Loona Construction Calculators
Work out exactly how much paint and primer a room needs. Measure the room in metres or feet, tell the calculator how many doors and windows to subtract, how many coats you plan and the coverage printed on the tin — it does the rest, including a sensible extra allowance for spills and touch-ups.
A typical interior emulsion covers about 10–12 m² per litre per coat on smooth walls. Two coats on 36.8 m² at 10 m²/L therefore needs about 7.4 litres before any waste allowance — this calculator adds the allowance for you.
You do not paint over them, so leaving them in overstates the paint you need. Standard sizes (door 1.89 m², window 1.8 m²) are subtracted automatically; for very large windows or patio doors, increase the window count.
On new plaster, bare drywall, stained walls or a strong colour change, yes — primer seals the surface so the topcoat covers evenly and you often save a third coat of expensive paint. On previously painted walls in good condition you can usually skip it.
The calculator first works out the paintable area, then divides by the coverage of your paint.
Wall area = 2 × (Length + Width) × Height − (Doors × 1.89 m²) − (Windows × 1.8 m²)
Paint needed = Wall area × Coats ÷ Coverage × (1 + Waste %)
Primer needed = Wall area ÷ Primer coverage × (1 + Waste %)
A standard interior door is taken as 0.9 m × 2.1 m (1.89 m², about 21 ft²) and a typical window as 1.5 m × 1.2 m (1.8 m², about 12 ft² of wall once the frame is counted). Imperial coverage in ft² per gallon is converted to m² per litre before calculating, so both systems give the same answer for the same room.
Yes. Switch the unit system to Imperial: dimensions in feet, coverage in ft² per gallon, and results in gallons (with litres shown alongside). The answer matches the metric calculation for the same room.
Two coats is the standard for a durable, even finish. A similar colour over a sound surface may take one; a bold colour change or fresh plaster usually takes two over primer, sometimes three.
Rollers and brushes hold paint, some is spilt, and you will want a little left for future touch-ups. 10% is a sensible default; use 15% for rough or textured walls.
Add this calculator to your own website or blog — it is completely free, needs no sign-up, and always shows the latest version. Copy the code below and paste it into your page where you want the calculator to appear. Keeping the small “Powered by Loona Automation” credit line under the embed helps keep this tool free — but you are free to style or remove it; the tool works either way.