Paint Area Calculator

A paint area calculator is the single most useful tool for stopping the two classic DIY decorator mistakes: buying far more paint than you need and watching the leftovers go hard in the shed, and buying too little and having to abandon a Saturday afternoon to drive back to the store for one more tin. Plug the wall heights, wall widths, door and window dimensions, and the number of coats into this calculator and it tells you exactly how many litres (or gallons) you need to buy, with a sensible waste allowance and a tin-break-even table that shows whether you should buy one 10 L tin or two 5 L tins or five 2.5 L tins.

Paint is sold by the litre in most countries and by the gallon in the United States, and the coverage rate varies by surface, paint quality, and application method. The default 10 m² per litre is the standard single-coat coverage that every major brand (Dulux, Crown, Leyland, Behr, Benjamin Moore, Sherwin-Williams) prints on the back of the can for a smooth drywall surface at roughly 100 µm wet-film thickness. Eggshell, satin, and semi-gloss paints go on slightly thinner per coat, so they cover 12 to 14 m² per litre; rough plaster, orange-peel texture, masonry, and bare brick absorb more, so they cover closer to 8 m² per litre. The preset selector on this calculator lets you switch between those common surfaces without re-typing the number.

How to Use the Paint Area Calculator

1. Pick the unit system, metric (metres, square metres, litres) or imperial (feet, square feet, gallons). The calculator converts internally so the math is identical regardless of which is chosen.
2. Switch to the Walls tab and enter the height of each wall, the width of each wall, and the number of identical walls. For a rectangular room, enter the height once and the width once, with a count of 4. For an L-shaped room, enter two lines (one per wall type) and use the count to mark how many walls of each shape there are.
3. Enter the door and window dimensions as height, width, and count. Each opening is multiplied by its count and subtracted from the gross wall area. Set the count to 0 if you have no doors or no windows.
4. Pick the number of coats. The default is 2, the most common repaint. Use 1 for a refresh over a similar colour, 3 for a dramatic colour change over white primer, or pick a custom count.
5. Choose a coverage preset. The default is smooth drywall at 10 m²/L. Other built-in options are eggshell and satin (12 m²/L), semi-gloss and gloss (14 m²/L), rough plaster or orange-peel texture (8 m²/L), bare masonry and brick (8 m²/L), and primer or sealer (9 m²/L). Pick "Custom" to enter any other coverage rate you want to use.
6. Set the wastage allowance with the slider. The default is 10%, which is appropriate for a roller-and-brush job on a regular rectangular room. Spraying typically needs 20%, and a beginner's first room typically needs 15% to 20% because of drips and corrections.
7. Read the result box for the net area, the multi-coat coverage demand, the effective coverage after waste, and the litres (or gallons) you need to buy. The tin-break-even table below that shows how many 1 L, 2.5 L, 5 L, and 10 L tins you would need and how much would be left over.
8. Switch to the Ceiling tab to calculate the ceiling area separately. Pick Rectangle or L-shape, enter the dimensions, choose the number of coats and the coverage, and the calculator outputs the litres needed for the ceiling.

The whole calculator runs in the browser. For the Paint Area Calculator, Inputs are not stored, not transmitted, and not associated with any account.

The Formula

The single closed-form formula that drives the result box is:

net_m² = Σ(wall_height × wall_width) − Σ(opening_height × opening_width)

Then:

litres_needed = ceil( net_m² × coats / (coverage_m²_per_L × (1 − waste_fraction)) )

Both directions work in any consistent unit. In metric, the coverage is in m² per litre; in imperial, the coverage is in square feet per gallon, and the answer is in gallons. The conversion factors between the two systems are exact by international definition: 1 m = 3.28083989501 ft (NIST SP 811), 1 gallon = 3.785411784 L (NIST SP 811), 1 ft² = 0.09290304 m² (exact).

The wastage fraction is a multiplier on the denominator that accounts for the paint that stays in the rim of the tin, on the roller, on the brush, and on the drop cloth. The default 10% is appropriate for a standard roller-and-brush job. A first-time painter with no experience typically needs 15% to 20%. A professional sprayer typically needs 20% to 30% because of overspray drift. The slider on the calculator covers 0% to 20%, which is the realistic range for DIY and most residential contractor work.

The ceiling tab uses the same formula but with a different area calculation. For a rectangular ceiling, the area is length × width. For an L-shape, the area is the sum of the two legs, where each leg is its own length × width. The two legs share one edge along the corner of the L, which is why the total area is the simple sum of the two rectangles, no overlap.

