Interior Paint Calculator
Calculate total gallons of interior paint needed for all rooms from wall dimensions and doors. Enter values for instant results with step-by-step formulas.
Reviewed by Abdullah, Technical Content Specialist
Formula
Gallons = (Net Wall Area x Coats) / Coverage Rate per Gallon
Net wall area is the total wall surface area (perimeter times height) minus door and window openings. Multiply by the number of coats, then divide by the coverage rate (typically 350-400 sq ft per gallon) to get the gallons needed. Round up to whole gallons.
Worked Examples
Example 1: Three-Bedroom House Interior
Problem:Calculate paint needed for 6 rooms averaging 14x12 ft with 8 ft ceilings, 2 doors and 2 windows per room, 2 coats, at 350 sq ft/gallon coverage.
Solution:Per room perimeter: 2 x (14 + 12) = 52 ft\nGross wall area per room: 52 x 8 = 416 sq ft\nDoor deduction: 2 x 21 = 42 sq ft\nWindow deduction: 2 x 15 = 30 sq ft\nNet per room: 416 - 42 - 30 = 344 sq ft\nTotal net: 344 x 6 = 2,064 sq ft\nWith 2 coats: 4,128 sq ft\nGallons: 4,128 / 350 = 11.8 gallons
Result:12 gallons paint | 6 gallons primer | Est. cost: $540 paint + $180 primer
Example 2: Single Large Living Room
Problem:Calculate paint for one 20x16 ft room with 9 ft ceilings, 1 door, 4 windows, 2 coats at 350 sq ft/gallon.
Solution:Perimeter: 2 x (20 + 16) = 72 ft\nGross wall area: 72 x 9 = 648 sq ft\nDoor deduction: 1 x 21 = 21 sq ft\nWindow deduction: 4 x 15 = 60 sq ft\nNet area: 648 - 21 - 60 = 567 sq ft\nWith 2 coats: 1,134 sq ft\nGallons: 1,134 / 350 = 3.24 gallons
Result:4 gallons paint | 2 gallons primer | Est. cost: $180 paint + $60 primer
Frequently Asked Questions
How do I calculate how much interior paint I need?
To calculate interior paint needs, measure the perimeter of each room and multiply by the wall height to get the gross wall area. Then subtract the area of all doors (approximately 21 square feet each for standard doors) and windows (approximately 15 square feet each for standard windows) to get the net paintable area. Multiply this net area by the number of coats you plan to apply, then divide by the paint coverage rate printed on the can, typically 350 to 400 square feet per gallon. Always round up to the next whole gallon and consider purchasing an extra gallon for touch-ups later. A standard 12x14 foot room with 8-foot ceilings, 2 doors, and 2 windows requires approximately 1.5 gallons per coat of paint.
How many coats of interior paint do I need?
Most interior paint jobs require two coats for optimal coverage, color depth, and durability, regardless of paint quality. A single coat often shows brush marks, roller lines, and uneven color density that become obvious as the paint dries and is viewed under different lighting conditions. When changing from a dark color to a light color, you may need three coats or a tinted primer plus two topcoats. Going from light to dark typically still requires two coats. High-quality paints with excellent hiding power can sometimes achieve acceptable results in one coat, but manufacturers usually specify these as one-coat paints and price them accordingly. Bathrooms and kitchens should always receive two full coats because the higher moisture and grease exposure demands a thicker, more durable paint film.
What is the coverage rate for interior paint?
Standard interior latex paint covers approximately 350 to 400 square feet per gallon on smooth, previously painted walls in good condition. This rate assumes a single coat applied with a roller at the proper thickness. However, several factors can significantly reduce coverage. Rough or textured surfaces like knockdown, orange peel, or popcorn textures increase surface area by 20 to 50 percent, reducing effective coverage to 200 to 300 square feet per gallon. Unpainted drywall or plaster absorbs more paint, reducing coverage to 250 to 300 square feet per gallon on the first coat. Dark or bold colors often have lower coverage rates because the pigments used are less opaque. Always check the specific coverage rate printed on the paint can, as it varies by brand and product line.
Do I need primer before painting interior walls?
Primer is recommended but not always mandatory for interior painting. You definitely need primer when painting over new drywall, as the paper face absorbs paint unevenly without primer, causing a blotchy appearance called flashing. Primer is also essential when painting over stains from water, smoke, markers, or grease to prevent them from bleeding through the topcoat. When making dramatic color changes, a tinted primer saves money compared to extra coats of expensive topcoat paint. Self-priming paints combine primer and paint in one product and work well for repainting walls in similar colors that are already in good condition. If you skip primer on bare drywall, you will typically need three coats of paint instead of two to achieve even coverage, which actually costs more than using primer plus two topcoats.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy