Home Improvement & Construction
Calculate paint gallons needed, project cost, and primer requirements for rooms, exteriors, and commercial spaces.
Coverage
350-400 sq ft/gal
Waste Buffer
+10% recommended
Reviewed by: CalculatorApp Construction & Home Improvement Team
Frequently Asked Questions
How many litres of paint do I need for a 12×12 ft room with 8 ft ceilings?
Walls: 2×(12+12)×8 = 384 ft² = 35.7 m². Ceiling: 12×12 = 144 ft² = 13.4 m². Less 2 doors (7.2 m²) + 2 windows (1.8 m²): net = 40.1 m². At 11 m²/L × 0.85 efficiency = 9.35 m²/L practical. Litres = 40.1/9.35 × 2 coats = 8.6 L → buy 2×5 L tins.
What is the spread rate of paint and why does it matter?
Spread rate is the area covered per litre at one coat, stated on the tin. It directly determines how much paint to buy. A paint with 10 m²/L covers twice the area of a 5 m²/L masonry paint per litre. Always use spread rate (not just price) to compare paint value.
How much paint do I need for a 10 m² feature wall — 2 coats?
Litres = 10 m² ÷ (11 m²/L × 0.85) × 2 = 2.14 L. Buy one 2.5 L tin. For deep/vibrant colours, coverage rate may drop to 8–9 m²/L — check the specific colour's spread rate on the manufacturer's data sheet.
Do I need to prime new plaster before painting?
Yes. New plaster is extremely porous and will suck moisture out of emulsion paint causing patchiness and adhesion failure. Apply one mist coat (emulsion diluted 3:1 with water) first. This seals the surface and allows the full-strength topcoats to bond and cure uniformly.
How much extra paint should I buy for touch-ups?
Buy 10% extra for interior projects and keep the remainder sealed in a labelled tin for touch-ups. Paint colour changes slightly on opening stored tins, so having your original batch is critical for invisible repairs. Note the tin batch number for future reference.
Can I use interior paint for exterior or vice versa?
No. Exterior paint contains UV stabilisers, mould inhibitors, and flexible binders to resist weathering and thermal movement. Interior paint lacks these and will fade, chalk, and flake outdoors within months. Exterior paint usually has higher VOC levels — only use in ventilated areas indoors.
What is the difference between vinyl matt and pure vinyl matt?
Pure vinyl/heavy vinyl matt/trade matt has higher PVA content, better opacity at higher coverage, and is more durable than standard vinyl matt. Trade versions typically cover 10–12 m²/L vs 9–10 m²/L for budget matts. Useful for rental properties where repainting frequency matters.
How do I calculate paint for a stairwell with vaulted ceiling?
Break the area into rectangles and triangles. For a landing-to-landing wall: measure height at each end and apply the trapezoid formula: area = (h1+h2)/2 × length. Sum all faces. Don't forget the ceiling soffits on landings. Add 15% wastage for awkward access and cut-in work.
Why does dark paint cover more poorly than white?
Dark pigments (carbon black, phthalocyanine blue) have lower hiding power per unit weight than titanium dioxide (white). Dark colours achieve opacity through depth of colour rather than scattering. This is why going from white to navy may need 2–3 coats, while going navy to navy (same colour) may need only 1.
How much does painting a house exterior cost to estimate materials?
Average house exterior in UK: 200–400 m². At masonry coverage 6 m²/L × 0.85 = 5.1 m²/L practical: 300 m² ÷ 5.1 = 58.8 L per coat × 2 = 117.6 L. At £10–15/L = £1,176–1,764 materials. Plus labour (trade painter: £250–500/day, typically 4–8 days for application).
What is the dry film thickness and why does it matter?
Dry film thickness (DFT) is the thickness of the paint layer once solvents have evaporated. Typical emulsion: 25–30 µm per coat. Anti-corrosion coatings: 50–200 µm. DFT determines durability, hiding power, and corrosion protection. Too thin = poor performance; too thick = sagging, cracking, or lifting on smooth surfaces.
How many gallons are in a litre of paint?
1 US gallon = 3.78541 litres, so 1 litre ≈ 0.2642 US gallons. This calculator lets you switch between metric (litres, m²/L) and imperial (gallons, ft²/gal) unit systems — it converts your surface area and coverage rate automatically rather than making you do the gallon-to-litre conversion by hand.
How many gallons is 50 litres of paint?
50 litres × 0.2642 = about 13.2 US gallons. For a large exterior job estimated at 50 L in metric mode, switching this calculator to imperial units gives you the equivalent gallon figure directly, using the imperial coverage rate (ft²/gal) instead of converting the litre result by hand.
How many gallons is 5 litres (5L) of paint?
5 litres × 0.2642 ≈ 1.32 US gallons — close to, but not exactly, a standard 1-gallon (3.785 L) US paint can. A 5 L tin (common in metric markets) is slightly larger than a US 1-gallon can.
How many gallons is 20 litres of paint?
20 litres × 0.2642 ≈ 5.28 US gallons. A 20 L pail is a common trade/bulk paint size in metric markets, roughly equivalent to five-and-a-quarter US gallon cans.
How much paint do I need per room?
A standard room (12x12 ft, 8 ft ceiling) needs approximately 1.5-2 gallons for two coats, minus door and window areas.
What is typical paint coverage per gallon?
Most paints cover 350-400 sq ft per gallon on smooth, previously painted surfaces. Rough or porous surfaces require more.
Do I need a primer coat?
Primer is recommended for new drywall, dark-to-light color changes, stained surfaces, and bare wood to ensure adhesion and color accuracy.
How many coats are typically needed?
Two coats are standard for most color changes. Dramatic color shifts may require three coats for even coverage.
What paint finish for bathrooms?
Semi-gloss or satin finishes resist moisture and are easier to clean, making them ideal for bathrooms, kitchens, and trim.
How do I calculate wall area?
Wall area = 2 × (length + width) × height. Subtract 20 sq ft per door and 15 sq ft per window.
What is the difference between flat and matte?
Both are low-sheen but flat has virtually no sheen; matte has a slight sheen. Flat hides imperfections better; matte is slightly more washable.
How long does exterior paint last?
Quality exterior paint lasts 7-10 years; the finish type, climate, and surface preparation significantly affect longevity.
Should I factor in waste?
Add 10% to your calculated amount for cuts-in, drips, touch-ups, and any overage on textured surfaces.
What is VOC in paint?
Volatile organic compounds (VOCs) are solvents released during drying. Low-VOC and zero-VOC options are healthier and meet strict emissions regulations.
How do I estimate paint cost?
Multiply gallon count by price per gallon; premium paints cost more upfront but often cover better and last longer.
Can leftover paint be stored?
Yes. Seal tightly, store at 50-80°F, away from freezing. Properly stored latex paint lasts 2-5 years; oil paint up to 15 years.
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