Worked Examples

Example 1, 4 × 3 m room, 2-coat repaint. A typical bedroom has four walls, each 4 m wide and 3 m tall. The gross wall area is 4 × (4 × 3) = 48 m². Subtract one door (2 × 0.9 = 1.8 m²) and one window (1.2 × 1.2 = 1.44 m²), giving 3.24 m² of deductions. The net area to paint is 48 − 3.24 = 44.76 m². At 2 coats, the demand is 89.52 m². With a 10% waste factor, the effective coverage is 10 × 0.9 = 9 m² per litre. The litres needed is ceil(89.52 / 9) = ceil(9.947) = 10 L. Buy one 10 L tin.

Example 2, 5 × 5 m room, 2-coat repaint. A larger room has four walls, each 5 m wide and 5 m tall. The gross wall area is 4 × (5 × 5) = 100 m². Subtract the same door and window openings (3.24 m²), giving a net area of 96.76 m². At 2 coats, the demand is 193.52 m². The litres needed is ceil(193.52 / 9) = ceil(21.502) = 22 L. Buy two 10 L tins, or three 5 L tins, or one 10 L + three 2.5 L tins.

Example 3, 12 × 12 ft room, 2-coat repaint. The American equivalent of Example 1 has four walls, each 12 ft wide and 8 ft tall. The gross wall area is 4 × (12 × 8) = 384 sq ft. Subtract one door (6.8 × 2.8 = 19.04 sq ft) and one window (4 × 3 = 12 sq ft), giving 31.04 sq ft of deductions. The net area to paint is 384 − 31.04 = 352.96 sq ft. At 2 coats, the demand is 705.92 sq ft. The default US coverage for smooth drywall is 350 sq ft per gallon, so with a 10% waste factor the effective coverage is 350 × 0.9 = 315 sq ft per gallon. The gallons needed is ceil(705.92 / 315) = ceil(2.241) = 3 gallons. Buy three 1-gallon cans, or one 5-gallon pail plus a 1-gallon can if you plan to repaint again later.

Example 4, single accent wall, 1 coat. A 4 m × 2.7 m accent wall behind a sofa, painted one coat in a bold colour over a primed drywall. The gross wall area is 1 × (4 × 2.7) = 10.8 m². There are no doors or windows on this wall. The net area is 10.8 m². At 1 coat, the demand is 10.8 m². With a 5% waste factor on smooth drywall, the litres needed is ceil(10.8 / (10 × 0.95)) = ceil(1.137) = 2 L. Buy one 2.5 L tin.

Example 5, L-shape ceiling, 1 coat. A 5 m × 4 m main rectangle plus a 2 m × 2 m extension. The area is 5 × 4 + 2 × 2 = 20 + 4 = 24 m². At 1 coat with the default coverage of 10 m²/L and 10% waste, the litres needed is ceil(24 / 9) = ceil(2.667) = 3 L. Buy one 5 L tin.

Where Paint Calculators Show Up

Paint estimators are needed anywhere a wall, a ceiling, or a surface is going to be coated with a coloured or protective film. The list below is the practical landscape.

DIY interior repaints. The most common use case. A homeowner or renter needs to estimate how many tins of emulsion to buy for a single room or a whole apartment. The calculator's quick presets cover the standard UK and US bedroom sizes (4 × 3 m, 5 × 5 m, 12 × 12 ft) and the standard interior door and window dimensions.

Professional painting quotes. A painting contractor needs to quote a job accurately so the customer is neither over-charged nor left with a half-tin of paint the contractor has to absorb. The break-even tin table is particularly useful here, it tells the contractor whether to spec one 10 L tin or two 5 L tins, which affects both the price quote and the supplier order.

Primer and sealer jobs. New plaster, new drywall, and bare masonry all need a primer coat before the topcoat. The primer preset on the calculator (9 m²/L) is the standard published coverage rate for diluted PVA primer on new plaster or drywall sealer on bare masonry. For unprimed surfaces, run the calculation once with the topcoat coverage and once with the primer coverage, then add the two requirements together.

Exterior masonry paint. Exterior walls in stone, brick, or rendered (stucco) need a different coverage rate than interior drywall. The masonry preset on the calculator (8 m²/L) is the standard coverage for mineral paint or exterior masonry paint on a primed surface. For bare, unprimed masonry, multiply the net area by 1.5 to account for the first coat's absorption.

Ceiling paint. Ceiling paint is typically a flat white emulsion with a high opacity (often called "dead flat" or "contract matt") and is applied in 1 coat over a similar colour or 2 coats over bare plaster. The ceiling tab on the calculator handles the area separately from the walls because most people use a different paint for the ceiling than for the walls.

Trim, doors, and skirting boards. Trim paint is gloss or semi-gloss and is applied with a brush rather than a roller. The coverage is typically 12 to 14 m² per litre because the brush transfers more paint to the surface than a roller does. For a complete estimate, add the trim area (in linear metres times the trim width) to the wall area and use the semi-gloss preset coverage.

Wood stain and varnish. The same formula works for transparent coatings, stain, varnish, wood oil, and Danish oil, by entering the appropriate coverage rate. Stain typically covers 12 to 16 m² per litre depending on the wood porosity; varnish typically covers 10 to 12 m² per litre. The custom coverage option on the calculator lets you enter any value.

Commercial and industrial coatings. Epoxy floor paint, anti-slip deck paint, swimming pool paint, and industrial coatings all have specific coverage rates published by the manufacturer. The calculator's custom coverage option accommodates any of these.

Common Mistakes

Forgetting to subtract doors and windows. The biggest single reason a DIY paint estimate is wrong. A standard 2 m × 0.9 m door is 1.8 m², and a standard 1.2 m × 1.2 m window is 1.44 m². With two doors and three windows, the deduction is 7.92 m², which is roughly 17% of a 4 × 3 m room's gross wall area. Always subtract the openings.

Underestimating wastage. The 10% default is fine for a professional with a good roller technique. A first-time DIY painter should use 15% to 20% because of drips, roller pickup, and the paint that stays in the rim of the tin. A sprayer should use 20% to 30% because of overspray drift.

Using the wrong coverage rate. The matte drywall preset (10 m²/L) does not apply to every surface. Eggshell, satin, and semi-gloss paints cover more area per litre (12 to 14 m²/L) because the wet film is thinner. Rough plaster, orange-peel texture, and bare masonry cover less (8 m²/L) because the surface absorbs more. Pick the preset that matches your actual surface, not the one that matches the paint tin you happened to pick up.

Forgetting the second coat. A refresh over a similar colour might genuinely need only 1 coat, but a colour change to a darker colour over a white wall needs 2 coats minimum, and a colour change to a lighter colour over a dark wall needs 3 coats. The default 2 coats is the correct assumption for most repaints.

Forgetting the ceiling. Many DIY painters forget to budget separately for the ceiling paint. The ceiling tab on this calculator handles the ceiling area separately so you can use a different paint (typically a dead-flat white) and a different coverage rate (typically 10 m²/L for a standard ceiling paint) than the walls.

Forgetting the primer. New plaster, new drywall, bare wood, and bare masonry all need a primer coat before the topcoat. The primer preset (9 m²/L) is the standard published rate. Run the calculation once with the topcoat and once with the primer, then add the two.

Forgetting to round up. The calculator always rounds up to a whole litre (or whole gallon) because nobody buys a half-tin of paint. If the calculation says 9.4 L, buy 10 L. If it says 21.3 L, buy 22 L. The break-even table shows the closest tin combination.

Mixing units mid-calculation. Pick metric or imperial, and stick with it. The calculator converts internally, but the inputs are still in the unit you chose. If you enter wall height in metres but coverage in square feet per gallon, the result will be off by a factor of about 3.28.

Coverage Rates by Surface

The default 10 m² per litre is the standard single-coat coverage on smooth drywall at 100 µm wet-film thickness. Real surfaces vary, and the table below shows the typical coverage range for common surfaces, expressed in m² per litre for a single coat.

Spraying uses 10% to 20% more paint than rolling due to overspray. Brushing uses 5% to 10% more than rolling due to less efficient transfer. The calculator's waste slider covers these adjustments.

Frequently Asked Questions

How much paint do I need for a 4 × 3 m room? A 4 × 3 m room with 2.4 m to 3 m ceilings has 4 walls totaling 4 × (4 × 3) = 48 m² of gross wall area. Subtract one door (1.8 m²) and one window (1.44 m²) for a net area of 44.76 m². At 2 coats with the default 10 m²/L coverage and 10% waste, you need ceil(44.76 × 2 / 9) = 10 L. Buy one 10 L tin or two 5 L tins.

How many square feet does a gallon of paint cover? A gallon of interior latex paint covers 350 to 400 square feet on smooth drywall at 100 µm wet-film thickness. Lower-quality paint covers closer to 300 sq ft per gallon, and higher-quality paint with higher solids content covers closer to 400 sq ft per gallon. The calculator's default of 350 sq ft per gallon is the industry standard estimate.

Do I need to prime first? Yes, in several cases: (1) New drywall requires a primer before paint. (2) New plaster needs a mist coat (diluted paint) followed by a primer. (3) Stained surfaces (water stains, smoke damage, tannin bleed) need a stain-blocking primer. (4) Dramatic colour changes (white to dark or vice versa) benefit from a tinted primer. (5) Bare wood, masonry, and metal need a primer compatible with the topcoat. For unprimed surfaces, add 25% to 50% more paint to the calculation.

How long does it take paint to dry? Latex paint: 1 to 2 hours to touch-dry, 4 hours to recoat. Oil-based paint: 6 to 8 hours to touch-dry, 24 hours to recoat. Humidity and temperature both affect drying time, high humidity slows drying, low temperature slows drying. Plan projects with adequate drying time between coats. Most paint failures are due to insufficient drying time between coats, leading to poor adhesion and visible brush marks.

