Paint Calculator

Estimate paint for four room walls

Enter the room's inside length, width, and wall height. Subtract the combined area of doors and windows, then set the number of coats and the coverage printed on your paint can.

The wall-area calculation

A rectangular room has two walls matching its length and two matching its width. Wall area is therefore 2 × (length + width) × height. Door and window area is subtracted once, then the remaining surface is multiplied by the number of coats. Dividing by the manufacturer's square-foot coverage gives the gallon estimate.

Worked example for a bedroom

Consider a 12-by-10-foot room with 8-foot walls. Its perimeter is 44 feet, producing 352 square feet of wall. Subtract 40 square feet for a door and window to get 312 paintable square feet. Two coats require 624 square feet of coverage. At 350 square feet per gallon, the estimate is 1.78 gallons, so buying two gallons is the practical starting point.

Coverage on the label matters

Coverage varies with paint, sheen, color change, surface texture, application method, and porosity. Fresh drywall, masonry, rough wood, and heavily patched walls can absorb more paint. A dramatic light-to-dark or dark-to-light change may need primer or another finish coat. Use the actual product label instead of treating 350 square feet as universal.

Measuring doors and windows

Multiply each opening's width by height and add the results. Small openings are sometimes left in the estimate because the extra paint provides a useful reserve. Do not subtract trim when it will receive the same paint. Closets, alcoves, partial walls, vaulted ceilings, and separate accent walls should be measured as additional rectangles.

Buying a sensible amount

Paint is sold in fixed container sizes, and tint batches can vary slightly. Round up to available cans and consider keeping some matching paint for touch-ups. On a large job, ask the retailer about mixing multiple cans together for color consistency. The estimate covers walls only; ceilings, baseboards, doors, cabinets, and trim need separate calculations.

Accuracy and limitations

This is a planning estimate, not a purchase guarantee. Measurements, spills, roller loading, overspray, surface condition, and leftover paint all affect real use. The calculator assumes straight rectangular walls and positive measurements. Confirm hazardous-material, ventilation, preparation, and drying guidance with the product manufacturer.