door and window openings, columns, built-in furniture. Then add waste:

• Tile / stone: 8%–10% (cutting and breakage, more for diagonal layouts)
• Wood flooring: 5%–8%
• Paint: about 10%, also converted via coverage per litre × number of coats
• Concrete: 5%–10% (uneven subgrade, compaction)

Order matters: (net area × (1 + waste rate)) — do not add waste before subtracting openings.

A 20 m² room minus a 0.2 m² doorway → net 19.8 m²; 600×600 mm tiles (0.36 m² each), 8% waste:

19.8 × 1.08 ÷ 0.36 = 59.4 → 60 tiles

Keep 2–3 spares from the same batch for future repairs. Tiles sell by the piece — round up, never order 59.4.

• Square the area factor: 1 ft = 0.3048 m but 1 ft² = 0.3048² ≈ 0.0929 m²; multiplying by 0.3048 understates by more than 3×.
• Cube the volume factor: 1 ft³ ≈ 0.0283 m³; 1 m³ = 1,000 L.
• Chinese customary units: 1 mu ≈ 666.7 m², not 1,000.

Imported material specs mix metric and imperial — confirm units before ordering; it is cheaper than returning stock.