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.
Save to project
Free to try — no sign-up
Plan the whole job — not just this calculation
Save results to a project: budget and shopping list build themselves, and the whole plan exports as a PDF. Draw your floor plan and room sizes pre-fill the calculators.
How the interior paint calculator works
- 1Wall area = sum of all wall surfaces (m²), minus windows and doors
- 2For trim, calculate trim area separately and run a separate paint calculation if it uses a different product, sheen or coverage rate
- 3Enter the coverage rate for the selected product; run separate calculations for emulsion, eggshell, gloss or trim paint if their coverage/coats differ
- 4Litres needed = area ÷ coverage rate × number of coats
- 5Add 5–10% for irregular surfaces, textured walls or colour changes
- 6Round up using the can size you enter, or compare available tin combinations manually from the result
Worked example
Open-plan living room: 45 m² (485 sq ft) wall area, 20 m² (215 sq ft) ceiling. Skirting and door frames 8 m² (86 sq ft) 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 — about 3 US gallons of emulsion plus a quart of 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?
Materials for a typical room come to 5–10 litres of paint — 1½–2½ US gallons — plus filler, tape and a roller sleeve — enter your own price per litre above and the calculator totals it exactly. If you are hiring the work out, expect labour to be the larger half of the bill by some way, and to rise sharply with preparation: filling, sanding and priming bare plaster takes far longer than rolling a finish coat over sound paint.
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.