Introduction
Figuring out how much paint you need for a project can be tricky. Buy too little and you'll make extra trips to the store. Buy too much and you'll waste money. Our Paint Calculator takes the guesswork out of the process. Just enter the size of your walls, the number of doors and windows, and the number of coats you plan to apply. The calculator will tell you exactly how many gallons of paint you need to get the job done right.
Most standard paints cover about 350 to 400 square feet per gallon on a smooth surface. This calculator uses that coverage rate to give you a reliable estimate. It also subtracts the area of doors and windows since you won't be painting over those. Whether you're painting a single room or an entire house, this tool helps you plan ahead and stay on budget. If you need to determine the exact dimensions of a room before you start, our Square Footage Calculator can help with that step.
How to Use Our Paint Calculator
Enter your room or surface dimensions, specify doors and windows, and choose your paint options. The calculator will tell you how much paint or stain you need to buy for your project.
Calculator Mode: Pick the tab that matches your project. "Quick Room" is for a fast single-room estimate. "Detailed Room" lets you add multiple rooms with exact measurements. "Exterior" is for the outside walls of your home. "Deck" covers deck floors, railings, and steps.
Unit System: Choose between Imperial (feet and inches) or Metric (meters and centimeters). All input fields and results will update to match the system you pick.
Room Length, Width, and Wall Height: Enter the length, width, and wall height of your room or surface. Use the two fields for each measurement — one for the larger unit (feet or meters) and one for the smaller unit (inches or centimeters).
Number of Doors: Enter how many doors are on the walls you plan to paint. The calculator subtracts about 21 square feet (or 1.95 square meters) for each door since you do not paint over them.
Number of Windows: Enter how many windows are on the walls you plan to paint. The calculator subtracts about 12 square feet (or 1.11 square meters) for each window.
Number of Coats: Select how many coats of paint you plan to apply. Most projects need 2 coats for good coverage, but you can choose 1 or 3 depending on the surface and color change.
Include Ceiling: Check this box if you also want to paint the ceiling. The calculator will add the ceiling area to your total paintable surface.
Advanced – Custom Coverage Rate: Click "Advanced" to set your own coverage rate if your paint covers more or less area per gallon than the default. The default is 400 square feet per gallon for interior walls, 350 for exterior walls, and 300 for deck stain or paint.
Railings and Steps (Deck Mode): Check the boxes to include railings and steps in your deck estimate. Enter the total railing length and height, and the number, width, depth, and rise of your steps. The calculator accounts for both sides of the railing.
Results: After you click "Calculate," the tool shows your total paintable area, area after deductions for doors and windows, total area with coats applied, the exact amount of paint needed, and the recommended number of whole gallons or liters to purchase.
How to Calculate Paint for Any Project
Figuring out how much paint to buy is one of the most common challenges in any painting project. Buy too little and you'll make extra trips to the store. Buy too much and you waste money. A paint calculator takes the guesswork out by using your room measurements, the number of doors and windows, and your chosen number of coats to tell you exactly how many gallons you need.
How Paint Coverage Works
A standard gallon of interior paint covers about 400 square feet of smooth wall surface with one coat. Exterior paint covers slightly less — around 350 square feet per gallon — because outdoor surfaces like wood siding, stucco, and brick are rougher and absorb more paint. Deck stain and deck paint cover even less, typically around 300 square feet per gallon, since wood decking is porous and heavily textured. These are average numbers. The actual coverage you get depends on the surface texture, the paint quality, and how you apply it.
Measuring Your Paintable Area
To find the total wall area of a room, you add up the length of all four walls (the perimeter) and multiply by the wall height. For a room that is 12 feet long and 10 feet wide with 8-foot ceilings, the perimeter is 44 feet, and the total wall area is 352 square feet. You then subtract the areas you won't paint — doors and windows. A standard interior door is about 21 square feet, and an average window is about 12 square feet. If that room has one door and two windows, you subtract 45 square feet, leaving 307 square feet of paintable wall space. Our Square Footage Calculator is a helpful companion tool for measuring rooms and surfaces accurately.
Why Two Coats Matter
Most painting jobs need two coats. The first coat seals the surface and provides a base layer. The second coat gives you full, even color and better durability. If you are painting a light color over a dark one, you may need three coats or a coat of primer first. When calculating paint, multiply your net paintable area by the number of coats. Using the example above, 307 square feet times 2 coats equals 614 square feet of coverage needed, which works out to about 1.5 gallons. Since paint is sold in whole gallons, you would buy 2 gallons.
Tips for Different Project Types
- Interior rooms: Don't forget to add the ceiling if you plan to paint it. Ceiling area equals the room length times the room width. If you're also replacing the drywall before painting, our Drywall Calculator can help you estimate materials for that step. Likewise, if you're considering wallpaper for an accent wall instead of paint, you can compare material needs easily.
- Exterior walls: Measure each wall section separately, especially if your home has gables or sections with different heights. Subtract for doors and windows just like you would indoors. If your exterior project also involves brickwork or a concrete foundation, plan those materials alongside your paint estimate.
- Decks: Calculate the floor area, then add railings and steps. Railings have two sides, so double the surface area. For each step, add the tread (the flat part you step on) and the riser (the vertical face).
Always Round Up
It is better to have a little extra paint than to run short. Running out of paint mid-wall can cause visible lap marks where wet and dry edges overlap. Most stores accept returns on unopened gallons, so rounding up to the next whole gallon is a safe and smart practice. Keep leftover paint sealed tightly for future touch-ups — it will stay usable for several years when stored properly in a cool, dry place.
Planning a larger renovation? You may also find these construction calculators useful: the Flooring Calculator for hardwood or laminate estimates, the Tile Calculator for bathrooms and kitchens, the Carpet Calculator for bedrooms, the Insulation Calculator for wall cavities, and the Epoxy Calculator for garage floors or countertops. Having accurate material estimates across every phase of your project keeps costs under control and reduces waste.