The single biggest mistake DIY painters make
It's not the cutting-in technique or the brush choice. It's miscalculating how much paint to buy. Walk into any hardware store and you'll see the clearance shelf stacked with 10-litre tins of unusual colours — evidence of overbuying. And nothing derails a weekend project faster than running dry halfway through the second coat.
The Paint Calculator does the heavy lifting, but understanding the logic behind it means you can sanity-check any estimate before you hit the checkout.
Coverage rates: what the tin actually means
Paint tins list coverage in square metres per litre (m²/L). A typical interior wall paint might claim 12–16 m²/L. But that figure assumes:
- A smooth, sealed surface
- A single coat
- Optimal application with a quality roller
- No significant colour change between old and new paint
In real-world conditions — textured surfaces, porous substrates, dark-to-light colour changes — effective coverage drops to 8–10 m²/L. Always use the lower end of the range for your estimate.
Step-by-step: calculating paint quantity
For a room, the process is:
- Measure wall area: Add up all wall lengths, multiply by ceiling height. For a standard 4m × 5m room with 2.7m ceilings: (4+5+4+5) × 2.7 = 48.6 m²
- Subtract doors and windows: A standard door is roughly 1.8 m², a standard window 1.4 m². Deduct these from your total.
- Divide by coverage rate: Using 10 m²/L as a conservative rate: 48.6 ÷ 10 = 4.86 litres per coat
- Multiply by number of coats: Two coats = 9.72 litres. Round up to 10 litres.
- Add a wastage buffer: Add 10% for cutting-in waste, touch-ups, and application inefficiency. Final estimate: 11 litres.
For ceilings, add the floor area (4 × 5 = 20 m²) and apply the same formula. Ceilings are often a separate product — ceiling flat white differs from wall paint and shouldn't be substituted.
When you need more coats than expected
Significant colour changes — particularly going from a deep colour to a light one — can require 3 or even 4 coats to achieve full hide. In these situations:
- Ask your paint supplier to tint the undercoat or primer to a mid-tone between old and new colour
- Use a high-hide paint formulation (look for 'maximum hide' or similar on the label)
- Consider a dedicated primer coat before your finish coats
Going from white to dark? Two coats of good quality paint is usually fine. Dark to white? Budget for three coats minimum, or your old colour will bleed through under certain lighting.
Exterior painting: different rules
Exterior surfaces have additional variables:
- Surface porosity: Bare timber, fibre cement, and render are all highly porous and require a suitable primer coat before any finish paint
- Weatherboard texture: Weatherboards have significantly more surface area than their flat projection measurement suggests — add 20–30% to account for overlaps and hidden surfaces
- Fascias and gutters: These require a different product (typically enamel or gloss) and separate calculations
For a full exterior repaint, it's worth investing in a decent brush and roller set. Professional painter brush and roller sets on Amazon Australia give a noticeably better finish than budget hardware-store equivalents and last multiple jobs if cleaned properly.
Trim, doors, and ceilings: separate estimates
Professional painters always calculate walls, ceilings, and trim separately — different products, different coverage rates, different application methods. Use the Paint Calculator for each surface type individually and add them together for your total shopping list.
Related calculations for bigger projects
If you're also doing concrete work (garage floor coating, path painting) or timber structures, use the Concrete Calculator and Timber Calculator to round out your materials estimate for the full project.