Interior Paint Calculator — Walls, Trim & Coverage
Calculate paint needed for interior walls, ceilings and trim. Get precise quantities in litres for emulsion, eggshell and gloss — broken down by surface type — so you buy the right amount without over-ordering.
How the interior paint calculator works
- 1Wall area = sum of all wall surfaces (m²), minus windows and doors
- 2Trim area = door and window frame perimeters × trim width (typically 0.1–0.15 m)
- 3Coverage rate: emulsion 10–12 m²/L, eggshell 12–14 m²/L, gloss 14–16 m²/L
- 4Litres needed = area ÷ coverage rate × number of coats
- 5Add 5–10% for irregular surfaces, textured walls or colour changes
- 6Round up to nearest available tin size
Worked example
Open-plan living room: 45 m² wall area, 20 m² ceiling. Skirting and door frames 8 m² total. 2 coats walls, 1 coat ceiling, 2 coats gloss on trim.
Walls: 45 ÷ 11 × 2 = 8.2 L emulsion. Ceiling: 20 ÷ 12 = 1.7 L. Trim: 8 ÷ 15 × 2 = 1.1 L gloss. Walls + ceiling: 9.9 L, or ~11 L including 10% allowance. Total: ~11 L emulsion + 1.1 L gloss.
Frequently asked questions
What sheen level should I use for interior walls?
Matt/flat finish hides surface imperfections best — ideal for living rooms and bedrooms. Eggshell is wipeable and suits hallways and kitchens. Satinwood and gloss are reserved for trim, doors and skirting boards.
How much does it cost to paint a room interior?
DIY materials typically cost £50–£150 per room (5–10 L paint at £10–£20/L). Professional painting adds labour at £150–£400 per room depending on size and preparation required. Use the calculator above to get your exact material cost.
What is the best brush or roller for interior walls?
For large wall areas, use a 9–12 inch roller with a medium (10–12 mm) nap for smooth to lightly textured walls. Cut in edges with a 2–2.5 inch angled brush. For ceilings, use a long-handle roller to avoid working overhead.
How long should I wait between coats of interior paint?
Water-based emulsion: 2–4 hours between coats at 20°C, 50% humidity. Oil-based paint: 16–24 hours. High-build or thick-film paints may require up to 8 hours. Always check the product datasheet — painting too soon causes lifting and poor adhesion.
Do I need to sand walls before repainting?
Lightly sand glossy surfaces with fine sandpaper (180–240 grit) to give the new paint a key. Fill any cracks or holes and sand flush. On previously painted walls in good condition, washing with sugar soap is often sufficient preparation.