Calculating how much paint you need for exterior walls is straightforward once you know your wall area and the paint's spread rate. Get it right and you'll avoid costly over-ordering or frustrating mid-job shortages. Here's a step-by-step guide tailored for New Zealand homes and conditions.
Step 1: Measure Your Wall Area
Start by measuring the perimeter of your home and multiplying it by the wall height (from ground level to the eaves). This gives you your gross wall area in square metres (m²).
Example:
- House perimeter: 40 m
- Wall height: 2.7 m
- Gross wall area: 40 × 2.7 = 108 m²
For gable ends (the triangular sections under a pitched roof), calculate the area of each triangle using: ½ × base × height, then add these to your total.
Step 2: Subtract Doors and Windows
Deduct the area of all windows and doors from your gross wall area. As a rough guide, use these approximate sizes:
| Opening | Approximate Area |
|---|---|
| Standard door | 2.0 m² |
| Small window | 0.5–1.0 m² |
| Large window | 1.5–2.5 m² |
| Sliding door/ranch slider | 3.5–5.0 m² |
Add up all your openings and subtract the total from your gross wall area to get your net paintable area.
Step 3: Find the Paint's Spread Rate
Every paint tin lists a spread rate (also called coverage rate) in m² per litre. This is typically found on the label or the manufacturer's technical data sheet. Exterior paints commonly cover somewhere in the range of 8–14 m² per litre per coat, but this varies significantly by:
- Surface texture — rough surfaces like roughcast, brick, or fibre cement absorb more paint than smooth weatherboards
- Paint type — primers and sealers often have lower spread rates than topcoats
- Application method — spraying can use more paint than rolling or brushing
Always check the specific product's data sheet. Resene, Dulux, and Wattyl all publish detailed technical information online for their exterior paint ranges.
Step 4: Calculate Litres Per Coat
Divide your net paintable area by the spread rate:
Litres needed (per coat) = Net area ÷ Spread rate
Example:
- Net area: 95 m²
- Spread rate: 10 m²/litre
- Litres per coat: 95 ÷ 10 = 9.5 litres
Step 5: Account for Number of Coats
Most exterior paint jobs require at least two topcoats, plus a primer or sealer coat if you're painting bare or weathered timber, new fibre cement (such as James Hardie products), or previously unpainted surfaces. Primers often have a lower spread rate, so calculate them separately using the primer's own coverage figure.
Total litres = Litres per coat × Number of coats
Using the example above with two topcoats: 9.5 × 2 = 19 litres of topcoat
Step 6: Add a Wastage Buffer
Always add 10–15% extra to your calculated total. This accounts for:
- Waste from brushing into corners and cutting in
- Uneven absorption on textured or weathered surfaces
- Touch-ups after the job is done
- NZ's variable weather — if rain interrupts your job, you may need to re-coat sooner
Final quantity = Total litres × 1.10 to 1.15
In our example: 19 × 1.10 = approximately 21 litres of topcoat.
NZ-Specific Considerations
New Zealand's climate varies enormously — from the humid, salty air of coastal areas to the UV-intense conditions of Central Otago and the wet winters of the West Coast. These factors affect both how much paint you need and which product you should choose:
- Coastal homes may need more frequent repainting and benefit from paints with added mould and salt resistance
- Rough-textured claddings (roughcast, brick, textured fibre cement) can use 20–30% more paint than smooth surfaces
- Bare or weathered timber weatherboards will absorb significantly more primer than previously painted surfaces
For homes with fibre cement cladding, always follow the manufacturer's priming and painting requirements — incorrect preparation can void warranties and may affect compliance with the NZ Building Code (particularly Clause E2: External Moisture).
BRANZ is an excellent NZ resource for guidance on exterior cladding maintenance and paint systems.
Useful Online Tools
Several NZ paint brands offer free online paint calculators that do the maths for you. Check the websites of Resene, Dulux, and Wattyl — these tools are a handy cross-check against your manual calculation. You can also find paints and primers at Bunnings, Mitre 10, and PlaceMakers.
When to Call a Professional
While painting exterior walls is generally a DIY-friendly task, there are situations where it's worth engaging a qualified painter:
- Multi-storey homes — working at height carries serious fall risks. WorkSafe NZ has clear guidelines on safe working at height that all DIYers should read before attempting work above one storey
- Significant surface repairs — rotted weatherboards, cracked roughcast, or failing cladding may indicate underlying moisture issues that need a professional assessment before repainting
- Heritage or complex claddings — some materials require specialist preparation and products
- Large or complex homes — a professional painter will calculate quantities accurately and can often source paint at trade pricing
You can find qualified, local painters and quantity surveyors on tradedirectory.co.nz to help with your project.
Site Safe
Licensed Building Practitioner (LBP)
ISO 9001 / 45001 / 14001
NZ Institute of Building (NZIOB)
