Paint Calculator

How this paint estimate works

The calculator finds the perimeter of a rectangular room (2 × (length + width)), multiplies it by wall height, and subtracts standard openings (21 sq ft per door, 15 sq ft per window). It then multiplies the remaining area by the number of coats, applies a waste allowance, and divides by the coverage rate printed on your paint can. The result is rounded up to a practical purchase quantity.

A complete room paint shopping list

Walls, ceiling, primer, trim enamel, and door paint are separate line items because they may use different products. Open Separate-product shopping list in the calculator to set each layer's coat count, label coverage, and unit price. The result rounds every product independently; leftover wall paint is not treated as available ceiling paint or primer.

• Walls: use net wall area after subtracting doors and windows.
• Ceiling: uses floor area and its own coating assumptions.
• Primer: covers walls and the included ceiling when selected.
• Trim: uses entered linear length × the visible 3-inch planning width.
• Doors: optionally adds one or two 21 sq ft faces per entered door.

Real example: Three-bedroom house interior

Here is a complete project walkthrough to show how the calculator works for a whole-house paint job, with 2 coats on smooth walls at 350 sq ft/gal and 10% waste.

Total paintable area: 2,258 sq ft. With 2 coats and 10% waste: 14.19 calculated gallons → buy 3 × 5-gallon buckets. The rounded purchase leaves about 0.81 gallon for touch-ups. At $42 per gallon of purchased capacity, the estimated paint cost is approximately $630. Use the multi-room feature for a per-colour container breakdown.

Paint coverage by surface type

Common paint container sizes

How should you round the result?

Paint is sold in fixed container sizes, so the calculated volume must be rounded to containers available locally. For example, 2.3 gallons can be covered by two one-gallon cans and two quart cans. A small surplus from the same batch is useful for touch-ups; compare unit prices, tinting rules, storage needs, and return policies before choosing a five-gallon bucket.

What changes the estimate?

• Surface porosity: New drywall can absorb 25–50% more paint than previously painted walls. Always prime bare drywall before painting.
• Colour change: Painting light over dark usually needs an extra coat or a tinted primer. Dark over light may need only one coat on smooth surfaces.
• Application method: Sprayers use 10–20% more paint than rollers due to overspray. Brushes use less but are slower for large areas.
• Texture depth: Heavy knockdown or popcorn texture can double the surface area compared to smooth walls.
• Paint quality: Premium paints (400+ sq ft/gal) cover significantly more than budget paints (250–300 sq ft/gal). Always check the label.
• Temperature and humidity: Extreme heat or humidity affects paint viscosity and coverage. Paint in 50–85°F with moderate humidity for best results.

Frequently asked questions

Should I buy extra paint "just in case"?

Yes — we recommend 10–15% extra. Different paint batches can vary slightly in colour, and you will need some for future touch-ups. Our calculator includes a 10% waste allowance by default.

What if I measure my room wrong?

Measure twice, paint once. For rectangular rooms, measure length and width at floor level. For wall height, measure in at least two spots — ceiling heights can vary by 1–3 inches. When in doubt, round up. It is far better to have extra paint than to run short mid-wall.

One coat or two coats?

Two coats is the industry standard for even coverage and true colour. One coat may be enough for a same-colour refresh on smooth, clean walls. Dark colours over light may need only one coat; light colours over dark almost always need two or more plus a primer.

Does paint quality affect coverage?

Yes, significantly. Premium paints often cover 400+ sq ft per gallon, while budget paints may cover only 250–300. Always check the manufacturer's stated coverage on the paint can — it is your most reliable number.

Can I use the same paint for walls and ceiling?

Ceiling paint is formulated differently — flatter sheen, less dripping, and often thicker. Plan it independently with the ceiling paint calculator when the product, coverage, colour, or coat count differs from the walls.

Example: a 12 × 12 room

A 12 ft by 12 ft room with 8 ft walls has 384 sq ft of gross wall area. Subtract one door (21 sq ft) and two standard windows (30 sq ft) to get 333 sq ft of net wall area. Two coats at 350 sq ft per gallon with 10% waste require 2.09 gallons, so plan to buy 2 one-gallon cans plus 1 quart can (2.25 gal total).

Common room-size estimates

Before you buy

• Confirm coverage on the specific product label — the most important number in any estimate.
• Use a lower coverage rate for textured or porous surfaces (see table above).
• Calculate repairs, stains, new drywall, or major colour changes separately with the primer calculator.
• Treat doors, trim, cabinets, and ceilings as separate calculations — they use different paints.
• Buy all paint for one colour at the same time to ensure batch consistency.
• Store leftover paint properly: seal the lid tightly and keep in a cool, dry place.