How Much Paint Do I Need? (Free Paint Calculator & Guide)
Buying too little paint means a frustrating mid-project run to the store, and a can mixed a week later rarely matches the first batch exactly. Buying too much means unused, wasted paint sitting in the garage. This guide breaks down the simple wall-area formula painters use to estimate exactly how many gallons a room needs, walks through a real example, and covers coverage rates, common mistakes, and interior vs exterior differences — then hands you off to our free Paint Calculator to run your own numbers instantly.
Why Use a Paint Calculator?
A paint calculator turns the raw dimensions of a room — length, height, and the doors and windows that interrupt the wall surface — into the exact number of gallons you should buy.
Instead of eyeballing a quantity at the store or guessing based on a similar past project, the calculator applies the same coverage math professional painters use, so you buy the right amount on the first trip and know upfront whether you need a second coat.
How to Calculate Paint
Paint estimating starts with the total wall area you need to cover:
Wall Area = Length × Height
Calculate this for every wall in the room, then add the totals together. If you are painting the ceiling too, calculate that separately as Room Length × Room Width.
Next, subtract the areas you will not be painting — doors and windows — since paint only covers surfaces you actually roll or brush:
Paintable Area = Total Wall Area − (Door Area + Window Area)
A standard interior door opening is about 20 square feet (roughly 3 ft × 6 ft 8 in), and a standard window is about 15 square feet — use your own measurements when possible for a more precise number.
Once you know the paintable area, divide it by your paint's coverage rate (in square feet per gallon, found on the can label) to get the gallons needed for one coat. Multiply by two if you are applying two coats, which is standard for most interior painting jobs.
Gallons Required = Paintable Area ÷ Coverage per Gallon
Example Calculation
Say you are painting a 12 ft × 15 ft room with 8 ft ceilings, one standard door, and two standard windows. Here is the math step by step:
Perimeter: (12 + 15) × 2 = 54 linear feet of wall
Total Wall Area: 54 ft × 8 ft = 432 square feet
Door Area: 1 × 20 sq ft = 20 square feet
Window Area: 2 × 15 sq ft = 30 square feet
Paintable Area: 432 − 20 − 30 = 382 square feet
Using an average coverage rate of 375 sq ft per gallon, one coat needs 382 ÷ 375 ≈ 1.02 gallons — round up to one gallon minimum, though most painters buy a full gallon with a little left over.
For a standard two-coat finish, double the paintable area before dividing: (382 × 2) ÷ 375 ≈ 2.04 gallons, so plan on two gallons for a clean two-coat job.
- Total Paintable Area: 382 sq ft
- Gallons Required (1 coat): ~1.02 gallons
- Gallons Required (2 coats): ~2.04 gallons
Typical Paint Coverage Table
| Sheen | Coverage per Gallon |
|---|---|
| Flat Paint | 350–400 sq ft |
| Eggshell | 350–400 sq ft |
| Satin | 350–400 sq ft |
| Semi-Gloss | 300–350 sq ft |
| High Gloss | 250–300 sq ft |
Interior vs Exterior Paint
| Factor | Interior Paint | Exterior Paint |
|---|---|---|
| Purpose | Formulated for indoor durability, washability, and low odor | Formulated to resist UV rays, rain, temperature swings, and mildew |
| Flexibility | Rigid finish, less flexible since it is not exposed to weather | More flexible binders to expand and contract with temperature changes |
| Coverage Rate | Typically 350–400 sq ft per gallon on smooth drywall | Often slightly lower, 300–400 sq ft per gallon, due to textured siding and stucco |
| Coats Needed | Usually two coats for full coverage and color accuracy | Two coats recommended, especially over bare wood or previously faded paint |
Common Painting Mistakes
A few small miscalculations account for most paint-shortage and paint-waste problems. Watch for these before you buy:
- Forgetting second coats — calculating only enough paint for one coat when the color and finish actually require two for even coverage.
- Not subtracting windows and doors — using the full wall area without removing openings, which inflates the paintable area and your gallon estimate.
- Buying too little paint — rounding down instead of up, or skipping a waste allowance, and running out mid-wall.
- Mixing paint batches — using cans from different production batches without intermixing them first, which can cause a visible color shift across the wall.
- Ignoring surface texture — assuming smooth-wall coverage rates on textured surfaces like stucco, brick, or heavily textured drywall, which soak up significantly more paint.
Get an instant estimate with the Paint Calculator
Enter your room dimensions, doors, and windows to get an instant breakdown of paintable area and exactly how many gallons to buy.
Frequently Asked Questions
How much does one gallon cover?
Most interior paints cover between 350 and 400 square feet per gallon in a single coat, depending on the sheen and the surface. Semi-gloss and high-gloss finishes cover less — closer to 250–350 square feet — since their formulas are thicker and more reflective.
Should I buy extra paint?
Yes — buying a little extra, or keeping enough for touch-ups, is worth the small added cost. Paint left over from the same batch can be resealed and stored for future touch-ups, and it saves you from a mismatched color if you run short mid-wall.
Do textured walls require more paint?
Yes. Textured surfaces like stucco, brick, popcorn ceilings, and heavily textured drywall have more surface area per square foot than a flat wall, so they typically need 20–50% more paint than the can's stated coverage rate suggests.
How many coats should I apply?
Two coats is the standard for most interior and exterior painting jobs, especially when changing colors or covering a darker base color. A single coat may be enough only when repainting the same color over a surface already in good condition.
Can I paint over dark colors?
Yes, but plan for extra coats or a tinted primer first. Covering a dark wall with a light color often takes two to three coats of paint (or one coat of primer plus two topcoats) to fully hide the original color without patchy coverage.
How long does paint last?
An unopened can of paint can last up to 10 years when stored in a cool, dry place. Once opened and resealed tightly, most latex and acrylic paints stay usable for 2–5 years, though it is worth stirring and checking consistency before reuse.