Exterior Paint Calculator
Calculate how many litres of exterior paint and primer you need for your facade. Yield adjusted by surface type, with 10% safety margin.
For a facade of 80 m² with smooth render and two coats of topcoat, you need about 20 L of exterior paint and 13 L of primer, safety margin included. On textured render (scraped or stippled), the same area requires 29 L; on porous surfaces like exposed brick or stone, 35 L. The difference — up to 75% more material for the same area — depends entirely on yield per surface type.
Yield per m²/L is the variable that governs the calculation. Smooth render yields 9 m² per litre per coat; textured render yields 6 m²/L; exposed brick or porous concrete yields 5 m²/L. Calculating with smooth-surface yield on a porous wall underestimates consumption by 44%, which on an 80 m² facade with two coats means 15 L short.
Primer has a fixed yield of 7 m²/L and is calculated separately from topcoat, with the same 10% safety margin. Enter your area, surface type, and number of coats; the calculator returns the exact quantities of paint and primer for one trip to the builder's merchant.
The explanation below uses metric units; the calculator works in either system.
The result updates automatically.
Perimeter × height, minus doors and windows
Filled in automatically from the surface type: about 9 m²/L on smooth render (about 97 sq ft/L). Check the label for the exact figure.
Total (paint + primer)
for 50 m² with 2 finish coats
- Primer/sealer
- 7.9 L
- Finish paint
- 12.2 L
- Safety margin
- 10% included
How it works
Formula
How the calculation works
How to calculate exterior paint for UK facades
Calculating exterior paint seems simple: area times yield. The problem is that most people use the wrong yield. The tin label shows a theoretical value under ideal laboratory conditions, on a smooth, uniform surface. A real facade has irregular render, patches where paint soaks in faster, and always some application waste. This calculator uses realistic yields by surface type, defined by EN 1062, and adds 10% safety margin automatically.
Real yield by surface type
Exterior paint yield varies with surface type. Smooth render (fine scraped or fine float) yields about 9 m² per litre per coat. Textured render (coarse scraped or stippled) drops to 6 m²/L because irregularities increase contact area and paint collects in the texture. Porous surfaces (exposed brick, natural stone, or porous concrete) yield 5 m²/L. Calculating with smooth-surface yield on a porous surface underestimates consumption by 44%. On a 100 m² facade, that means roughly 7 litres short in your estimate.
When to use primer or sealer
Primer is not optional on new surfaces or porous ones. Its role is to penetrate the surface, consolidate loose particles, and create a base to which topcoat adheres properly. Without primer on new render or degraded masonry, topcoat absorption is uneven, resulting in visible patches and shorter durability. Primer yield is lower than topcoat yield (7 m²/L) because its formulation is thinner and penetrates deeper. This calculator treats primer separately, so you know exactly how many litres of each product to buy in one visit.
How to measure facade area correctly
The area entered should be the real painted area, not the gross area. Start with facade perimeter multiplied by usable height, then subtract areas not being painted: doors, windows, and other pre-finished elements. A standard door is about 2 m², a sliding window 1.5 m², and a single-hung window 1 m². On facades with many openings, this subtraction can represent 10 to 20% of gross area. Failing to subtract openings means buying excess paint, which is harmless. Failing to measure the full facade means running out mid-project, which is the worst outcome.
Number of coats for exterior work
On existing facades with paint in good condition, one coat over primer may suffice if the new colour is close to the old. For significant colour change, stain cover, or unpainted masonry, two coats are the minimum recommended. Three coats are needed for heavily degraded paint, render with residual dampness, or dark colours over very light surfaces. This calculator allows 1 to 3 coats, and the total litres includes all coats plus primer, if selected.
Common mistakes in exterior painting
The most frequent error is buying exactly the measured amount, with no margin. This calculator already includes 10%, but those calculating by hand often round down. With 10% margin, an 80 m² facade with two coats on smooth render needs roughly 18 litres instead of the theoretical 17.8. The difference seems small, but it is the difference between finishing and returning to the merchant with the risk of colour mismatch from a different batch.
The second costly mistake is mixing batches. Paint of the same brand name but different production batches can show visible tonal variation in natural light. Buy everything from the same batch and keep one closed tin for future touch-ups.
