Paint Calculator

Calculate exactly how many gallons of paint you need for any room. Instant results with coverage rates and cost estimates.

Wall measurements

Measure length and height of each wall to paint

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~20 sq ft each

~15 sq ft each

$

Master Guide to Calculating Paint Requirements for Perfect Coverage

Knowing exactly how much paint to buy saves money, prevents mid-project store runs, and ensures consistent color matching from the same production batch. Whether you're refreshing a single room or painting your entire home, accurate paint calculations help you budget effectively and purchase the right quantity the first time. Running short means potential color mismatches between batches, while over-purchasing wastes money on paint you'll never use.

The foundation of paint calculation involves measuring wall dimensions and applying coverage rates. Start by measuring the length and height of each wall in feet, multiply length by height to get square footage for each wall, add all wall areas together for total coverage area, then subtract 20 square feet per door and 15 square feet per standard window. One gallon of quality interior paint typically covers 350-400 square feet per coat on smooth, primed surfaces. Divide your total square footage by the coverage rate, multiply by the number of coats needed, and round up to the nearest gallon.

Understanding Paint Coverage Rates and Variables

Paint coverage varies significantly based on several factors. Premium quality latex paint covers 400-450 square feet per gallon with excellent hiding power, while standard economy paint covers 300-350 square feet per gallon and may require additional coats. Surface texture dramatically impacts coverage—smooth drywall allows maximum coverage, textured or popcorn surfaces can reduce coverage by 25-40%, and unpainted drywall absorbs more paint, reducing coverage to 250-300 square feet per gallon.

Color and application method also affect how far your paint goes. Painting dark walls with light colors requires 2-3 coats for full coverage, while painting light colors over white or beige walls often needs just 1-2 coats. Spray application uses 20-30% more paint than brush or roller application due to overspray, while roller application provides the best balance of coverage and efficiency. Primer coats typically cover 300-350 square feet per gallon because primers are designed to be absorbed into the surface, creating a better bond for topcoats.

How Many Coats Do You Really Need?

Most professional painters recommend two coats for durability and even color coverage. One coat might suffice only when painting the exact same color over clean, well-maintained existing paint, or when using paint-and-primer combination products on light color changes. Two coats are standard for color changes within the same color family, painting over primed new drywall, and ensuring professional-looking results with uniform sheen and depth.

Three coats become necessary in specific situations: covering dark colors (navy, red, dark green) with white or light colors, painting over stains, water damage, or glossy surfaces, dramatic color transformations like bright yellow over deep purple, and when using lower-quality paint with poor hiding power. Many professionals apply tinted primer as the first coat when making dramatic color changes, which reduces the number of topcoats needed to two instead of three, saving both time and money while achieving better results.

Accounting for Doors, Windows, and Other Openings

Subtracting doors and windows from your calculations provides more accurate paint estimates. A standard interior door measures approximately 20 square feet (3 feet wide by 6.5-7 feet tall), while average windows cover about 15 square feet (3 feet wide by 5 feet tall). For a typical room with 2 doors and 3 windows, that's 85 square feet you won't be painting. However, many professionals recommend buying slightly extra paint anyway—about 10% more—to account for wastage, touch-ups, and future repairs.

This extra 10% often balances out with the areas you subtract, meaning if you calculate 400 square feet minus 70 square feet of openings equals 330 square feet, then add 10% back for contingency, you end up near 365 square feet—close to one gallon of paint. Other areas that don't require paint include fireplaces, built-in shelving, large mirrors or artwork that will be rehung, and areas behind large furniture that won't be moved. Keep detailed measurements and calculations for each room, as this helps when you need to order additional paint or plan future painting projects in similar spaces.

Frequently Asked Questions

How much paint do I need to paint a room?
To calculate paint needed, measure wall length and height to get square footage for each wall, add all wall areas together, subtract 20 square feet per door and 15 square feet per window, divide by your paint coverage (typically 350-400 sq ft per gallon), and multiply by number of coats. For example, a 12×15 room with 8-foot ceilings has approximately 432 sq ft of wall space. Minus 2 doors (40 sq ft) and 2 windows (30 sq ft) equals 362 sq ft. At 400 sq ft per gallon with 2 coats, you need 2 gallons.
How many square feet does one gallon of paint cover?
One gallon of paint typically covers 350-400 square feet for one coat on smooth, primed surfaces. Actual coverage varies by paint type: premium latex paint covers 400-450 sq ft per gallon, standard latex covers 350-400 sq ft, primer covers 300-350 sq ft due to higher absorption, and textured or porous surfaces reduce coverage to 250-350 sq ft. Darker colors over light walls may need extra coats. Always check the paint can label for manufacturer-specific coverage rates.
Do I really need to subtract doors and windows from calculations?
Yes, subtracting doors and windows provides more accurate estimates and prevents over-purchasing. A standard door measures approximately 20 square feet (3 ft × 6.5 ft), while an average window covers about 15 square feet (3 ft × 5 ft). For a room with 2 doors and 3 windows, that saves 85 square feet of unnecessary paint. However, many professionals add 10% extra paint for touch-ups, meaning you may end up with the right amount anyway after subtracting openings.
How many coats of paint should I apply?
Most painting projects require 2 coats for even coverage and durability. One coat is sufficient only when painting the same color over a well-maintained existing paint. Two coats are standard for most color changes, new drywall, and professional results. Three coats are needed when covering dark colors with light paint, painting over stains or water damage, or dramatic color changes like red to white. Primer plus one coat works for minor color changes on properly prepared surfaces.
Should I buy paint by the gallon or quart?
Buy gallons for rooms and large areas, as it costs less per ounce than quarts. One gallon equals four quarts and typically costs 25-40% less than buying four quarts separately. Purchase quarts for small projects like accent walls, bathrooms, closets, touch-ups and repairs, or testing colors. For projects needing 1-1.5 gallons total, buying 2 gallons is more economical than mixing gallons and quarts, plus you have paint for future touch-ups.
How do I calculate paint for ceilings and trim?
Calculate ceiling paint by multiplying room length times width (same as floor square footage), then dividing by 400 sq ft per gallon. Ceiling paint typically covers 350-400 sq ft per gallon. For trim and molding, measure the linear feet of all baseboards, door frames, and window casings, multiply by the average height (usually 4-6 inches or 0.33-0.5 feet), divide by 350 sq ft per gallon. One gallon of trim paint typically covers 120-150 linear feet of standard baseboard.
What if my paint estimate seems too high or too low?
Verify your measurements are in feet (not inches), confirm you entered wall heights correctly (typical is 8-10 feet), check that you selected the right number of coats, and ensure you chose the correct paint type as coverage varies. Textured walls, rough surfaces, or unpainted drywall absorb more paint and reduce coverage by 25-50%. If switching from dark to light colors, you may need an extra coat beyond the estimate. When in doubt, buy an extra gallon—unopened paint can usually be returned to most stores.
How much does paint cost per gallon?
Paint costs vary widely by quality and brand. Basic contractor-grade paint costs $20-30 per gallon, mid-grade interior paint runs $30-50 per gallon, premium brands like Benjamin Moore or Sherwin-Williams cost $50-80 per gallon, and specialty paints (exterior, bathroom, kitchen) range $45-90 per gallon. Better paint typically covers better, requiring fewer coats, and provides superior durability. For whole-house projects, buying 5-gallon buckets saves $5-15 per gallon compared to individual gallons.

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