How to use the paint calculator
- Measure the total width of all walls you plan to paint and enter it in feet.
- Enter the wall height from floor to ceiling.
- Set the number of standard doors and windows to subtract their area automatically.
- Choose how many coats you intend to apply and confirm the coverage rating on your paint can.
- Read the result to see both the exact gallons of paint required and the whole number of cans to buy.
How the paint calculation works
The math behind this paint calculator is straightforward. First, multiply the total wall width by the height to get the gross area. Then subtract the area taken up by openings: each standard door removes 21 square feet (3 ft by 7 ft) and each standard window removes 15 square feet (3 ft by 5 ft). Multiply the remaining net area by the number of coats, then divide by the coverage rate printed on your paint can.
For example, a room with 40 ft of total wall width and 8 ft ceilings has 320 sq ft of gross wall area. Subtract one door (21 sq ft) and two windows (30 sq ft) to get 269 sq ft of net paintable area. Two coats means 538 sq ft total. At 350 sq ft per gallon, you need 538 / 350 = 1.537 gallons, so you should buy 2 gallons.
Standard coverage rates by paint type
Coverage varies by sheen, brand, and surface texture. Rough surfaces like stucco absorb more paint, while smooth drywall requires less. Always check the label on your specific product, but these ranges are reliable starting points for estimating how much paint you need.
| Paint type | Typical coverage (sq ft per gallon) |
|---|---|
| Interior flat or matte latex | 350 to 400 |
| Interior eggshell or satin | 350 to 400 |
| Interior semi-gloss or gloss | 300 to 350 |
| Exterior acrylic latex | 250 to 350 |
| Primer (drywall or masonry) | 200 to 300 |
Common mistakes when estimating paint
The most frequent error is forgetting to account for multiple coats. A single coat rarely provides even color, especially when painting over a darker shade or applying a light color to bare drywall. Most professional painters recommend two coats for walls and three for ceilings or dramatic color changes.
Another mistake is ignoring trim. Baseboards, crown molding, and door casings require separate paint, usually a semi-gloss or gloss finish. Measure trim linearly rather than by square footage, and estimate roughly one quart per 100 linear feet for a single coat.
Finally, people often forget that textured walls consume significantly more paint. Popcorn ceilings, knockdown textures, and rough stucco can reduce effective coverage by 20 to 50 percent compared to smooth drywall.
Tips for buying the right amount
Always round up to the next whole gallon. Paint stores cannot sell partial cans, and having a small surplus is far better than running short mid-project. Leftover paint is useful for touch-ups later.
If your calculation lands just above a whole number, such as 2.1 gallons, consider whether you can be slightly more efficient with your roller technique or if the surface is smoother than average. In most cases, buying 3 gallons is the safer choice because batch variations between cans can cause visible differences on large walls.
For projects involving multiple rooms, calculate each room separately using this paint calculator, then sum the totals. Buying in five-gallon pails is often cheaper per gallon than individual one-gallon cans, provided you are using the same color throughout.
When to use primer first
Primer seals porous surfaces, blocks stains, and improves adhesion. You need a dedicated primer when painting new drywall, covering dark colors with lighter ones, or working over glossy finishes that have not been sanded. Primer coverage is typically lower than topcoat coverage, often around 200 to 300 sq ft per gallon, so run a separate calculation for it.
Some modern paints include built-in primer, marketed as paint-and-primer-in-one products. These work well on previously painted walls in good condition but may still require a separate primer coat on raw wood, metal, or heavily stained surfaces.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| Small bedroom | Total wall width (ft): 36; Wall height (ft): 8; Number of doors: 1; Number of windows: 1; Number of coats: 2; Coverage per gallon (sq ft): 350 | Gross wall area: 288 sq ft; Net paintable area: 252 sq ft; Total area with coats: 504 sq ft; Gallons needed (exact): 1.44; Gallons to buy: 2 |
| Large living room | Total wall width (ft): 60; Wall height (ft): 9; Number of doors: 2; Number of windows: 3; Number of coats: 2; Coverage per gallon (sq ft): 400 | Gross wall area: 540 sq ft; Net paintable area: 453 sq ft; Total area with coats: 906 sq ft; Gallons needed (exact): 2.265; Gallons to buy: 3 |
| Hallway, no windows | Total wall width (ft): 20; Wall height (ft): 8; Number of doors: 3; Number of windows: 0; Number of coats: 1; Coverage per gallon (sq ft): 350 | Gross wall area: 160 sq ft; Net paintable area: 97 sq ft; Total area with coats: 97 sq ft; Gallons needed (exact): 0.277; Gallons to buy: 1 |
| Garage walls | Total wall width (ft): 80; Wall height (ft): 10; Number of doors: 1; Number of windows: 0; Number of coats: 2; Coverage per gallon (sq ft): 300 | Gross wall area: 800 sq ft; Net paintable area: 779 sq ft; Total area with coats: 1,558 sq ft; Gallons needed (exact): 5.193; Gallons to buy: 6 |
FAQ
How many square feet does a gallon of paint cover?
A standard gallon of interior latex paint covers approximately 350 to 400 square feet on smooth, primed drywall. Textured or unprimed surfaces will reduce that number.
Should I calculate for one coat or two?
Plan for two coats in almost all situations. One coat is only sufficient when repainting a wall with the exact same color and sheen, and even then, two coats provide better durability.
Does this paint calculator include ceilings?
No. Ceilings are calculated separately because they use different techniques and sometimes different paint. To include a ceiling, measure its length and width, multiply them together, and add that area to your total before dividing by coverage.
How do I measure irregular walls?
Break irregular walls into rectangles and triangles. Calculate the area of each shape separately, then add them together to get the total wall width equivalent at your ceiling height.
Do I need extra paint for textured walls?
Yes. Textured surfaces like stucco or popcorn ceilings absorb more paint. Reduce your expected coverage rate by 20 to 50 percent depending on the roughness of the texture.
Is it better to buy too much or too little paint?
It is always better to buy slightly more. Running out mid-wall forces you to wait for drying, which can leave lap marks. Leftover paint can be sealed and stored for future touch-ups.