Paint and Project Calculators
Quick answer
To estimate paint by hand, measure each wall’s length times height, subtract doors and windows, and total the square footage. One gallon covers roughly 350 square feet per coat on a smooth, primed surface. Assume two coats for most color changes and add 10 percent for waste. Divide your total by 350, round up, and you have your gallon count.
Key takeaways
- Plan on about 350 square feet per gallon per coat on smooth primed surfaces.
- Assume two coats and add 10 percent waste.
- Texture and porosity drop real-world coverage, so round up.
- Interactive calculators are coming online soon; the hand method works today.
The short answer
Buying paint is a math problem most people solve by guessing, which is how you end up with a half-empty gallon or a third trip to the store. The core formula is simple: wall area equals length times height, minus doors and windows. A gallon covers about 350 square feet per coat on a smooth, primed surface, most jobs need two coats, and you should add 10 percent for waste and touch-ups. Work through that and you will rarely be off by more than a gallon.
This page is the manual version of the calculators we are building. The interactive tools will do the arithmetic for you; the assumptions behind them are the same ones below, so the numbers will agree. Start with our start here guide if you are planning a whole project, and check the paint reviews once you know how much you need.
Calculators coming online
These tools are in development and will publish as they are finished — coming soon, not yet linked:
- Paint calculator (walls and ceilings)
- Wall paint calculator
- Cabinet paint calculator
- Paint cost calculator
- Roller cover quantity calculator
- Painting time calculator
- Painting supplies checklist
Each tool will use the same coverage assumptions below, so a hand estimate today and a calculator estimate next month should land on the same gallon count.
How to estimate paint by hand
Use these formulas when the calculators are not handy. A tape measure and a phone calculator are all you need.
- Find each wall’s area. Wall area = length × height. Measure in feet. Add the four walls together for the room total.
- Subtract openings. Deduct the area of doors and windows. A standard door is about 21 square feet; a typical window is about 12 to 15 square feet, but measure yours when it matters.
- Add the ceiling if you are painting it. Ceiling area = length × width of the room.
- Multiply for coats. Two coats is the standard for most color changes and repairs. Multiply your total paintable area by two.
- Add 10 percent waste. Multiply by 1.1 to cover trim, touch-ups, and what stays in the roller and tray.
- Divide by coverage. Divide by about 350 square feet per gallon per coat, then round up to the nearest gallon.
Example: a 12-by-12-foot room with 8-foot ceilings has four walls totaling 384 square feet. Subtract one door (21 square feet) and one window (14 square feet) to get 349 square feet. Two coats and 10 percent waste bring it to about 768 square feet, or roughly three gallons after rounding up.
Coverage assumptions we use
Coverage is not one number. It falls as the surface gets rougher and more porous. These are the standard industry ranges we build into every estimate, stated per gallon per coat.
| Surface texture | Square feet per gallon per coat | What it looks like in practice |
|---|---|---|
| Smooth, primed | 350–400 | New or primed drywall, smooth plaster |
| Medium texture | 250–350 | Standard drywall with light texture, previously painted walls |
| Rough or porous | 200–300 | Heavy texture, bare wood, brick, stucco, bare drywall |
Use the low end when the surface is bare, textured, or very porous, and the high end on smooth, sealed, primed surfaces. Bare surfaces also drink primer, so plan primer as its own coat rather than assuming the topcoat will stretch further.
How to measure a room
Accurate measurement beats every trick for buying the right amount. Do it once, slowly, and the estimate takes care of itself.
- Sketch the room. Draw a rough floor plan and label each wall A, B, C, D. This keeps you from measuring the same wall twice.
- Measure each wall’s length and height. Use a tape measure and round up to the nearest half foot. Ceiling height is usually consistent, but measure at both ends of a wall on older homes.
- Measure doors and windows. Record width and height of each opening so you can subtract its area.
- Measure the ceiling. Length times width, straight across the room.
- Note the surface type. Write down whether each wall is smooth, textured, bare, or previously painted, so you pick the right coverage range.
For trim, doors, and cabinets, the math is different — those use square footage by surface rather than wall area. Our cabinet painting guide covers the surfaces that break the standard formula.
Common estimating mistakes
The two most expensive errors are both easy to avoid. Forgetting the second coat is the first: most color changes and repairs need two, and buying for one leaves you short and forces a color-match trip. Ignoring texture is the second: a rough or porous wall can cut coverage by a third, so using the smooth 400-square-foot figure on bare drywall means buying too little. Round up on gallons and store the leftover, labeled, for touch-ups.
Also watch for subtracting openings you are actually painting around — trim and frames still need paint, and a tiny rounding error over four walls turns into a short gallon. When the surface needs work first, our surface preparation page will tell you whether primer changes the math.
Frequently asked questions
How much does one gallon of paint cover?
How many gallons do I need for a 12×12 room?
Should I always buy extra paint?
Do ceilings count the same as walls?
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