How Much Paint Do You Need? Gallons & Cost Calculator

Wall Area Minus Exclusions, Times Coats

Variables Explained

Coverage, Cut-Ins & Second Coats

• 1Worked example: a 12 × 15 ft room with 8 ft ceilings, one door (21 sq ft), two windows (30 sq ft), two coats → (2 × 27 × 8 − 51) × 2 = 762 sq ft; at 350 sq ft per gallon that's 2.18, so buy 3 gallons.
• 2One gallon of paint typically covers roughly 350-400 square feet of smooth surface, or about 10 m² per liter. Measure carefully and buy 10-15% more than calculated for touch-ups.
• 3Primer improves adhesion and seals porous surfaces like bare drywall or fresh plaster, so it can actually reduce the number of finish coats you need and save paint overall.
• 4Paint dries fastest in warm, dry conditions around 50-85°F (10-30°C). Avoid direct sunlight and high humidity, which can cause streaking, blistering, and uneven drying.
• 5Coverage drops on textured or porous surfaces, which can absorb up to 25% more paint than smooth walls, so factor that in when estimating total volume.
• 6Two thin coats typically look better and last longer than one thick coat — allow the first coat to dry fully before applying the second for an even finish.

Paint Aisle Vocabulary

Coverage Rate

The surface area that one liter (or gallon) of paint can cover in a single coat, measured in m²/L. Depends on paint type, surface porosity, and application method.

Primer

A preparatory coating applied before the finish paint. Primer improves adhesion, seals porous surfaces, and provides a uniform base that reduces the number of finish coats needed.

Sheen

The level of gloss in dried paint: flat (no shine, hides imperfections), eggshell (slight sheen, washable), satin (soft sheen, durable), semi-gloss (noticeable shine, very washable), and high-gloss (mirror-like, for trim).

VOC (Volatile Organic Compounds)

Chemicals released by paint as it dries. Low-VOC and zero-VOC paints minimize indoor air pollution and are recommended for interior use, especially in occupied spaces.

Cut-in

The technique of painting edges, corners, and areas around trim with a brush before rolling the main wall area. Proper cut-in eliminates the need for painter's tape in skilled hands.

Manufacturer Coverage Spec Sources

Our calculations are based on established financial and scientific formulas. The following sources inform the default values and methodology used in this calculator:

• Painting and Decorators Contractors Association (PDCA) — Industry standards and painting guides
• U.S. Environmental Protection Agency (EPA) — Volatile Organic Compounds impact on indoor air quality

How Much Paint Do I Need for a Wall?

One gallon of paint typically covers 350 to 400 square feet (32 to 37 square meters) with a single coat. Most walls need two coats, so divide your total wall area by 175 to 200 square feet per gallon to estimate paint needed for two coats.

Calculation Tips

• • Measure wall height × width for each wall, then add them together
• • Subtract doors (21 sq ft each) and windows (15 sq ft each) from total area
• • Textured walls may require 10-20% more paint
• • Dark colors over light walls may need 3 coats
• • Always buy 10% extra for touch-ups and mistakes
• • Primer is needed for bare drywall or drastic color changes

Quick Formula

Total Paint (gallons) = (Total Wall Area - Doors - Windows) ÷ 175

Example: Bedroom 12×12 ft with 8 ft ceiling:
• 4 walls × (12 × 8) = 384 sq ft
• Minus 1 door (21) + 2 windows (30) = 333 sq ft
• 333 ÷ 175 = 1.9 gallons (buy 2 gallons)

How Much Paint Do You Need for Different Room Sizes?

Use this quick reference to estimate paint needs for common room sizes. All estimates assume 8ft ceilings, standard doors and windows, and two coats of paint with 350 sq ft per gallon coverage.

Remember to add 10-15% extra for touch-ups and roller absorption. For open-concept rooms, measure each wall individually. Ceiling paint is separate — a standard ceiling takes about 1 gallon per 400 sq ft.

How to Measure a Room for Paint in 5 Steps

Painting contractors measure the same way every time. Run through these five steps and you will buy the right number of gallons — not the ballpark figure that leaves you with half a wall unpainted:

1. Measure each wall's width and multiply by ceiling height. A 12×14 ft room with 8 ft ceilings: (12 + 14) × 2 walls each = perimeter 52 ft × 8 ft = 416 sq ft of wall.
2. Subtract openings by their standard sizes. Doors ≈ 21 sq ft, windows ≈ 15 sq ft. Two doors and two windows take the room to 344 sq ft.
3. Multiply by the number of coats. Two coats over a mid-tone: 344 × 2 = 688 sq ft of coverage to buy for.
4. Divide by real coverage, not the can's best case. One gallon covers 350–400 sq ft on smooth primed walls; rough or heavily textured walls behave closer to 300. At 350: 688 ÷ 350 ≈ 2 gallons.
5. Round up to whole gallons, never partially. Partial batches can drift in color, and touch-ups later need the same batch. One spare quart per room covers drips and future scuffs.

Ceiling: treat it as its own rectangle (length × width) and add 1 coat of ceiling paint at the same coverage rate. The calculator above walks through the same arithmetic with your exact room count, coat count, and per-gallon coverage.

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