What is the difference between flat, satin, semi-gloss, and gloss paint? Flat (matte) hides imperfections but is not washable. Best for low-traffic ceilings. Satin (or eggshell in the US) has a subtle sheen, good washability, and is the most common finish for living-room walls. Semi-gloss has a noticeable sheen, excellent washability, and is the standard for kitchens, bathrooms, and trim. Gloss has the highest sheen, is very durable, and is traditional for woodwork and front doors, but it shows every surface imperfection. Most walls use eggshell or satin; kitchens, bathrooms, and trim use semi-gloss.

How do I calculate paint for an exterior project? For exterior walls: length × height for each wall, minus doors and windows. For trim: linear feet of soffits, fascia, and trim. For siding (lap, board-and-batten): the linear feet times the exposure height. Rough surfaces (stucco, cedar shake) need 30% to 50% more paint than smooth surfaces. Most exterior projects need 1 to 2 gallons per 100 to 150 sq ft of wall area, depending on surface type and number of coats.

Should I buy one big tin or several small tins? The break-even table on the calculator shows the answer. A 10 L tin is normally cheaper per litre than a 5 L tin, but it is also more paint than you need for a small room. The general rule: buy one tin of the size that is just larger than the total demand, then a second tin of the smallest size if you need to top up. For a 4 × 3 m room needing 10 L, buy one 10 L tin. For a 5 × 5 m room needing 22 L, buy two 10 L tins. For a small accent wall needing 2 L, buy one 2.5 L tin.

What if my walls are not rectangular? The calculator treats each wall as a rectangle. For a triangular gable wall, calculate the area as base × height / 2 and add it to the deductions-free total manually. For a wall with a sloped ceiling, break it into a rectangle and a triangle and add the two areas. For a circular wall (a tower or a turret), use 2 × π × radius × height. The wall-only tab handles any wall shape for which you can compute the area in square metres.

Can I use this calculator for ceiling paint? Yes. Switch to the Ceiling tab and pick Rectangle or L-shape. Enter the room dimensions, choose the number of coats (typically 1 for a refresh or 2 for a new colour), pick the ceiling paint coverage (typically 10 m²/L for standard dead-flat ceiling emulsion), and read the litres needed. The ceiling area is separate from the wall area so you can use a different paint and a different coverage rate.

How accurate is the 10% waste factor? For a professional with a good roller technique, 10% is accurate to within 2%. For a DIY painter, 10% is accurate to within 5%. For a sprayer, 10% is too low, use 20% to 30% instead. For a beginner's first room, use 15% to 20%. The slider on the calculator covers 0% to 20%, which is the realistic range.

What about textured walls, dado rails, and picture rails? Texture (orange peel, knockdown, popcorn) reduces the effective coverage by 15% to 30%. Pick the rough preset (8 m²/L) for textured walls. Dado rails and picture rails are usually painted with the trim colour (semi-gloss) and can be estimated as linear feet times the rail width. A 100 mm-wide dado rail adds 0.1 m² of painted area per linear metre; a 50 mm-wide picture rail adds 0.05 m² per linear metre.

Is one coat of paint enough? Only in three cases: (1) a refresh over a very similar colour, (2) a paint-and-primer-in-one product on a primed surface, (3) a ceiling paint refresh over a clean white. For all other repaints, two coats is the standard. For a dramatic colour change or a bare porous surface, three coats is not uncommon.

What is the difference between a litre and a quart? One litre is slightly larger than one US quart (1 L = 1.056688 qt). The calculator uses litres and gallons, which are the two standard paint-selling units in the world. A US gallon is exactly 3.785411784 L (NIST SP 811). If you need to convert to quarts, multiply the litre total by 1.0567.

References

• British Coatings Federation (BCF), Typical coverage rates for interior and exterior paints on common substrates.
• PPCA (Paint & Painting Contractors Association, UK), Practical coverage guide for trade painters, including wFT/dFT relationships.
• NACE / SSPC, Protective coatings coverage standards for industrial and marine applications.
• Wikipedia, "Paint", Aggregation of published coverage rates by surface and application method.
• NIST Special Publication 811 (Guide for the Use of the SI Units), exact conversion factors: 1 ft = 0.3048 m, 1 gal = 3.785411784 L.
• BIPM SI Brochure (9th edition, 2019), Sections 2.3.2 and 2.3.3 on non-SI units accepted for use with the SI.
• ASTM D344, Standard Test Method for Relative Hiding Power of Paints by the Visual Evaluation of Brushouts.
• ISO 6504-3:2006, Paints and varnishes, Determination of hiding power, Part 3: Determination of contrast ratio of light-coloured paints at a fixed spreading rate.