🧱 Building & decorating
How much tile do I need? Paint and concrete coverage
Three jobs, one shape of arithmetic: measure an area, multiply by a depth or a number of coats, then add waste. The waste factor is the part people skip, and it is the part that decides whether you finish the job on Saturday or drive back to the shop.
The three sums
Every coverage question in this guide reduces to one of three multiplications. Tile is area ÷ tile area, with a waste allowance. Paint is wall area × coats ÷ coverage per litre. Concrete is length × width × depth. Everything else is unit conversion and rounding up to what the shop actually sells.
Measure in metres and millimetres, then convert once at the end. Mixing feet and metres inside one calculation is the most common source of a job that comes up short.
Tile: area, tile size, boxes
Tile area is width × height of the tile in metres. A 300 × 300 mm tile is 0.09 m²; a 600 × 600 mm tile is 0.36 m²; a 100 × 300 mm metro tile is 0.03 m². Divide the wall or floor area by that number to get the tile count, then add waste.
Grout joints slightly reduce the number of tiles needed, but only by 1–3% at typical 2–3 mm joints, which is well inside the waste allowance. Do not bother subtracting it.
| Layout | Waste | Why |
|---|---|---|
| Straight lay, square room | 10% | Edge cuts, breakages, one spare box for later repairs |
| Straight lay, many corners or pipes | 12–15% | More cuts around fittings and door reveals |
| Diagonal (45°) | 15% | Every perimeter tile is a triangle, so offcuts rarely fit again |
| Herringbone or chevron | 15–20% | Pattern geometry makes most offcuts unusable |
| Large format over 800 mm | 15% | Fragile, hard to re-cut, and pattern-matched |
Worked example. A floor 4.2 m × 3.0 m is 12.6 m². With 300 × 300 mm tiles that is 12.6 ÷ 0.09 = 140 tiles. Add 10% for a straight lay: 154 tiles. If the box holds 11 tiles, that is 154 ÷ 11 = 14 boxes exactly. If the shop only sells boxes of 10, buy 16 boxes (160 tiles) and keep the spare.
For a wall, subtract the door and window openings before dividing. A 1.8 m² door and a 1.5 m² window in a 12 m² wall leave 8.7 m² of tiling, not 12.
Grout: the small number that gets forgotten
Grout quantity per square metre depends on tile size and joint dimensions: kg/m² = (A + B) ÷ (A × B) × joint width × joint depth × 1.6, with A and B in metres and the density of cementitious grout taken as about 1.6 kg/L.
For 300 × 300 mm tiles with a 3 mm joint and 8 mm depth: (0.3 + 0.3) ÷ (0.3 × 0.3) = 6.67 m of joint per m², times 0.003 × 0.008 m = 0.00016 m³, times 1,600 kg/m³ = 0.26 kg/m². A 12.6 m² floor needs about 3.2 kg, so one 5 kg bag is enough for the floor but not the walls as well.
Epoxy grout is denser and sold by volume, so use the manufacturer's chart rather than this estimate.
Paint: area, coats, coverage
Emulsion on a smooth, sealed wall covers 10–12 m² per litre per coat. That is the same figure as the 350–400 sq ft per US gallon printed on many American tins: 400 sq ft is 37.2 m², and 37.2 ÷ 3.785 L = 9.8 m²/L.
Porous surfaces drink paint. New plaster, bare wood and previously unpainted masonry can need 20–30% more for the first coat, and a primer coat is cheaper than a third finish coat.
Worked example. A room 4 m × 3 m with 2.4 m ceilings. Perimeter = 2 × (4 + 3) = 14 m, so wall area = 14 × 2.4 = 33.6 m². Deduct a 1.8 m² door and a 1.8 m² window: 30 m². Two coats at 11 m²/L = 60 ÷ 11 = 5.5 L. Buy two 2.5 L tins plus one more, or three 2.5 L tins (7.5 L) if the colour is a strong one that needs an extra coat.
Ceilings are a separate area: 4 × 3 = 12 m², and they usually take one coat of a flatter paint. Skirting, doors and trim are measured in linear metres and painted in gloss or satin, which covers 12–16 m²/L.
Concrete: volume, depth and bags
Concrete is ordered by volume: cubic metres in most of the world, cubic yards in the US. 1 yd³ = 27 ft³ = 0.764555 m³, so cubic yards = cubic metres ÷ 0.7646.
Worked example. A patio 4 m × 3 m at 100 mm deep is 4 × 3 × 0.1 = 1.2 m³. That is 1.2 ÷ 0.7646 = 1.57 yd³. Add 10% for an uneven sub-base and spillage: 1.32 m³ or 1.73 yd³.
| Bag | Yield | Bags per m³ | Bags per yd³ |
|---|---|---|---|
| 80 lb (36.3 kg) | 0.60 ft³ (0.017 m³) | ≈ 59 | ≈ 45 |
| 60 lb (27.2 kg) | 0.45 ft³ (0.013 m³) | ≈ 79 | ≈ 60 |
| 50 lb (22.7 kg) | 0.375 ft³ (0.011 m³) | ≈ 94 | ≈ 72 |
| 25 kg (metric) | 0.0125 m³ | ≈ 80 | ≈ 61 |
Bag yields are nominal. Mixing by hand leaves more in the bucket than a mixer does, and a dry mix packs less densely, so the last bag of a big pour rarely stretches as far as the first.
| Use | Depth | Notes |
|---|---|---|
| Garden path, stepping stones | 75–100 mm | Compacted sub-base essential |
| Patio, shed base | 100 mm | Mesh for spans over about 1.5 m |
| Footpath for cars | 100–125 mm | Reinforcement recommended |
| Driveway | 125–150 mm | Thicker at the road edge; consider air entrainment in freezing climates |
| Post footing | Depth below frost line | Diameter typically 3× post width |
Mixing your own concrete
A general-purpose mix by volume is 1 part cement to 2 parts sand to 3 parts coarse aggregate, with water at roughly half the cement weight. That is a strong mix for a slab; footings and fence posts tolerate leaner ratios. Adding water beyond the design ratio makes the mix easier to place and weaker to set — a common reason a small slab cracks.
If you are mixing bags rather than buying aggregate, follow the bag's stated yield and add 5–10% for waste. If you are ordering ready-mix, order in cubic metres or yards and expect a minimum load charge; a part-load is often more expensive per unit than a full one.
Rounding rules that save a second trip
Round tile up to whole boxes, never part boxes. Round paint up to the can sizes actually sold, and keep a small tin of the same batch for touch-ups. Round concrete up by 5–10% — a short pour is the only mistake here that cannot be fixed by buying one more unit, because a cold joint is permanent.
Keep the receipts and the batch codes. Tile from a different firing batch can differ visibly, and paint mixed in a different base can be a shade off, which is exactly the situation the spare box and spare tin exist for.
Frequently asked questions
Sources
- Manufacturer coverage and bag-yield data sheets for bagged concrete and cementitious grout.
- Portland Cement Association — concrete mix proportions, slab thickness and curing guidance.
- NIST Special Publication 811 — the cubic yard, cubic foot and pound definitions used here.
- Paint-industry coverage guidance of 350–400 sq ft per US gallon, converted to metric.
- Tile-fixing guidance on joint widths and waste allowances for bond patterns.