Projects & Trades
Paint Calculator
Estimate how much interior paint you need for walls, ceilings, multiple coats, and one or more rooms.
Room dimensions
Surfaces to paint
Doors and windows
The defaults assume a 3 × 7 ft door and roughly 15 square feet per window. Change these values if you know the actual opening sizes.
Paint settings
Paint coverage varies by product, surface, color change, texture, and application method. Use the coverage printed on your paint can when available.
Estimated amount to buy
Enter your room dimensions to calculate an estimate.
How the paint estimate is calculated
For walls, the calculator adds the lengths of all four walls and multiplies that perimeter by the wall height. Door and window areas are then subtracted from the wall area.
If the ceiling is included, its area is calculated from the room length and width and added to the paintable surface area.
The paintable area is multiplied by the number of coats, then divided by the selected paint coverage rate to estimate gallons required. Your extra-paint allowance is added at the end.
How much paint do I need?
Paint requirements depend on the size of the room, the number of coats, the amount of wall area occupied by doors and windows, and the coverage rate of the paint you are using.
Two coats require approximately twice as much paint as one coat when all other conditions remain the same.
Paint coverage per gallon
This calculator starts with a coverage rate of 350 square feet per gallon. Actual coverage varies, so use the manufacturer's stated coverage whenever possible.
What this estimate does not include
Primer, trim paint, doors being painted, cabinets, closets, unusual architectural features, and exterior surfaces are not included unless their area is represented in the measurements you enter.
Interior Paint Calculator Guide
An interior paint calculator helps estimate how much paint you need for walls, ceilings, and rooms based on the dimensions of the space, the number of coats, and the coverage rate of the paint.
This calculator estimates paintable square footage, subtracts doors and windows from wall area, accounts for multiple coats, and converts the result into gallons of paint.
How Much Paint Do I Need?
The amount of paint you need depends on the total surface area being painted, the number of coats, the coverage rate of the paint, and whether you are painting walls, ceilings, or both.
For walls, the calculator uses the room perimeter and wall height to determine the gross wall area.
2 × (Room Length + Room Width) × Wall Height = Wall Area
Door and window areas are then subtracted from the wall area. If the ceiling is included, its area is calculated separately and added to the total.
Interior Paint Calculator for a Room
To estimate paint for a room, enter the room length, room width, and wall height. Then choose whether you are painting the walls, the ceiling, or both.
Enter the number of doors and windows so their approximate area can be deducted from the wall surface. If you know the actual opening sizes, you can change the default door and window area values.
Paint Calculator for Walls
When calculating paint for walls only, the calculator determines the area of all four walls and subtracts the area occupied by doors and windows.
This is useful for bedrooms, living rooms, offices, hallways, and other spaces where the main goal is estimating wall paint.
Paint Calculator for Ceilings
Ceiling area is calculated by multiplying the room length by the room width. When you choose the ceiling-only option, wall dimensions are not included in the paint quantity.
Room Length × Room Width = Ceiling Area
If you are painting both walls and the ceiling with the same paint and coverage assumptions, choose the walls-and-ceiling option.
How Many Gallons of Paint Do I Need?
Once the total paintable area is calculated, the calculator multiplies that area by the number of coats and divides by the coverage rate of the paint.
Total Paintable Area × Number of Coats ÷ Coverage per Gallon = Gallons Needed
The calculator then adds your selected extra-paint allowance to estimate the amount to purchase.
Paint Coverage per Gallon
Paint coverage varies by product, surface texture, color change, application method, and manufacturer. This calculator begins with a default coverage rate of 350 square feet per gallon.
For a more accurate estimate, use the coverage rate printed on the paint can or provided by the manufacturer.
Rough, porous, or heavily textured surfaces may require more paint than smooth surfaces. Large color changes may also require additional coats.
One Coat Versus Two Coats of Paint
The number of coats has a major effect on paint quantity. Two coats require approximately twice the surface coverage of one coat when all other factors remain the same.
Some projects may require only one finish coat, while others may require two or more coats depending on the existing color, new color, paint quality, surface condition, and desired finish.
Doors and Windows
Doors and windows reduce the amount of wall surface that needs wall paint, so their area is subtracted from the wall calculation.
The calculator begins with an estimated 21 square feet per door and 15 square feet per window. These defaults can be changed if you know the actual dimensions of the openings.
If the doors themselves will also be painted, calculate that paint separately rather than subtracting the door area from the project.
Multiple Rooms
If several rooms have identical dimensions and the same paint requirements, use the number-of-rooms field to calculate them together.
Rooms with different dimensions, ceiling heights, opening sizes, or paint colors should generally be calculated separately.
Why Add Extra Paint?
A small amount of extra paint can account for measuring differences, variation in real-world coverage, touch-ups, spills, and paint that remains in rollers, brushes, trays, and containers.
The calculator includes a 10% extra-paint allowance by default. You can change that percentage to match your project.
Should Primer Be Included?
Primer is not included in the paint quantity shown by this calculator. Primer coverage and the number of primer coats can differ from the finish paint.
Primer may be useful on new drywall, bare surfaces, repaired areas, porous materials, stains, or major color changes. If primer is required, estimate it separately using the product's stated coverage.
Gallons, Quarts, and Liters
The calculator reports an exact estimated quantity in gallons and also shows an equivalent volume in liters.
If buying only one-gallon cans, the calculator rounds the adjusted quantity up to the next whole gallon can. Depending on the amount needed, a combination of gallon and quart containers may reduce leftover paint.
Tips for a Better Paint Estimate
Measure the room rather than relying only on nominal room dimensions. Ceiling height, wall projections, alcoves, large windows, and unusual room shapes can affect the actual paintable area.
Check the coverage rate for the exact paint product you plan to use. Product coverage can vary, and manufacturer information is generally more useful than a generic assumption.
If different surfaces will use different colors or products, calculate them separately instead of combining everything into one estimate.
Paint Calculator Disclaimer
This calculator provides planning and purchasing estimates. Actual paint requirements may vary because of surface texture, porosity, application method, color changes, paint formulation, number of coats, measuring differences, and job-site conditions.