Paint Calculator
Litres or gallons of paint needed for a wall or room
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What this calculator computes
The paint calculator estimates the volume of paint needed to cover a wall, room, or exterior surface to a specified number of coats. Inputs are the wall area (length × height for a single wall, or perimeter × height for a whole room) and the paint's coverage rate, which manufacturers print on the tin and varies with paint chemistry, surface porosity, and application method. Typical coverage rates are 10–14 m² per litre (400–560 ft² per US gallon) for matt and silk emulsion on smooth previously-painted walls, dropping to 6–8 m² per litre on rough or unpainted plaster, and 12 m² per litre for exterior masonry paint on smooth render. The calculator returns the volume needed in litres and US gallons, separated into the first coat and any additional coats so the user can plan the order: typically a 5 L tin handles a 50 m² single-coat job and a 2.5 L tin handles 25 m² with margin. Doors, windows, and other openings should be subtracted from the wall area before the calculation; a typical interior door is about 2 m² and a typical UK domestic window is 1.5–2 m². The calculator allows entering the deduction directly, so the result is the net paintable area times coverage. Coverage rate is the single most variable input — bare drywall and fresh plaster soak up the first coat at 6–8 m²/L, while a previously painted satin or eggshell surface gives the rated 10–14 m²/L.
Calculator
The formula
Formula
paint_volume = (wall_area × coats) / coverage_rate
Worked example
When to use this calculator
Use this calculator before buying paint for any interior or exterior decoration project — bedrooms, living rooms, hallways, exterior walls, ceilings, and feature walls. The most common use is sizing emulsion paint for residential redecoration, where ordering exactly the calculated quantity often leaves no margin for cutting in around trim, second-coat touch-ups, or the inevitable spilled tin. A 10–15% safety margin on top of the calculated figure is sensible for medium-sized rooms, climbing to 20% for new plaster or bare-board substrates that absorb the first coat unevenly. The calculator does not account for primer or undercoat (bare wood and new plaster need a separate primer coat at 8–10 m²/L), nor for trim and woodwork paint, which is purchased separately in smaller tins. For ceilings, calculate area as length × width independently of the wall calculation. For exterior masonry, the rough rendered surface usually halves the rated coverage of smooth-wall paint.
Common input mistakes
- Using the rated coverage on bare or previously unpainted surfaces. Manufacturers' coverage figures (often 12 m²/L or 14 m²/L) assume a smooth previously-painted surface that does not absorb paint into the substrate. Bare drywall, fresh plaster, and rough masonry can drop the first-coat coverage to 6–8 m²/L, so the first coat needs nearly twice as much paint as the rated figure suggests. The second coat then runs at the rated coverage.
- Forgetting to deduct doors and windows. A standard interior door is about 2 m² and a window roughly 1.5–2 m². For a typical bedroom with one door and one window, that is a 3.5–4 m² deduction, which on a 35 m² wall area is 10% of the total. Painting the gross area produces a result 10% high and the surplus paint either dries in the tin or has to be returned.
Frequently asked questions
How much paint do I need for one room?
For a typical UK bedroom (12–15 m² floor, 2.4 m ceiling, one door, one window), two coats of silk emulsion needs about 5–7 L. For a larger lounge (20–25 m² floor) two coats needs 8–10 L. The exact figure depends on coverage rate (10–14 m²/L for emulsion on previously-painted walls, lower on bare plaster) and the number of openings deducted. Ceiling paint is calculated separately as length × width.
What is the paint coverage rate?
Coverage rate is the area one litre or gallon of paint covers to a specified film thickness, printed on the tin. Standard interior emulsion is 10–14 m² per litre (400–560 ft² per US gallon) on smooth surfaces; bathroom and kitchen paints with mildew-resistant additives sit at 8–12 m²/L; exterior masonry paint at 6–10 m²/L on rough render. Bare or freshly plastered surfaces always cover less on the first coat than the rated figure suggests.
Do I need primer or undercoat?
Bare wood, fresh plaster, and surfaces switching from oil-based to water-based paint need a primer or undercoat before the topcoat to ensure adhesion and a uniform finish. Primer coverage is typically 8–10 m²/L, and one coat is enough on most domestic surfaces. Surfaces being repainted in the same product as the existing coat usually skip primer and apply two coats of topcoat directly. Specialist primers (stain-blocking, alkali-resisting) are needed for water-stained, smoke-stained, or freshly-rendered surfaces.
Is one coat enough?
One coat is rarely enough for a uniform finish, particularly when changing colours significantly or painting over a darker existing colour. Two coats is the standard for new decoration, and three coats is occasionally needed for deep colour changes (red over white, dark grey over pale yellow). Some "one-coat" paints achieve full coverage in a single application but typically apply at half the area-per-litre of a standard emulsion, so the total paint volume is similar.
Should I add a safety margin?
Yes — add 10–15% on top of the calculated figure for ordinary domestic redecoration, and 20% for first-time decorating or awkward layouts. Tins of paint do not store well once opened, skinning and developing foul odours within 6–12 months, so over-ordering produces waste. Under-ordering produces a tedious mid-project trip back to the merchant and risks a slight colour-batch mismatch if the production code changes between deliveries. Order at the higher end of the safety range for projects where the substrate is unfamiliar.