Work out exactly how much paint and how many tiles your project needs. Free calculators plus the formulas, waste allowances, and mistakes that cost you money.
To work out paint, you divide the wall area by the coverage rate on the tin and multiply by the number of coats. To work out tiles, you divide the surface area by the area of one tile and add a waste allowance. Both are simple the mistakes come from the parts people skip: subtracting openings, allowing for waste, and remembering that primer and second coats change everything. Paint Calculator → | Tile Calculator → How Much Paint Do I Need? The formula: Paint needed = (Wall area ÷ Coverage per litre) × Number of coats Step 1 — Measure the wall area. Height × width for each wall, then add them together. For a room, an easy shortcut: (room perimeter × wall height). Step 2 — Subtract openings. Doors and windows aren't painted. A standard interior door is roughly 1.8 m², a typical window around 1.5 m². Skipping this step on a room with three windows and two doors overestimates by a meaningful amount. Step 3 — Divide by the coverage rate. Coverage is printed on the tin, typically 10–14 m² per litre for interior emulsion. Use the manufacturer's number, not a generic one — coverage varies significantly by paint type and finish. Step 4 — Multiply by coats. Two coats is standard. Three if you're going light over dark, or covering a bold colour. Worked example: a 4 m × 5 m room with 2.5 m ceilings, one door, two windows. Perimeter: (4 + 5) × 2 = 18 m Wall area: 18 × 2.5 = 45 m² Minus openings: 45 − 1.8 − 3.0 = 40.2 m² At 12 m²/L: 40.2 ÷ 12 = 3.35 L per coat Two coats: 6.7 litres What people get wrong with paint Forgetting primer. Bare plaster, new drywall, and patched areas absorb far more paint than a sealed surface. Primer first, or budget an extra coat. Ignoring the surface texture. Textured or porous walls consume noticeably more paint than smooth ones. If your walls are rough, treat the tin's coverage figure as optimistic. Buying exactly the calculated amount. Add roughly 10% for touch-ups later. More importantly, paint batches vary slightly in colour — matching a top-up tin months later is harder than it sounds. Forgetting the ceiling. It's a separate calculation: room length × width. How Many Tiles Do I Need? The formula: Tiles needed = (Surface area ÷ Area of one tile) × Waste factor Step 1 — Measure the surface. Length × width for floors. For walls, height × width, minus doors and windows. Step 2 — Calculate one tile's area. A 30 cm × 60 cm tile is 0.3 × 0.6 = 0.18 m². Step 3 — Divide. Surface area ÷ tile area = tiles required before waste. Step 4 — Add waste allowance. This is the step that decides whether your project stalls: Layout Waste allowance Straight/grid layout, simple rectangular room 10% Diagonal layout 15% Herringbone or complex pattern 20% Room with many cuts, alcoves, or angles 15–20% Large-format tiles (higher breakage risk) 15% Worked example: a 3 m × 4 m floor with 30×60 cm tiles, straight layout. Area: 12 m² Tile area: 0.18 m² Tiles: 12 ÷ 0.18 = 66.7 → 67 tiles Plus 10% waste: 74 tiles Then divide by tiles per box to get the number of boxes, and round up you can only buy whole boxes. What people get wrong with tiles Buying too little and reordering. Tile batches (dye lots) vary in shade. A reorder weeks later can arrive visibly different from the first batch. Buy everything at once. Ignoring grout lines. With small tiles and wide grout joints, the joints occupy real surface area this works in your favour slightly, but don't rely on it to cover a thin margin. Forgetting spare tiles for the future. Keep several from the original batch. When one cracks in three years, an exact-match replacement is otherwise almost impossible to find. Not accounting for the pattern. Herringbone and diagonal layouts generate far more offcuts than straight grids hence the higher waste percentages above. Frequently Asked Questions How much paint do I need for one room? Multiply the room perimeter by the wall height, subtract door and window areas, divide by the paint's coverage rate (usually 10–14 m² per litre), then multiply by the number of coats. For a typical 4×5 m room with 2.5 m ceilings, two coats needs roughly 6–7 litres. The paint calculator does this instantly. How much extra tile should I buy for waste? Add 10% for a straight layout in a simple room, 15% for diagonal layouts or rooms with many cuts, and 20% for herringbone and other complex patterns. Does one litre of paint really cover 12 square metres? That figure assumes a smooth, sealed, previously painted surface. Bare plaster, textured walls, and porous surfaces absorb considerably more, so real coverage on those is lower. Should I calculate paint per coat or total? Calculate the area first, then multiply by coats at the end. Two coats is standard for most repaints; three when covering a dark colour with a lighter one. Do I subtract windows and doors when calculating tiles for a wall? Yes subtract any area that won't be tiled. But still apply the waste percentage to the remaining area, since cuts around openings generate extra offcuts.