Paint Calculator
Enter your room size and how many coats you plan to apply. The calculator subtracts doors and windows, adds the ceiling if you want, and tells you how many gallons to buy and what it will cost.
How to use the paint calculator
- Measure the length and width of the floor and the height of the walls.
- Count doors and windows. The calculator removes about 21 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window.
- Choose the number of coats. Two coats is normal for a color change; one coat can be enough when repainting the same color.
- Type the coverage from your paint can and the price. The result updates instantly.
The paint formula
Wall area = 2 × (length + width) × height − doors − windows
Paint (gallons) = (wall area + ceiling area) × coats ÷ coverage per gallon
Example
A 12 × 10 ft bedroom with 8 ft walls has 2 × (12 + 10) × 8 = 352 sq ft of wall. Take away one door (21 sq ft) and two windows (30 sq ft) and you have 301 sq ft. Two coats need 602 sq ft of coverage. At 350 sq ft per gallon that is 1.72 gallons, so you buy 2 gallons. Add the ceiling (120 sq ft) and you need 2.4 gallons — buy 3, or 2 gallons plus a quart.
Tips to get the amount right
- Rough or porous walls (new drywall, brick, textured plaster) can drink 20–30% more paint. Use a lower coverage number, such as 250–300 sq ft.
- Dark over light, or light over dark, often needs a primer coat or a third coat.
- Keep a little extra from the same batch for touch-ups; colors can vary slightly between batches.
- Trim and doors are usually painted in a different sheen. A quart covers about 85–100 sq ft.
Frequently asked questions
How many square feet does a gallon of paint cover?
Most interior paints cover about 350–400 square feet per gallon for one coat on smooth, primed walls. Rough or unprimed surfaces can drop to 250–300 square feet.
How much paint do I need for a 12x12 room?
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. After one door and two windows (about 51 sq ft) you have 333 sq ft. Two coats at 350 sq ft per gallon need about 1.9 gallons, so buy 2 gallons for the walls.
Do I need primer?
Use primer on new drywall, bare wood, stains, glossy surfaces, or when making a big color change. Many 'paint and primer in one' products still need two coats.
Should I buy gallons or a 5-gallon bucket?
If you need four or more gallons of the same color, a 5-gallon bucket is usually cheaper per gallon and keeps the color consistent.
Does the calculator include the ceiling?
Only if you choose 'walls + ceiling'. Ceilings are usually painted with flat ceiling paint, so you may want to calculate them separately.