How Paint Coverage Is Calculated
The paintable area is calculated by working out the total surface area of the surfaces you are painting, then subtracting the area of doors and windows that will not be painted with the main colour.
Formulas used:
- Wall area: 2 × (L + W) × H (perimeter of room × ceiling height)
- Ceiling area: L × W
- Door deduction: Each door ≈ 20 sq ft (standard 32"×80" door)
- Window deduction: Each window ≈ 15 sq ft (standard double-hung window)
- Gallons: (Paintable area × coats) ÷ coverage per gallon, rounded up to next quarter-gallon
Standard coverage for quality interior latex paint is 350–400 sq ft per gallon on smooth surfaces. On rough surfaces (textured walls, bare drywall, masonry) coverage drops to 200–300 sq ft per gallon — adjust the coverage input accordingly.
Worked example: 14 × 12 ft bedroom
The walls of a 14 × 12 ft bedroom with 8 ft ceilings, one door and two windows are getting 2 coats of paint rated at 350 sq ft per gallon, priced at $45 a gallon.
- Gross wall area: 2 × (14 + 12) × 8 = 416 sq ft
- Less 1 door (20 sq ft) and 2 windows (30 sq ft): 366 sq ft net
- 366 × 2 coats ÷ 350 = 2.09 gallons, rounded up to 2.25 gallons
- Estimated cost: $101.25
In practice that is 2 gallons plus 1 quart. Paint the ceiling with a separate product: switch the surface to Ceiling only and run the numbers again for it.
Choosing the Right Number of Coats
The number of coats needed depends on several factors:
- 1 coat: Only appropriate for touch-ups over the same colour, or when using a paint+primer on a previously painted surface with the same sheen. Rarely gives complete coverage.
- 2 coats: Standard for most repaints. Always required when changing colour, switching sheen levels or painting over a repair patch.
- 3 coats: Recommended when painting over dark colours with a light colour (especially white over red or deep blue), on raw drywall without a dedicated primer coat, or for high-traffic areas like hallways.
The primer question: If you are painting bare drywall or plaster, count a primer coat separately. Primer has lower coverage (250–300 sq ft/gallon) and is not interchangeable with paint in the calculator. Add a separate calculation for the primer.
Tips for Estimating and Buying Paint
Buying the right quantity saves both money and the hassle of return trips mid-project:
- Round up, not down: The calculator rounds up to the nearest quarter-gallon. Buy that amount; having a small amount left over is useful for touch-ups.
- Batch consistency: If the job requires multiple cans of the same colour, buy them all at the same time from the same batch. Tint batches can vary slightly; box (mix) all cans together before starting.
- Sheen affects coverage: Flat/matte paint covers better (more solids) than semi-gloss or gloss. High-sheen paints may need an extra coat.
- Write down the formula: Note the paint formula (on the lid) and the store code so you can match it exactly for touch-ups years later.
- Cost estimator: Enter your price per gallon to get a total material cost estimate. Professional painters typically mark up materials by 20–30% on top of labour.
Frequently asked questions
How many gallons of paint do I need for a 12x12 room?
A 12×12 room with 8 ft ceilings has approximately 384 sq ft of wall area. Subtract two standard doors (40 sq ft) and two windows (30 sq ft) = 314 sq ft net. At 350 sq ft/gallon and 2 coats, you need 628 sq ft ÷ 350 = 1.8 gallons, rounded up to 2 gallons of wall paint. Add 1 gallon for the ceiling (144 sq ft at 2 coats = 0.82 gallons).
Does paint coverage differ between brands?
Yes. Premium paints typically cover 350–400 sq ft per gallon due to higher solids content. Budget paints may cover only 250–300 sq ft. Check the can label for the manufacturer's coverage rating and use that number in the coverage field. High-quality paints also tend to cover in fewer coats, so the total cost per sq ft is often similar.
Why does the calculator round up to the nearest quarter-gallon?
Most paint stores sell paint in quarts (0.25 gallon) and gallons. Rounding up to the nearest quarter-gallon means you are always buying a standard container size without wasting money on a full extra gallon when you only need a quart.
Should I include the ceiling in the same calculation as the walls?
Usually no — ceiling paint is a different product (flat sheen, often bright white) and has different coverage than wall paint. Use the "Both" option to estimate total area, but buy the paints separately. Ceiling paint is almost always applied in 1 coat over an existing white ceiling, 2 coats if repainting a non-white ceiling.
How much paint does trim and baseboard require?
Trim (door casings, window trim, baseboards) uses about 1 quart per standard room for a single coat. Semi-gloss or gloss paint is standard for trim. For a room with 4 door casings, 2 window casings and 48 linear feet of baseboard, plan on 0.5–1 gallon of trim paint at 2 coats.