Paint Calculator
Calculate exactly how many gallons of paint you need for any room based on wall area, openings, number of coats, and paint coverage.
How to use this paint calculator
- Measure room length, width, and ceiling height in feet. For rooms with irregular ceilings, use the average height.
- Count all doors and windows to subtract their area — standard interior doors are 21 sq ft and average windows are 15 sq ft.
- Choose your coat count — two coats is standard for most interior projects, especially color changes.
- Enter the coverage per gallon from your paint can label — typically 350–400 sq ft for premium paints.
- Round up to the nearest whole gallon when purchasing, and buy 10% extra for touch-ups.
Formula
Paintable area = (2 × (L + W) × H) − (doors × 21 sq ft + windows × 15 sq ft). Gallons = paintable area × coats ÷ coverage per gallon.
About the Paint Calculator
Paint is one of the most cost-effective home improvement investments available — transforming the feel of a room for a fraction of the cost of new furniture or flooring. Yet many DIY painters underestimate or overestimate their paint needs, leading to either frustrating mid-project runs back to the store or significant waste from unused gallons.
The paint calculator uses contractor-standard math: wall perimeter times ceiling height gives gross wall area, from which standard door and window areas are subtracted to yield paintable surface. This approach is accurate for standard rectangular rooms. For rooms with vaulted ceilings, angled walls, or complex geometry, measure each wall section individually and add the areas together before dividing by coverage.
Paint quality matters more than most DIY painters realize. Premium paints (Benjamin Moore, Sherwin-Williams, PPG premium lines) typically provide better coverage per gallon, richer color, superior durability, and better washability. The price difference between a budget gallon at $25 and a premium gallon at $65 is often recovered in labor savings — better coverage means fewer coats, and better durability means repainting less frequently. For rooms you will keep for many years, premium paint is usually the better investment.
Application method significantly affects coverage and finish quality. A quality 3/8-inch nap roller is ideal for smooth to lightly textured walls. A 1/2-inch or 3/4-inch nap handles heavier textures. Brushes are essential for cutting in at corners, edges, and trim — use angled sash brushes for control. Sprayers provide the fastest coverage and finest finish for large areas but require significant masking and preparation. For most DIY room painting projects, roller and brush application is the most practical and forgiving method.
Frequently asked questions
+How much paint do I need for a 12×12 room?
A standard 12×12 room with 8-foot ceilings has a wall perimeter of 48 feet, giving a total wall area of 384 square feet. Subtracting one standard door (21 sq ft) and two windows (30 sq ft total) leaves approximately 333 square feet of paintable surface. At 350 sq ft per gallon, one coat requires just under 1 gallon. For the standard two coats, you need approximately 1.9 gallons — so buy 2 gallons. For textured walls or a dramatic color change, buy 3 gallons to be safe.
+Does one gallon of paint really cover 350 square feet?
Coverage varies significantly based on paint quality, surface condition, and application method. Premium interior paints typically cover 350–400 sq ft per gallon on smooth, properly primed surfaces. Lower-quality paints may only cover 250–300 sq ft. Textured surfaces (orange peel, knockdown, heavy stipple) can reduce coverage by 15–25% because paint fills texture valleys. New drywall or bare wood is highly porous and absorbs significantly more paint — always prime these surfaces first or use a paint-and-primer product. The coverage printed on the can assumes ideal smooth-surface conditions.
+How many coats of paint do I need?
Two coats is the professional standard for most interior paint projects. One coat is sometimes sufficient for touch-ups on the exact same color with matching paint, or for very light colors over white. Three or more coats may be needed when covering dark colors with light paint (especially yellows over reds or deep blues), painting over high-gloss surfaces without sanding, or when using very low-opacity designer colors. Using a tinted primer for dramatic color changes can reduce the number of finish coats needed from three to two, saving both paint cost and time.
+Should I buy extra paint and how much?
Yes — always buy 10–15% more than the calculated amount. Extra paint serves several purposes: it covers any calculation errors or unusual absorption, it handles drips and mistakes during application, and most importantly, it provides touch-up paint for the future. If you need to touch up scuffs or nail holes years later, paint from a different batch may not match exactly due to sheen differences and tinting lot variations. Store leftover paint in the original can with a tight lid, labeled with the room and purchase date. Properly stored, interior latex paint remains usable for 2–5 years.
+What is the difference between sheen levels and which should I use?
Paint sheen ranges from flat/matte through eggshell, satin, semi-gloss, and high-gloss. Flat and matte have no shine and hide surface imperfections best — ideal for ceilings and low-traffic adult bedrooms. Eggshell has a slight sheen and is washable — the most popular choice for living rooms and dining rooms. Satin has more sheen and is more durable and cleanable — ideal for hallways, kids' rooms, and family spaces that need regular wiping. Semi-gloss is highly washable and moisture-resistant — the standard choice for kitchens, bathrooms, and trim. High-gloss is extremely durable and shiny — used for doors, cabinets, and furniture finishes where maximum durability is needed.
+Should I prime before painting?
Priming is essential in several situations: painting new drywall (drywall's paper surface is highly absorbent and requires sealing), painting over dark colors with lighter ones (primer dramatically reduces number of finish coats needed), painting over stains (water stains, smoke damage, marker — use stain-blocking primer), painting previously bare or stripped wood, or painting over glossy surfaces (degloss primer or light sanding improves adhesion). For painting over existing paint in good condition with a similar color, priming is optional but still improves the final result. Many premium paints include primer properties in the formulation, though separate priming still produces superior results for challenging situations.