Last updated: March 2026
How to Calculate Paint for a Room
Calculating paint accurately saves money and reduces waste. The basic formula is straightforward: measure your walls, subtract openings, multiply by the number of coats, and divide by your paint’s coverage rate. Here is exactly how to do it step by step.
One gallon of interior paint typically covers 350-400 square feet. The average room requires 1.5-2 gallons, and most paint projects need two coats for proper coverage.
Step 1: Measure your walls. For a rectangular room, measure the length and width. The total wall area is the perimeter (2 × length + 2 × width) multiplied by the ceiling height. A 12’ × 10’ room with 8-foot ceilings has a perimeter of 44 feet and a total wall area of 352 square feet.
Step 2: Subtract doors and windows. A standard interior door opening is roughly 21 square feet, and a typical window is about 12 square feet. For our example room with one door and two windows, subtract 45 square feet to get 307 square feet of paintable wall.
Step 3: Factor in coats. Two coats is the professional standard. Multiply your paintable area by the number of coats. Our example becomes 614 square feet of coverage needed.
Step 4: Divide by coverage. Standard interior latex covers about 350 square feet per gallon on smooth surfaces. So 614 ÷ 350 = 1.75 gallons. Round up to 2 gallons to ensure you have enough for touch-ups.
Common Paint Calculation Mistakes
Forgetting to account for texture. Rough or textured walls can use 20–30% more paint than smooth drywall. If your walls have knockdown, orange peel, or popcorn texture, reduce your coverage estimate from 350 to about 250–300 sq ft per gallon.
Skipping primer. Painting over dark colors, new drywall, or patched areas without primer wastes paint. You may need three or more topcoats to achieve even coverage when one coat of primer plus two topcoats would have been cheaper and faster.
Buying exact quantities. Always round up and buy at least 10% extra. You will need paint for touch-ups, and having leftover paint from the same batch ensures perfect color matching for future repairs.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 square feet of wall area. After subtracting a standard door (21 sq ft) and two windows (24 sq ft), you have roughly 339 square feet of paintable surface. With two coats at 350 sq ft/gallon coverage, you need about 2 gallons of paint.
How many square feet does a gallon of paint cover?
A gallon of standard interior latex paint covers approximately 350-400 square feet on smooth surfaces with one coat. Textured walls, porous surfaces, and dark-to-light color changes may reduce coverage to 250-300 square feet per gallon.
Should I subtract doors and windows when calculating paint?
Yes, always subtract doors and windows. A standard interior door is about 21 square feet (3' x 7') and a typical window is about 12 square feet (3' x 4'). Skipping this step can lead you to buy 10-20% more paint than you actually need.
Is one coat of paint enough?
One coat is rarely sufficient. Two coats is the professional standard for even coverage, proper color depth, and durability. You may get away with one coat only when painting the same color or using a premium paint-and-primer combo over a similar shade.
How do I calculate paint for a ceiling?
Ceiling paint is calculated by multiplying length times width of the room. A 12x10 room has a 120 sq ft ceiling. Ceilings typically need two coats of flat/matte paint. Use our calculator and toggle 'Include ceiling' for an automatic estimate.
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