What this calculator does
The Paint Coverage and Cost Calculator estimates how much paint you need for a single wall, a whole room of four walls, a ceiling, trim, or a directly entered area — then tells you how many gallons (or liters) to buy and the total cost. It is a universal geometry tool: defaults follow common US painting practice (square feet, gallons, about 350 ft² per gallon), but a Metric mode works in meters, square meters, and liters.
How to use it
Pick your project type and unit system. Enter the dimensions for that project — each dimension has its own unit dropdown, so you can mix feet and inches if needed. For walls and rooms, add the number of doors and windows to subtract standard openings (a door is assumed to be 21 ft² and a window 15 ft²). Set the coverage per container, the number of coats, and optionally a price per gallon or liter to get an estimated cost.
The formula explained
First the paintable area \(A\) is computed from your inputs and openings are subtracted. The exact paint volume is $$V = \frac{A \times \text{coats}}{\text{coverage}}$$ Because you can only buy whole containers, the gallons to buy is the ceiling of \(V\). Total cost is that rounded number of containers multiplied by your price: $$\text{Cost} = \lceil V \rceil \times P$$
Worked example
A room 12 ft by 10 ft with 8 ft walls, 2 doors and 2 windows: perimeter = \(2 \times (12 + 10) = 44\) ft, gross wall area = \(44 \times 8 = 352\) ft². Subtract 2 doors (42 ft²) and 2 windows (30 ft²) to get 280 ft². With 2 coats at 350 ft²/gallon: $$V = \frac{280 \times 2}{350} = 1.6 \text{ gallons}$$ so buy 2 gallons. At $30/gallon the estimated cost is $60.
FAQ
Why round up to whole gallons? Paint is sold in whole containers, so the purchase quantity is always rounded up. The exact decimal is shown too, so you know how much you actually use.
Are the door and window sizes editable? They use standard assumptions (3×7 ft door, 3×5 ft window). If your openings differ, use the Calculated Area mode and subtract openings yourself.
Why is the ceiling not included for a room? The room mode paints only the four walls. Add a separate ceiling calculation if you also paint the ceiling.
