How to Calculate Paint Coverage for Any Room
In This Article
Why Accurate Paint Estimation Matters
Estimating the amount of paint needed for a room is a common DIY challenge. Buying too little means extra trips to the store and potentially mismatched batches. Buying too much wastes money and creates disposal problems. An accurate calculation saves time, money, and frustration.
The basic formula is simple: divide the total paintable area by the coverage rate of your paint. But several factors affect both the area calculation and the coverage rate, which this guide walks through step by step.
Step 1: Measure the Room
Start by measuring the length and width of the room in metres or feet. Calculate the perimeter by adding all wall lengths. Multiply the perimeter by the ceiling height to get the total wall area. For example, a 5 × 4 m room with 2.5 m ceilings has a perimeter of 18 m, and wall area of 45 m².
Subtract the areas of windows and doors. A standard door is about 2 m², and a typical window is 1.5 m². For the ceiling, multiply length by width (20 m² in our example). For trim, measure the linear length of baseboards, crown moulding, and door/window frames.
Step 2: Know Your Paint Coverage
Most interior paints cover 10-12 m² per litre (or 350-400 ft² per gallon) per coat. This coverage rate assumes a smooth, primed surface. Rough or porous surfaces like textured walls or unpainted drywall may require significantly more paint – sometimes 20-30% more.
Divide your total paintable area by the coverage rate to get the litres needed per coat. Multiply by the number of coats (usually 2 for good coverage). For our example: 45 m² of walls ÷ 11 m²/L = 4.1 L per coat, so about 8.2 L for two coats.
Factors That Affect Paint Usage
Dark colours may require more coats for full coverage, especially when covering light walls or making drastic colour changes. The type of paint also matters – matte finishes tend to hide imperfections better but may require more coats for washability.
Application method affects usage too. Spraying wastes the most paint (up to 30% overspray), rollers use a moderate amount, and brushes are most efficient for small areas. Temperature and humidity can affect drying time and coverage, so follow the manufacturer's recommendations.