The third mistake is painting onto damp surfaces. Paint applied over damp render peels easily. In the UK, avoid painting within 48 hours of heavy rain, especially on north-facing facades, which dry more slowly. To check for dampness before painting, press your palm against the render for 10 seconds; if your hand is damp, the surface is not ready.
Surface preparation is non-negotiable
Surface preparation is equally critical to paint quality itself. Any loose paint, algal growth, or powdery render must be removed before priming. A well-prepared substrate ensures proper adhesion and longevity. Many project failures trace to skipped or rushed prep, not poor paint quality. Building Standards (England, Scotland, Wales) and the Building Regulations for Northern Ireland set minimum performance requirements for exterior finishes and require that materials be suitable for their intended use and applied according to the manufacturer's instructions. Follow these requirements exactly; they exist to protect your property from weathering and moisture damage.
Facade assessment before repainting
Before committing to a repaint, assess the existing finish carefully. If the current paint is chalking (white powder on your fingertip after rubbing), it has failed and must be stripped back to sound render or prime coat, depending on severity. If there is algal growth, clean with a soft brush or low-pressure water; high-pressure washing can damage the substrate and trap moisture inside. If there are cracks wider than about 2 mm, they may indicate movement or subsidence; consult a surveyor before painting over them, as the underlying problem must be addressed first. Similarly, if there is obvious damp or staining, identify and remedy the source (leaking gutters, poor drainage, bridged damp-proof course) before repainting; paint applied to damp substrates will fail within months.
Chemical safety and health
Exterior paint contains solvents and biocides. Always work in well-ventilated conditions and wear appropriate personal protective equipment, including gloves and eye protection. Follow the product data sheet for application temperature and humidity ranges; applying paint in unsuitable conditions leads to poor adhesion and premature failure. If the existing paint is lead-based (common on older properties), proper containment and disposal procedures apply; consult the Health and Safety Executive guidance or a specialist contractor. Never assume what the existing finish contains — test it if the property predates 1992.
The Building Research Establishment (BRE) offers detailed guidance on facade assessment, paint selection for UK climates, and durability expectations for different exposure conditions.
Frequently asked questions
How many litres of exterior paint are needed per square metre?
It depends on surface type and number of coats. With two coats and 10% margin already included: smooth render requires 0.24 L/m², textured render 0.37 L/m², and porous surfaces 0.44 L/m². For other coat counts, the calculator adjusts automatically.
What is the difference between exterior and interior paint?
Exterior paint is formulated to resist temperature swings, moisture, UV, and algae or mould growth. Its resins are more flexible, allowing it to move slightly with minor render cracks without peeling. Interior paint lacks this resilience and deteriorates rapidly outdoors.
Does the yield printed on the tin match this calculator?
Not exactly. Tin yield is the theoretical value under ideal conditions: smooth surface, controlled temperature, no application loss. Real-world yield on site is typically 10 to 20% lower. This calculator uses on-site values from EN 1062, with 10% safety margin already built in.
Should I use primer before painting the facade?
On new render, heavily porous surfaces, or where render is degraded, primer improves adhesion and reduces topcoat consumption. On older facades with sound paint, it can be skipped if the surface is clean and non-flaking. When in doubt, apply primer: the cost is low and the risk of topcoat peeling on poorly prepared surfaces is real.
What is the most common mistake when buying exterior paint?
Buying exactly the calculated amount with no margin. If paint runs out before the final coat and the batch is unavailable, the replacement may have a different tone, even with the same product name. Always buy 10 to 15% more and keep one closed tin for future touch-ups.
How long does exterior paint last in the UK climate?
With proper surface preparation and correct application, most exterior paints last 8 to 12 years. On north-facing facades with little sunlight, algal growth may appear before colour fading becomes obvious. On south-facing facades, colour fade is the most common wear pattern. Check manufacturers' guidance for your climate zone.
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By RivoCalc · Reviewed under our editorial methodology · Updated 9 May 2026
This calculator provides estimates for planning purposes only. Actual paint requirements may vary based on surface condition, preparation method, local climate, and application technique. Always follow the paint manufacturer's instructions and product data sheets. Surface preparation is critical to paint durability and must be carried out in accordance with the Building Regulations and Building Standards applicable to your region. For facades with special conditions (rising damp, active algal growth, structural movement, lead-based paint, or heritage status), consult a qualified surveyor or specialist contractor before commencing work.