Paint calculator

How It Is Calculated

This calculator estimates the number of gallons of paint you need and the total cost. The calculation follows these steps:

1. Find the total area to be painted by multiplying the surface area by the number of coats: A = S × C, where S is the surface area (sq ft) and C is the number of coats.
2. Divide the total area by the paint coverage per gallon to get the raw gallons: G_raw = A / R, where R is the coverage (sq ft/gal).
3. Apply the waste factor: G_waste = G_raw × (1 + W/100), where W is the waste percentage.
4. Round up to the nearest whole gallon because paint is sold in whole gallons: G_needed = ceil(G_waste).
5. Calculate the total cost: P = G_needed × D, where D is the cost per gallon ($).

Assumptions

• The surface area input is the total area to be painted, excluding areas not painted (e.g., windows, doors) unless already subtracted.
• Paint coverage per gallon is based on manufacturer's estimate for a smooth, primed surface; rough or porous surfaces may require more paint.
• The waste factor accounts for spillage, touch-ups, and uneven application; typical values are 10–15%.
• Paint is purchased in whole gallons only; partial gallons are not considered.
• Number of coats is as specified; two coats are typical for most projects.

Worked Example

Suppose you have a surface area of 400 sq ft, you plan to apply 2 coats, the paint covers 350 sq ft per gallon, you allow a 10% waste factor, and the paint costs $40 per gallon.

1. Total area: A = 400 × 2 = 800 sq ft.
2. Raw gallons: G_raw = 800 / 350 ≈ 2.2857 gal.
3. With waste: G_waste = 2.2857 × (1 + 10/100) = 2.2857 × 1.1 ≈ 2.5143 gal.
4. Gallons needed (rounded up): G_needed = ceil(2.5143) = 3 gal.
5. Total cost: P = 3 × 40 = $120.

Thus, you need 3 gallons of paint at a total cost of $120.

Practical Tips

• Always measure your surface area carefully, subtracting windows and doors. For walls, multiply length by height; for ceilings, length by width.
• Buy all paint at once to ensure color consistency. If you need more than one gallon, mix them together in a large bucket to avoid slight color variations.
• Consider using a primer, especially on new drywall or dark colors. Primer can improve coverage and reduce the number of paint coats needed.
• If you are also working on other projects like decking or roofing, plan your purchases together to save on trips.
• Keep leftover paint for touch-ups; label the can with the room and date.