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A paint calculator answers a question that seems simple until you are standing in a hardware aisle holding two tins of slightly different white. How much paint do I actually need? Enter your room dimensions, and a paint calculator returns the litres or gallons required, accounting for wall area, doors, windows, and the number of coats. Whether you are repainting a single bedroom or estimating a full exterior job, the right calculator prevents the two most common and expensive mistakes: running out mid-wall and paying premium prices for a second trip, or buying forty percent more than you needed and watching the excess harden in the shed. This guide explains the formula, the coverage rates, the cost estimates, and the regional differences that determine what your painting project will actually require.
At its core, a paint calculator solves a volume problem from area measurements. It measures the surface you intend to paint, subtracts the areas you will not paint, multiplies by the number of coats, and divides by the coverage rate of the paint you have chosen. The result is a quantity in litres or gallons.
The wall area formula for a standard rectangular room is straightforward:
For example, a room measuring 14 feet by 12 feet with a 9-foot ceiling has a perimeter of 52 feet and a wall area of 468 square feet. If you are also painting the ceiling, add length times width: 14 × 12 = 168 square feet. The total paintable surface is 636 square feet.
Next, subtract the openings. A standard door is approximately 21 square feet (about 2 square metres) and a window averages 15 square feet (about 1.4 square metres). In our example room, one door and two windows reduce the wall area by 51 square feet, bringing the net paintable wall area to 417 square feet. If you plan to paint the door and trim in the same colour, do not subtract the door.
Then divide by the coverage rate. A standard interior paint covers roughly 350 to 400 square feet per gallon per coat, or around 12 to 14 square metres per litre. Using 400 square feet per gallon for a single coat: 417 ÷ 400 = 1.04 gallons. Multiply by two coats: 2.08 gallons. Round up to the nearest purchasable can size, which in this case is three gallons if buying individually, or a two-gallon can plus a quart. A free paint calculator performs this sequence instantly and handles the unit conversions between feet, metres, gallons, and litres.
Coverage rate is the single most variable input in the calculation. It depends on the paint product, the surface texture, and the application method. The theoretical coverage printed on the tin assumes a smooth, non-porous surface and ideal application conditions. Real-world coverage is almost always lower.
The following table summarises typical coverage rates for common paint types across the major English-speaking markets. These figures are practical estimates, not laboratory conditions. Always check the product data sheet for the specific paint you intend to buy.
| Paint Type | Coverage (sq ft/gal) | Coverage (m²/L) | Typical Coats |
|---|---|---|---|
| Interior emulsion (standard) | 350–400 | 10–12 | 2 |
| Interior emulsion (premium) | 400–450 | 12–14 | 2 |
| Exterior acrylic | 300–350 | 8–10 | 2 |
| Primer / sealer | 300–350 | 8–10 | 1 |
| Trim / gloss enamel | 400–450 | 12–14 | 2 |
| Ceiling flat white | 350–400 | 10–12 | 2 |
UK emulsion paints typically cover around 12 to 14 square metres per litre, with premium trade formulations reaching 16 to 17 square metres per litre on smooth surfaces[reference:0][reference:1]. Australian paints often list coverage closer to 16 square metres per litre per coat for interior low-sheen products[reference:2]. Indian paints vary widely by brand and finish, with luxury emulsions covering 110 to 130 square feet per litre practically, and standard emulsions covering 105 to 120 square feet per litre after accounting for textured walls[reference:3].
Surface texture matters enormously. A smooth, previously painted wall will achieve close to the theoretical coverage. Bare plaster, rough masonry, or textured finishes absorb significantly more paint and may require 15 to 20 percent additional product. If you are painting over a surface that has never been painted, reduce your coverage estimate accordingly and add a primer coat.
Once you know the quantity, the next step is the budget. Paint prices vary substantially by country, by brand tier, and by pack size. The following estimates are indicative for standard interior emulsion in 2026 and should be treated as a starting point rather than a quote.
| Country | Budget Paint | Mid-Range Paint | Premium Paint |
|---|---|---|---|
| India (per litre) | ₹120–200 | ₹250–400 | ₹500–800 |
| United Kingdom (per litre) | £4–6 | £7–10 | £12–18 |
| United States (per gallon) | $20–30 | $35–50 | $55–80 |
| Canada (per 3.78 L) | C$40–55 | C$60–80 | C$90–120 |
| Australia (per litre) | A$8–12 | A$14–20 | A$22–35 |
In India, interior emulsion prices range from around ₹180 to ₹355 per litre depending on the brand and finish, with premium luxury paints reaching ₹500 to ₹800 per litre[reference:4][reference:5]. A full 2 BHK flat of approximately 700 square feet requires about 26.7 litres of premium emulsion at two coats, costing between ₹11,500 and ₹13,000 in paint alone[reference:6]. Labour and preparation add significantly to that figure.
Canadian pricing for a standard 3.78-litre can runs from about C$40 for budget paint to C$90–120 for premium formulations[reference:7]. Australian exterior acrylic roof paint costs around A$173 for 10 litres, with 20 litres of exterior acrylic emulsion at approximately A$179[reference:8]. UK trade paints from brands like Dulux Trade can cost £28.90 to £36.12 per 2.5 to 5 litres excluding VAT[reference:9].
These paint costs are only part of the project budget. A paint calculator with cost estimate typically includes only the paint itself. Primer, sandpaper, masking tape, brushes, rollers, trays, and drop sheets add a further 15 to 25 percent to material costs. Professional labour, where required, can double or triple the total.
