Paint Calculator: Estimate How Much Paint You Need

Few things are more frustrating than running out of paint halfway through a project or having gallons of leftover paint you will never use. Our Paint Calculator estimates the exact amount of paint needed for any room or surface based on wall dimensions, number of doors and windows, paint coverage, and the number of coats. Save money and avoid multiple trips to the hardware store.

How Paint Calculations Work

The basic formula: total paintable area = wall perimeter × wall height - door areas - window areas. Each door subtracts approximately 20 square feet, each window subtracts approximately 15 square feet. A gallon of paint typically covers 350-400 square feet per coat. Most rooms need two coats for even coverage.

For the ceiling, multiply length × width to get the area. Ceilings typically need one coat of ceiling paint unless covering a dark color. For trim, measure the linear footage of baseboards, crown molding, and door/window casings. Trim paint covers about 100-150 linear feet per quart.

Using the Paint Calculator

Enter the room length, width, and ceiling height. Enter the number of doors, windows, and their approximate sizes. Select the number of coats (typically 2) and the paint coverage rate (default 350 sq ft per gallon). The calculator shows paint needed for walls, ceiling, and trim separately, with recommendations for the number of gallons and quarts.

The calculator also estimates primer needs — primer is recommended when painting over dark colors, bare drywall, or when making a drastic color change. Primer coverage is similar to paint at 300-350 sq ft per gallon.

Paint Coverage Guidelines

Wall Paint

One gallon covers 350-400 square feet per coat. Flat and matte finishes cover slightly more area than eggshell or satin. Textured surfaces (popcorn ceilings, knock-down walls) may require 20-30% more paint due to increased surface area.

Ceiling Paint

Ceiling paint is thinner and may cover slightly more area (400-450 sq ft per gallon). Flat white ceiling paint is standard. Darker ceiling colors may require more coats.

Trim Paint

Semi-gloss or gloss paint for baseboards, doors, and trim. Coverage depends on the application method. A quart typically covers 100-150 linear feet of standard 3-inch trim.

Primer

Covers 300-350 sq ft per gallon. Use primer when: painting over dark colors (especially red, black, or dark blue), covering stains or water damage, painting new drywall, or making a significant color change (light to dark or dark to light).

Paint Estimation Tips

Buy all at once: Purchase all paint at the same time to ensure color consistency. Different batches can have subtle color variations.
Round up: Always round up to the nearest gallon. Leftover paint is useful for touch-ups.
Save leftover paint: Label leftover paint with the room name and date. Store in a cool, dry place for future touch-ups.
Consider spray application: Spraying uses 20-30% more paint than rolling due to overspray. Adjust estimates accordingly.
Textured surfaces need more: Brick, stucco, and textured walls absorb significantly more paint than smooth drywall.
Quality matters: Higher-quality paint covers better and often requires fewer coats, saving money overall.

Real-World Example

A 12×14 foot bedroom with 8-foot ceilings, one door, and two windows:

Wall perimeter: 2 × (12 + 14) = 52 feet
Total wall area: 52 × 8 = 416 sq ft
Subtract door (20 sq ft): -20
Subtract windows (2 × 15): -30
Net wall area: 366 sq ft
Paint needed (2 coats): 366 × 2 / 350 = 2.09 gallons → 2 gallons + 1 quart
Ceiling area: 12 × 14 = 168 sq ft → 1 gallon (covers one coat)
Trim (baseboards only, 52 linear ft): ~1 quart
Primer (if needed): 366 / 350 = 1.05 → 1 gallon

Total paint purchase: 2 gallons wall paint, 1 gallon ceiling paint, 1 quart trim paint. If using primer: 1 gallon primer.

Start Calculating

Use our Paint Calculator below to estimate paint quantities for your next painting project. Also check our Flooring Calculator for floor finishes and our Square Footage Calculator for general measurements.