How much paint do you actually need for a room in Australia?

Paint quantity is one of those things that seems straightforward until you're standing in Bunnings trying to decide between a 4 L and a 10 L tin of a colour you can't return once opened. Get it wrong and you're either making a second trip for a top-up or storing leftover paint in the garage for a decade.

I've painted enough rooms to know the calculation matters — and that most people underestimate how much they need, particularly when covering a dark colour with a light one or painting bare plaster for the first time.

The basic formula

Start with your paintable area in square metres. For walls: add up the perimeter of the room, multiply by the ceiling height, then subtract doors (roughly 2 m²each) and windows (roughly 1.5 m² each). For ceilings: length × width.

Then divide by your paint's coverage rate, which you'll find on the tin. Most standard interior paints cover 12–16 m² per litre per coat on a smooth, previously painted surface. That's the number from the tin — real-world coverage is usually lower due to surface texture, absorption and brush-versus-roller application method.

For a two-coat job (which is standard), multiply by two. Then add 10% for waste, touch-ups and the fact that real surfaces are never perfectly smooth.

Coverage rates by paint type

The coverage rate on the tin is for ideal conditions — smooth surface, consistent application, single coat. Real-world rates are lower. Here's a practical guide:

• Premium interior acrylic (Dulux Wash&Wear, Taubmans Endure): 12–14 m²/L per coat on previously painted surfaces. Thick formula, good hiding power.
• Standard interior acrylic: 10–12 m²/L per coat. Fine for low-traffic areas.
• Ceiling paint: 12–16 m²/L per coat. Usually thicker consistency than wall paint — buy ceiling paint specifically, not wall paint for ceilings.
• Primer/sealer: 8–12 m²/L. Always use a primer on bare plaster, new plasterboard, or when making a dramatic colour change. Skipping primer on bare surfaces will cost you an extra coat of topcoat.
• Enamel/semi-gloss (doors, trims, skirtings): 12–14 m²/L but applied thinner — use the lower end of the range for planning.

How many coats do you actually need?

This is the variable most people get wrong — assuming two coats is always enough. Sometimes it is. Often it isn't.

• Previously painted surface, similar colour: 2 coats of quality paint is usually enough.
• Light colour over dark (e.g. white over charcoal): 3 coats minimum, possibly 4. This is where people run short. Tint your undercoat or use a high-hide primer first to reduce the number of topcoats needed.
• Dark colour over light: 2 coats usually sufficient — dark pigments have good hiding power.
• Bare plaster or new plasterboard: 1 coat of sealer/primer + 2 coats topcoat. The plaster will absorb the first coat completely — don't skip the primer and expect to get away with two topcoats.
• Textured surfaces (render, rough plaster): Add 20–30% to your quantity estimate. Texture dramatically increases surface area and absorbs more paint.

Australian tin sizes and what to buy

Australian paint is sold in standard tin sizes: 1 L, 2 L, 4 L, 10 L and 15 L. The 10 L tin almost always offers the best value per litre for any job over about 25 m² — the price-per-litre drops significantly at the 10 L size compared to buying multiple 4 L tins.

One thing worth knowing: always buy from the same batch number if you're buying multiple tins. Colour-matched paint can vary slightly between batch runs, and the difference is visible on the wall. The batch number is printed on the lid. If you need more paint mid-job, take your empty tin back to the store so they can match the batch as closely as possible.

For colour-matched paints (custom colours mixed in-store), buy slightly more than you think you need upfront. Getting an exact colour match on a second visit is possible but not guaranteed — the mixing ratios involve small amounts of multiple tints and the result can vary slightly.

Feature walls — the planning trap

Feature walls are where DIY paint jobs most often go wrong on quantity. A few things to know:

• A feature wall in a contrasting colour to the rest of the room needs to be calculated separately — don't lump it in with the main wall colour.
• Dark feature wall colours (deep navy, charcoal, forest green) often need 3 coats on a previously light-coloured wall. Budget for it.
• If painting a feature wall and the adjacent walls in different sheens (e.g. feature in satin, others in low-sheen), buy them separately and keep clear of which tin is which during the job.
• For a 3 m × 2.4 m feature wall in a deep colour over a light base: expect to need about 2–3 L for 3 coats — more if the surface is textured.

Queensland-specific tip — humidity and drying time

In Queensland's subtropical climate, humidity affects paint drying time significantly. High humidity (above 85%) slows drying and can cause paint to blush — a milky, cloudy appearance in the finish. During Queensland's wet season (November through April), paint in the coolest part of the day, ensure good ventilation, and allow longer drying time between coats than the tin specifies. If you're painting in a humid room (bathroom, laundry), use a paint formulated for wet areas — standard interior acrylic isn't mould-resistant.

Common mistakes and how to avoid them

• Buying too little and having to make a second run mid-job. Always round up to the next tin size. The difference in cost between a 4 L and a 10 L is far less than the time of making a second trip and the risk of a colour batch mismatch.
• Using wall paint on ceilings. Ceiling paint is formulated differently — it's thicker so it doesn't drip, and it dries flat to hide surface imperfections. Using wall paint on a ceiling gives an inferior result.
• Skipping prep. Paint adheres to clean, lightly sanded surfaces. Washing walls with sugar soap before painting and sanding glossy surfaces improves adhesion dramatically. Skipping prep means the paint won't last.
• Not stirring the paint. Pigment settles in the tin — stir thoroughly before use and occasionally during, particularly with deep colours where the pigment is heavy.

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