Estimating paint, tiles and concrete without running short
Material estimates fail in predictable ways. How to work out what a job needs, how much waste to allow, and why running short mid-pour is costly.
Every material estimate is the same two-step calculation: work out the area or volume, then divide by what one unit covers. What separates a good estimate from a frustrating trip back to the merchant is the third step everyone skips — deciding, deliberately, how much extra to buy.
Paint: area × coats ÷ spread rate
Start with the wall area. A 12-metre run of wall 2.4 m high is 28.8 m². Two coats means covering 57.6 m². At a typical spread rate of 11 m² per litre, that is about 5.2 litres — so a 5-litre tin leaves you slightly short, and this is exactly the situation where people convince themselves the second coat will stretch.
Spread rates on tins assume good conditions. Reality reduces them considerably:
- Porous or new plaster drinks the first coat — some of it never becomes visible finish.
- Textured surfaces have more actual area than their measured dimensions suggest.
- Dark to light changes routinely need three coats, not two.
- Rollers and trays keep a surprising amount that never reaches the wall.
Tiles: count them, then add for cuts
A 4 m × 3 m floor is 12 m². A 30 cm tile covers 0.09 m². That is 134 tiles to cover the bare floor — and 134 is not the number to order.
Every tile meeting a wall gets cut, and the offcut is usually unusable. Add breakages in transit and during laying, plus a few spares put aside for future repairs, and the standard 10% allowance takes you to 147 tiles, or 15 boxes of ten.
| Layout | Waste allowance | Why |
|---|---|---|
| Straight, square room | 10% | Edge cuts and breakages only |
| Diagonal | 15% | Every perimeter tile is cut at an angle |
| Herringbone or complex pattern | 15–20% | Frequent cuts, more unusable offcuts |
| Large-format tiles | 15% | Fewer tiles, but each waste piece is bigger |
The spares matter more than people expect. Tile batches vary in shade, and a discontinued line cannot be matched at all — so the handful you keep in the garage may be the only way to repair a cracked tile in five years without redoing the floor.
Concrete: the one you cannot get wrong
A 4 m × 3 m slab at 10 cm thick is 1.2 m³. With the standard 10% allowance that becomes 1.32 m³ — roughly 72 bags of 40 kg premix, since each bag yields only about 0.0185 m³ once mixed.
Concrete deserves more caution than any other material here, because the failure mode is different in kind. Run short on paint and you buy another tin. Run short mid-pour and the first section begins curing before the rest arrives, leaving a cold joint — a permanent line of weakness through the slab that no amount of finishing will remove.
The habits that make estimates hold
- Measure the room, not the plan. Rooms are rarely square, and walls are rarely the length the drawing claims.
- Work in one unit system. Mixing centimetres and metres mid-calculation is the most common source of order-of-magnitude errors.
- Buy the same batch at once. Paint, tile and even premix vary slightly between production runs.
- Round up to whole units. You cannot buy 0.4 of a tin or 4.7 bags, so the shop will round for you anyway.