Paint Calculator

1. Calculate the paintable surface area.
2. Subtract doors and windows that won't be painted.
3. Multiply by the number of coats.
4. Divide by the paint's coverage rate.
5. Add any extra allowance you choose.
6. Round up to the container sizes available.

Paint Required = (Net Area × Coats) ÷ Coverage

Example: a 12 × 10 ft room with 8 ft walls has 352 ft² of wall. Subtracting one 3 × 7 ft door and two 3 × 4 ft windows leaves 307 ft². Two coats need 614 ft² of coverage; at 350 ft² per gallon that's about 1.75 gallons, or 1.84 gallons with a 5% allowance — two 1-gallon cans.

For a rectangular room:

Perimeter = 2 × (Length + Width)

Wall Area = Perimeter × Height

Then subtract openings that won't receive the same paint.

Subtract doors and windows only when they won't receive the same paint. For small openings or complex rooms, you may prefer to leave them included as a conservative planning estimate. The calculator never assumes a standard door or window size — enter your own measurements or a total opening area.

Coverage varies by:

• Product
• Substrate
• Texture
• Porosity
• Application method
• Color change

Current Behr guidance lists roughly 250–400 ft² per gallon for several paint products. The calculator's 350 ft²/gal (about 8.59 m²/L) is an example planning value only — use the actual label or specification for your chosen paint.

The calculator doesn't decide this. Actual coat count can depend on the paint product, the current surface, the old and new color, primer and the finish you want. You enter the planned coat count; two coats is a starting default, not a rule.

Primer uses the same area and coverage idea:

Primer Required = (Net Area × Primer Coats) ÷ Primer Coverage

Primer is optional and kept separate from finish paint. Don't assume primer is always required — or that a “paint and primer” product always removes the need for it.

Ceiling area for a rectangular room is Length × Width. Coverage and coat requirements can differ from wall paint, so BuildCalcWorks lets you set separate coats, coverage, containers and price for the ceiling.

Exterior quantity uses the same surface-area principle — add each wall and any triangular gable (0.5 × base × height), then subtract openings. Real-world coverage can vary with siding texture, masonry, porosity, application and product. Current Behr exterior guidance also uses roughly 250–400 ft² per gallon as a product-dependent reference. The calculator doesn't judge weather suitability or preparation needs.

Paint may be sold in quarts, gallons, larger pails or metric containers such as 1, 2.5, 3, 5 and 10 liters. Select the sizes actually available for your product; the calculator always rounds containers up and shows how much extra paint you'd have left.

You can estimate cost from paint containers, primer, trim paint and optional supplies such as tape, rollers and drop cloths. This is a material estimate only.