What This Calculator Does
This tool estimates how much paint you need to cover a rectangular wall. Enter the wall's length and height in feet, the coverage rating of your paint (square feet per gallon), and how many coats you plan to apply. It returns the number of gallons required, the wall area, and a rounded-up suggestion of how many cans to buy.
How to Use It
Measure your wall's length and height. Find the coverage rating on your paint can label — most interior paints cover about 350–400 sq ft per gallon. Choose the number of coats (two is typical for full color change or new drywall). The calculator multiplies wall area by coats, then divides by coverage.
The Formula Explained
The painted surface is area = length \(\times\) height. Multiplying by the number of coats gives the total surface the paint must cover. Dividing that total by the coverage per gallon gives the gallons needed:
$$\text{Gallons} = \frac{\text{Length (ft)} \times \text{Height (ft)} \times \text{Coats}}{\text{Coverage (sq ft/gal)}}$$Because paint is sold in whole cans, the "Whole Cans to Buy" row rounds the gallons up to the next whole number.
Worked Example
A wall 12 ft long and 8 ft high has an area of 96 sq ft. With paint rated at 350 sq ft per gallon and a single coat: \(96 \div 350 \approx 0.27\) gallons — so you would buy 1 can. For two coats, the total area is 192 sq ft and you would need about 0.55 gallons, still one gallon can.
FAQ
Should I subtract doors and windows? For a quick estimate you can, but leaving them in adds a small safety margin for touch-ups and waste.
What coverage value should I use? Check the can; smooth surfaces cover more, while textured or porous walls cover less, so round down if unsure.
Why round up the cans? Paint comes in fixed sizes, so you must buy at least enough whole gallons to cover the wall.
