Buying too much paint wastes money; buying too little means a trip back to the hardware store mid-job — and the risk the new tin won't match the original batch. Getting the quantity right before you start takes five minutes with the right formula, and it can save you a meaningful amount on a whole-house repaint.
Try it yourself: Use our free Paint Coverage Calculator to calculate exactly how many litres of paint you need for any room or surface — including doors, trim, and ceiling.
The Basic Paint Coverage Formula
Litres needed = Surface area (m²) ÷ Coverage rate (m²/L) × Number of coats
Wall area = (Room perimeter × Wall height) − Door and window area
Room perimeter = 2 × (Length + Width)
Example: Standard bedroom (4m × 3.5m, 2.7m ceiling height)
- Perimeter = 2 × (4 + 3.5) = 15m
- Wall area = 15 × 2.7 = 40.5 m²
- Less one door (1.8m × 0.9m = 1.62 m²) and one window (1.2m × 1.0m = 1.2 m²)
- Net wall area = 40.5 − 1.62 − 1.2 = 37.68 m²
- Two coats at 12 m²/L: 37.68 ÷ 12 × 2 = 6.3 litres
Paint Coverage Rates by Paint Type
Coverage rate — how many square metres a litre of paint covers — varies by product. Always check the manufacturer's specification on the tin, but these are standard guides:
| Paint Type | Typical Coverage | Best For |
|---|---|---|
| Interior flat / matt | 14–16 m²/L | Ceilings, low-traffic walls |
| Interior low-sheen | 12–15 m²/L | Bedrooms, living areas |
| Interior semi-gloss | 10–14 m²/L | Kitchens, bathrooms, halls |
| Interior gloss | 10–12 m²/L | Trim, skirting, doors |
| Exterior flat / low-sheen | 10–14 m²/L | Weatherboard, render |
| Exterior gloss | 10–12 m²/L | Fascia, gutters, doors |
| Primer / undercoat | 10–13 m²/L | Bare surfaces, colour changes |
How Many Coats Do You Need?
The number of coats depends on several factors:
- Same colour, good existing paint: 1–2 coats typically sufficient
- Colour change (light to dark or dark to light): 2–3 coats. Going from dark to light is the hardest — a quality tinted primer significantly reduces the number of topcoats needed.
- Bare plaster, new plaster, or repairs: 1 coat sealer/primer + 2 topcoats
- Previously unpainted or raw timber: 1 coat primer + 1 undercoat + 2 topcoats
- Highly porous surfaces (old render, brick): Apply a diluted first coat (20% water) to seal, then 1–2 full coats
Surface Area Quick Reference
| Item | Typical Area (m²) |
|---|---|
| Standard interior door (per side) | 1.8 |
| Front door (per side) | 2.0 |
| Standard window (including frame) | 1.2–1.8 |
| Skirtings per linear metre | 0.14–0.18 |
| Ceiling (3.5m × 4m room) | 14 |
| Ceiling (4m × 5m room) | 20 |
| Average house exterior (Aus brick veneer) | 180–250 |
Choosing the Right Sheen Level
Paint sheen affects both the look and practicality of the finish. The higher the sheen, the more durable and washable the surface — but the more surface imperfections it will highlight.
- Flat / matt: No sheen. Hides imperfections best. Hard to wash. Best for ceilings and low-traffic walls.
- Low-sheen / eggshell: Slightly reflective. The most popular for Australian living areas and bedrooms. More washable than flat.
- Satin: Noticeable sheen. Good for hallways, kids' rooms, and rental properties — durable and cleanable.
- Semi-gloss: Obvious sheen. Standard for bathrooms, kitchens, laundries, and trims. Very washable, resists moisture.
- Gloss: High sheen. Best for trim, skirting boards, doors, and areas that take a lot of wear. Shows application marks most readily.
How to Reduce Paint Waste
- Buy in the right tin size: Paint comes in 1L, 2L, 4L, 10L, and 15L tins. Calculate your total litres needed, then round up to the nearest available size — not to the next size up. Leftover paint from an unneeded extra tin can't always be returned.
- Keep batch numbers consistent: Buy all tins for a job from the same batch where possible. Slight colour variation between batches is common in custom-mixed paints.
- Store leftovers correctly: Partially used tins last 1–3 years if sealed properly. Press plastic wrap against the paint surface before replacing the lid to minimise skin formation.
- Order an extra 10% buffer: For touch-ups and inevitable miscalculation — especially on textured or heavily porous surfaces.
Frequently Asked Questions
How much does it cost to paint a room in Australia?
A standard 15 m² bedroom with two coats costs approximately $80–$130 in paint materials (quality paint runs $60–$90 per 4L tin). Professional painter labour adds $30–$60 per m² for walls, meaning a full room repaint often costs $600–$1,500 professionally done including materials.
Do I need to prime before painting?
Primer is needed for: bare surfaces (new plaster, bare timber, masonry), dramatic colour changes (particularly dark to light), stain-blocking (water damage, nicotine staining, knots in timber), and when switching from oil-based to water-based paint. For most repaints over the same colour, a quality topcoat without primer is fine.
How do I calculate paint for a sloped ceiling?
Multiply the floor area by a factor of 1.1–1.3 depending on the pitch. For a steep cathedral ceiling, measure the actual slope length rather than using floor dimensions.
What's the coverage difference between roller and brush application?
Roller application typically provides better coverage per litre than brush application because it creates a more even, thinner film. Sprayer application is the most efficient but requires careful masking and more technical skill to avoid drips.
Calculate your paint quantities now
Use our Paint Coverage Calculator to enter your room dimensions and get an exact litre count — including separate calculations for ceiling, walls, and trim.
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