Paint Calculator

Understanding Paint Coverage

One gallon of interior paint covers approximately 350–400 square feet of wall area per coat. This calculator deducts 20 sq ft per standard door and 15 sq ft per standard window — the "net paintable area." Coverage drops to 300–350 sq ft per gallon when painting over dark colours or raw drywall because more paint is absorbed or required for opacity. Cheap paint is often false economy: higher-quality paints with higher pigment content cover in fewer coats, and the labour time of an extra coat on a large room far exceeds the paint cost difference. For ceilings, use flat finish paint — it hides imperfections better than satin or eggshell.

Primer: When You Need It

Primer is necessary when painting over raw drywall (it seals the porous surface), going dramatically lighter (e.g., dark red to white), painting over glossy surfaces (primer creates tooth for adhesion), covering stains or smoke damage, or transitioning between paint types (oil to latex). You don't need primer when painting a similar colour over existing latex paint in good condition. Tinted primer — primed and colour-matched to your final colour — reduces the number of topcoats needed when covering dark walls. For most standard repaints over existing paint in decent condition, primer adds cost and time without meaningful benefit.

Interior vs Exterior Paint

Interior and exterior paints are formulated for different environments and should not be substituted. Interior paints optimise for scrubbability, low VOC off-gassing, and resistance to staining. Exterior paints contain UV stabilisers, mould inhibitors, and additives that help them flex with temperature changes — properties that make them unsuitable for indoor use due to higher VOC content and off-gassing. Exterior paint used indoors can release fumes for months; interior paint used outdoors will chalk, fade, and peel within 1–2 seasons. Finish matters outdoors: satin and semi-gloss resist moisture and mildew better than flat.

Wall Prep for a Lasting Finish

70% of a good paint job is preparation. Fill nail holes and cracks with lightweight spackle, let dry, and sand smooth. Clean walls with a trisodium phosphate (TSP) solution or sugar soap to remove grease, particularly in kitchens and near light switches. Sand glossy surfaces lightly with 120-grit sandpaper to improve adhesion. Tape trim, ceiling lines, and any areas you want sharp edges on — or cut in freehand with a high-quality angled brush if you have a steady hand. Skipping prep is the most common reason paint peels or shows imperfections, regardless of paint quality.