Paint Calculator

Whether you are refreshing a tired living room, painting a new bedroom, or tackling a whole-house renovation, knowing exactly how much paint to buy is one of the most important steps in any painting project. Buy too little and you will face frustrating mid-project trips to the hardware store, risking a colour mismatch between batches. Buy too much and you end up with expensive leftover paint that clutters your garage for years. The Paint Calculator eliminates that guesswork by calculating your exact paint requirements based on the dimensions of your room and your chosen paint's coverage rate.

Paint coverage — the number of square feet a single gallon can cover — varies significantly by product. Budget flat paints may cover only 250–300 sq ft per gallon on a smooth, primed surface, while premium self-priming paints can stretch to 400–450 sq ft. The industry standard most professionals use is 350 sq ft per gallon, which accounts for typical wall texture and normal absorption. Always check the label of the specific paint you intend to purchase for the manufacturer's stated coverage.

The number of coats required depends on several factors: the colour you are covering, the colour you are applying, and the paint type. Switching from a dark colour to a light one almost always requires at least two coats, sometimes three. Painting over bare drywall or new plaster typically requires a primer coat followed by two finish coats. Going from one similar shade to another may allow you to get away with a single coat if the paint is high-quality and self-priming. As a general rule of thumb, budget for two coats on most residential projects.

This calculator subtracts the area of standard doors (approximately 20 sq ft each) and windows (approximately 15 sq ft each) from your total wall area before computing how many gallons you need. This produces a more accurate estimate than simply multiplying wall area without deductions. In practice, many professional painters skip these deductions as a built-in buffer for touch-ups, edges, and trim; the choice is yours.

For ceilings, run the calculator separately by treating the ceiling as a single rectangular surface (length × width) with one wall of that height. Most ceilings need only one coat of ceiling-specific paint. Trim, baseboards, and doors are usually painted with a semi-gloss or gloss paint and should also be measured and calculated separately.

Once you have your gallon estimate, always round up to the nearest whole gallon. It is wise to keep at least one extra quart or half-gallon of your exact paint colour for future touch-ups — walls get scuffed, and matching paint years later when the formula is discontinued is notoriously difficult. Store leftover paint properly in a sealed container in a climate-controlled space for best longevity.

Planning your paint purchase carefully also helps you compare prices across brands and finishes. A premium paint at $60 per gallon that covers 400 sq ft may end up cheaper than a $35 budget paint covering only 250 sq ft when you factor in total gallons needed and the labour saved by needing fewer coats. Use this calculator as a starting point, then add roughly 10% extra for waste, touch-ups, and any miscalculation in your measurements.