Paint Coverage Calculator

How Paint Coverage Is Calculated

The standard coverage rate for interior latex paint is 350 to 400 square feet per gallon on smooth surfaces. This calculator uses 375 square feet per gallon as the baseline, which accounts for the typical variation between brands and surface textures. One gallon covers roughly 35 square metres. Premium paints with higher pigment density often cover 400 or more square feet, while budget paints may need an extra coat to achieve full coverage.

The calculator computes wall area from the room perimeter (2 times length plus 2 times width) multiplied by wall height. It then subtracts standard-size deductions for doors (21 square feet each, based on a 3 by 7 foot door) and windows (12 square feet each, based on a 3 by 4 foot window). If you are also painting the ceiling, it adds the floor area (length times width). Trim and baseboards add the perimeter multiplied by a half-foot strip height.

How Many Coats Do You Actually Need

One coat is rarely sufficient for a finished look unless you are touching up an existing colour with the exact same paint. Two coats is the standard for most repainting projects: the first coat seals the surface and provides base coverage, the second coat delivers even colour and full opacity. Three coats are recommended when painting a dark colour over a light one, or vice versa, and when using certain reds, oranges, or yellows that have poor hiding power.

If you are covering new drywall or bare wood, add a coat of primer before your colour coats. Primer is cheaper than paint and provides a better base for adhesion and colour accuracy. Self-priming paints claim to eliminate this step, but professional painters still recommend separate primer on new surfaces for the best results.

Factors That Affect Paint Usage

• Surface texture: Textured walls, popcorn ceilings, and stucco absorb more paint than smooth drywall. Add 20 to 30 percent for heavily textured surfaces.
• Colour change: Going from dark to light (or light to dark) reduces effective coverage. Budget for an extra coat or use a tinted primer.
• Application method: Rollers use paint most efficiently. Brushes use slightly more due to absorption. Sprayers can waste 20 to 40 percent through overspray unless you are experienced.
• Paint quality: Higher-quality paints have more pigment and binders, covering in fewer coats. The per-gallon cost is higher but total project cost is often lower because you buy fewer gallons.

Walls, Ceiling, and Trim

This calculator lets you choose which surfaces to paint. In many projects, walls and trim are painted different colours, and the ceiling gets a flat white. Calculate each separately with the appropriate colour and sheen. Ceiling paint is typically sold in flat finish with high hiding power and lower splatter formulation. Trim paint is usually semi-gloss or satin for durability and easy cleaning.

Money-Saving Tips

• Buy slightly more than you need: Having a quarter gallon left over for touch-ups is better than buying a whole extra gallon later when the batch might not match perfectly.
• Mix all cans together: Pour all your cans into a larger container and stir (called boxing). This ensures uniform colour even if individual cans vary slightly.
• Store leftover paint properly: Seal cans tightly and store them upside down in a climate-controlled space. Properly stored latex paint lasts up to 10 years.
• Return unopened cans: Most hardware stores accept returns of unopened, unmixed paint within a reasonable period. Buy one extra can as insurance and return it if unused.