Paint Calculator

How This Calculator Works

Paint needed is a room’s wall area, minus doors and windows, divided by how much area one gallon covers per coat. Enter a room’s length, width, and wall height, plus how many doors and windows it has and how many coats you’re planning, and this calculator estimates how much paint to buy, along with a rough cost range and a few things worth considering before you start.

Doors and windows don’t get painted (at least not with wall paint), so their area is subtracted from the total before estimating how much paint you need — otherwise the estimate would run high for any room with more than the average opening or two.

The Formula

Wall area, then paint volume:

$$\vG{\text{Gross Wall Area}} = \vF{\text{Perimeter}} \times \vC{\text{Wall Height}} = 2 \times (\vA{\text{Length}} + \vB{\text{Width}}) \times \vC{\text{Height}}$$ $$\vH{\text{Net Wall Area}} = \vG{\text{Gross Wall Area}} - (\vD{\text{Doors}} \times {\sim}20 \text{ sq ft}) - (\vE{\text{Windows}} \times {\sim}15 \text{ sq ft})$$ $$\text{Paint Needed} = \frac{\vH{\text{Net Wall Area}} \times \text{Number of Coats}}{\text{Coverage per Gallon}}$$

The coverage figure — about 350 square feet per gallon per coat — is the paint industry’s widely-cited estimate for a smooth interior wall; textured walls or very porous surfaces may need more.

Worked Example

A 13ft × 11.5ft room (about 4m × 3.5m) with 7.9ft walls, 1 door, 1 window, and 2 coats:

1. Perimeter: $2 \times (\vA{13} + \vB{11.5}) = \vF{49} \text{ ft}$.
2. Gross wall area: $\vF{49} \times \vC{7.9} \approx \vG{387} \text{ sq ft}$.
3. Net wall area, after subtracting the door and window: $\vG{387} - 20 - 15 = \vH{352} \text{ sq ft}$.
4. Paint needed: $\vH{352} \times 2 \div 350 \approx 2 \text{ gallons}$ — rounded up to whole gallons for what to actually buy, that’s 3 gallons (paint is sold in whole cans, and running short mid-coat is worse than a little extra left over).