Paint Calculator
Calculate exactly how many gallons of paint you need to paint the walls of any room.
On this page
Wall Surface Measurement
Measure the length of your walls and the ceiling height. Don't forget to count your doors and windows so the calculator can subtract them from your total paintable area!
How to Use the Paint Calculator
Buying too much paint wastes money, and buying too little means an annoying mid-project trip to the hardware store (where the second batch might have a slightly different color tint!). Use our calculator to get it right the first time.
- Measure the room: Use a tape measure to find the length and width of the room in feet.
- Check the ceiling height: Most standard rooms are 8 or 9 feet tall.
- Count openings: Enter the number of doors and windows in the room. We will automatically subtract 21 sq ft per door and 15 sq ft per window.
- Select coats: Always plan for 2 coats unless you are just doing minor touchups in the same color.
Wall Painting Formula
The math behind painting a room involves calculating the total surface area of all four walls, subtracting the areas you won't paint (doors and windows), and dividing by the standard coverage rate of a gallon of paint.
Step 1: Calculate Total Wall Area
Wall Area = 2 × (Length + Width) × Height
Step 2: Subtract Openings
Net Area = Wall Area − (Doors × 21) − (Windows × 15)
Step 3: Calculate Gallons
Gallons = (Net Area × Number of Coats) ÷ 350 sq ft
Painting Example Calculation
You want to paint a bedroom that is 12 feet long, 10 feet wide, with 8-foot ceilings. The room has 1 door and 2 windows. You want to apply 2 coats of paint.
- Wall Area: 2 × (12 + 10) × 8 = 352 square feet.
- Subtract Openings: 352 − 21 (for 1 door) − 30 (for 2 windows) = 301 square feet (Net Area).
- Total Paint Area (2 coats): 301 × 2 = 602 square feet.
- Gallons Needed: 602 ÷ 350 = 1.72 gallons.
Conclusion: You will need to purchase 2 gallons of paint.
Pro Painting Tips
- Always use primer on new drywall: If you are painting brand new drywall or spackle, you must apply a coat of PVA drywall primer first. Otherwise, the porous drywall will aggressively suck the moisture out of your expensive topcoat paint, leaving a patchy finish.
- Mix multiple gallons together: If a room requires 3 gallons of paint, mix all 3 gallons into a large 5-gallon bucket before you start. This process is called "boxing" and ensures that if one can was tinted slightly differently at the store, the final color on your wall remains perfectly consistent.
- Paint the trim first: Professional painters usually paint the baseboards and window trim first. Once the trim is completely dry, they tape it off and roll the walls. It is much easier to tape a flat baseboard than to perfectly tape the textured edge of a wall.
Frequently Asked Questions
How much area does one gallon of paint cover?
One gallon of high-quality interior paint generally covers about 350 to 400 square feet on smooth walls. If your walls are heavily textured, porous (like bare drywall), or if you are using low-quality paint, you may only get 250 to 300 square feet of coverage.
Do I always need two coats of paint?
In almost all cases, yes. Two coats provide the truest color, the best durability, and the most even finish. You might get away with one coat if you are repainting the exact same color over a wall that is already in great condition.
How much paint do I need for the ceiling?
You can find your ceiling square footage by simply multiplying the length of the room by the width. If the ceiling area is 200 square feet, you will need approximately 1 gallon of ceiling paint for a single coat, or 2 gallons for two coats.
Should I subtract doors and windows?
Yes, to avoid overbuying paint. A standard interior door takes up about 21 square feet, and a standard window takes up about 15 square feet. Our calculator automatically subtracts these averages from your total wall area.