Two full coats is the professional standard for almost every interior painting project. One coat leaves thin patches, shows roller marks, and fails to achieve the durability the paint is formulated to provide. The second coat builds film thickness and ensures even colour.
There are exceptions. Fresh plaster or bare wood requires a primer coat followed by two topcoats, making three passes in total. A dramatic colour change — painting white over dark red, for example — may need three topcoats to achieve full opacity. The same applies to strong colour transitions in the opposite direction. If you are painting a ceiling, one coat of dedicated ceiling paint is sometimes sufficient because ceilings are not subject to the same wear and tear as walls, but two coats still give a more uniform finish.
Exterior paint calculations follow the same logic but with three important adjustments. First, you measure each side of the building separately rather than treating the perimeter as a single continuous surface. Second, you add the area of gable ends, which are triangular and require a separate calculation: half the base times the height. Third, exterior surfaces are rougher, whether they are rendered masonry, weatherboard, or fibre cement. Practical coverage drops by 10 to 20 percent compared to interior painting.
Exterior projects also almost always require a primer. Bare wood, weathered masonry, and previously unpainted surfaces need a primer or sealer coat to ensure adhesion and to prevent the topcoat from being absorbed unevenly. A date difference calculator is not relevant to paint quantity, but if you are planning a project timeline, it helps you work out drying intervals between coats. Most exterior acrylics require four to six hours between coats, and longer in humid or cool conditions.
Even with a calculator, avoidable errors creep in. These are the ones that cost the most money and cause the most frustration.
Paint is sold in different standard pack sizes depending on where you are. In the United States, the standard can is one gallon (3.78 litres), with quarts for smaller quantities. In the UK and much of Europe, paint is sold in 1-litre, 2.5-litre, 5-litre, and 10-litre tins. In India, common sizes are 1 litre, 4 litres, 10 litres, and 20 litres. Australia uses 1-litre, 4-litre, 10-litre, and 15-litre containers.
Terminology also varies. What Americans call “emulsion” or “latex” paint is called “emulsion” in the UK and “plastic emulsion” in India. “Eggshell” in the US refers to a low-sheen finish; in the UK, it traditionally refers to a solvent-based mid-sheen paint for woodwork. “Gloss” and “satin” are broadly consistent, but the level of sheen can differ between brands and regions. A paint calculator should let you select your local unit system — feet and gallons, metres and litres — so you are not converting manually and introducing errors.
The same calculator serves different purposes depending on the project type.
Interior room repainting. Measure the perimeter, ceiling height, and subtract openings. Two coats of emulsion on walls, plus a separate ceiling calculation if required. This is the most common use case and the one the default calculator handles best.
New construction. Add a primer coat to the calculation. Bare plaster and new drywall absorb paint aggressively. Reduce coverage by 15 to 20 percent for the first coat. Plan for one primer plus two topcoats on all surfaces.
Exterior house painting. Measure each elevation separately, add gables, and apply a 10 to 20 percent coverage reduction for surface roughness. Include primer for bare or weathered surfaces.
Fence and deck staining. Stain coverage is different from paint coverage. A paint calculator designed for walls may not account for the slatted or grooved surfaces of fences and decks. Measure the total surface area including both sides if you are staining both, and check the stain can for its specific coverage rate.
Feature walls. A single accent wall in a darker or bolder colour needs less paint than a full room. Measure only that wall, subtract any openings on it, and calculate two coats. If the base wall colour is light and the feature colour is dark, allow for three coats.
A standard 10x10 foot room with 9-foot ceilings has about 300 square feet of wall area after subtracting a door and window. At 350 square feet per gallon per coat and two coats, you need roughly 1.7 gallons. Buy two gallons to be safe. If you are also painting the ceiling, add 100 square feet and a separate calculation.
It depends on the paint. Standard UK emulsion covers around 12 to 14 square metres per litre. Premium paints can reach 16 to 17 square metres per litre on smooth surfaces. Indian emulsions often cover 10 to 12 square metres per litre practically. Always check the tin for the specific product coverage before calculating.
Two full coats is the professional standard for most projects. Fresh plaster or bare wood may need a primer plus two topcoats. A dramatic colour change from dark to light may need three coats for full opacity. One coat is rarely sufficient for a durable finish.
Yes. A standard door is about 21 square feet and a window averages 15 square feet. Subtracting these prevents overestimating paint by 10 to 15 percent. If you plan to paint the door and trim in the same colour, do not subtract the door from the wall area.
A paint calculator tells you how much paint you need in litres or gallons. A paint cost calculator takes that quantity and multiplies it by the price per litre or gallon, adding labour and materials for a total project estimate. The paint calculator on Calculator200 provides both quantity and cost.
Yes, but exterior calculations need adjustments. Measure each side of the house separately. Add gable areas if present. Exterior surfaces are rougher, so reduce coverage by 10 to 20 percent. Primer is often required for bare wood or masonry, adding one more coat to the calculation.
Ceiling area is simply length times width. Add this to your wall area if you are painting the ceiling. Ceiling paint is usually a separate flat white product with slightly different coverage, so calculate it separately and buy the appropriate product.
In summary, a paint calculator turns a guess into a number you can act on. It handles the arithmetic of wall area, subtracts the openings you will not paint, applies the coverage rate of your chosen product, and multiplies by the coats you intend to apply. The output is a quantity in litres or gallons, and with a cost estimate, a budget figure for the paint itself. Whether you are calculating paint for a single room or a full exterior repaint, the free paint calculator above gives you a precise starting point. Use it before you buy, check the tin for the actual coverage rate, and always keep a little extra for touch-ups. A well-calculated paint job wastes neither paint nor money.