Exterior Paint Calculator

How to Calculate Exterior Paint for Your House

Painting the outside of your house is one of the highest-impact home improvements you can make — it protects your walls from weather damage and completely transforms the kerb appeal. For interior rooms, our paint calculator covers wall and ceiling estimates with the same formula. The challenge is working out exactly how much paint to buy. Too little means a return trip to the paint shop mid-project; too much means wasted money sitting in tins in your garage.

Our exterior paint calculator takes the guesswork out of the process. Enter your wall dimensions, choose your surface type, and get an instant estimate for paint volume, primer, can sizes, and total cost.

The Exterior Paint Formula

The formula behind the calculator is straightforward:

Paint Needed (L) = (Net Wall Area x Number of Coats) / Coverage Rate x Waste Factor

Where net wall area equals the gross wall area minus windows and doors:

Net Wall Area = (Wall Height x Wall Length x Number of Walls) - (Windows x 1.5 m²) - (Doors x 1.9 m²)

The coverage rate and waste factor depend on your surface type. Rough and porous surfaces absorb more paint, so the coverage rate drops and you should increase the waste allowance.

Coverage Rates by Surface Type

Different exterior surfaces absorb paint at very different rates. Using the wrong coverage figure is the most common reason DIYers run out of paint halfway through a wall.

These figures assume a mid-range masonry paint or exterior wood paint. Premium paints with higher solid content may stretch 10-15% further per litre.

How Much Paint for Different House Sizes

This table gives ballpark figures for standard 2-coat applications on smooth render, with 15% waste. Your actual requirement will vary based on the number of windows, doors, and the surface texture.

Paint costs are based on mid-range masonry paint at approximately EUR 5-7/L. Premium paints can cost EUR 8-12/L.

Step-by-Step: Measuring Your House for Paint

Getting accurate measurements is the key to an accurate paint estimate.

1. Measure wall height — Measure from the base of the wall to the soffit (underside of the roof overhang). For most two-storey houses this is 5-6 metres. Include any gable ends as a separate calculation using the triangular area formula (base x height / 2).

2. Measure wall length — Measure each wall you plan to paint. For a semi-detached house, you typically paint the front, back, and one side. For a detached house, measure all four sides.

3. Count windows and doors — Each standard window subtracts about 1.5 m² and each door about 1.9 m² from the paintable area. Count carefully — a house with 8 windows and 2 doors has about 15.8 m² of openings.

4. Identify your surface — Is it smooth render, textured render, bare brick, painted brick, or wood cladding? This determines the coverage rate.

5. Choose your coats — 2 coats is standard. Use 3 if covering a dark colour or painting bare masonry for the first time.

Choosing the Right Exterior Paint

Not all exterior paints are the same. The three main categories are:

Masonry paint is designed for rendered walls, brick, and concrete. It is water-based, breathable, and flexible enough to cope with the slight expansion and contraction of masonry in changing temperatures. Look for products with a 15-year guarantee.

Exterior wood paint or stain is formulated for timber cladding, fascias, and window frames. It contains UV filters and fungicide to prevent greying and mould. Available in opaque paint or semi-transparent stain finishes.

Brick paint is a specialist formulation for painting bare brick. It penetrates the porous surface better than standard masonry paint and provides a breathable, waterproof finish. Use it when painting brick for the first time.

Common Exterior Paint Mistakes

Avoid these pitfalls to get a professional finish:

• Skipping the pressure wash — Dirt, algae, and loose material prevent paint adhesion. Always pressure wash and let the wall dry for at least 48 hours before painting.
• Painting in the wrong weather — Paint needs 4-6 hours of dry conditions above 10°C to cure. Rain within 2 hours of application can ruin the finish.
• Ignoring cracks and gaps — Fill hairline cracks with flexible exterior filler before painting. Water enters unfilled cracks and lifts the paint from behind.
• Using interior paint outside — Interior paint lacks UV stabilisers, fungicide, and the flexibility needed for exterior temperature swings. It will crack, peel, and fade within a year.
• Thin coats to save paint — Two proper coats at the correct spread rate last years longer than three thin, stretched coats. Follow the manufacturer's coverage guidance.

While you are working on the exterior, consider the rest of the outside of your home. Our roofing calculator helps estimate tiles and battens if the roof also needs attention, and our fence calculator covers panels, posts, and paint for garden boundaries.

How to Reduce Exterior Paint Waste

The 15% waste factor in our calculator accounts for paint left in trays, on rollers, in crevices, and from uneven application. You can reduce waste by:

• Using a paint grid instead of a tray — less paint pooling at the bottom
• Applying paint with an airless sprayer for large flat areas — more even coverage, less roller stipple
• Buying the right nap length for your surface — 18 mm for textured masonry, 12 mm for smooth render
• Keeping the paint well-stirred throughout the job — settled paint gives inconsistent coverage

If you are painting a large house or multiple surfaces, consider getting professional quotes. A professional painter can often buy trade paint at lower prices and will complete the job faster with less waste.