Free Paint Calculator – Estimate Paint for Any Room
Stop guessing at the hardware store. Enter your room dimensions and instantly see how many gallons of paint you need — for walls, ceilings, and trim.
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Paint Calculator
Enter your room details for an instant, accurate paint estimate. Results update in real time.
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Common Paint Questions — Answered
The most searched paint quantity questions, answered with direct data — no fluff.
How much paint for a 12×12 room?
A 12×12 room with 9-ft ceilings has ~334 paintable sq ft after deducting a door and two windows. Two coats at 350 sq ft/gal = 2 gallons.
What does 1 gallon of paint cover?
One gallon of standard interior latex paint covers 350–400 sq ft on smooth walls per coat. Exterior and textured surfaces: 250–350 sq ft/gal.
How many coats of paint do I need?
2 coats is the standard for full, even coverage. Dark-to-light color changes may require 3. One coat is only acceptable for minor touch-ups over an identical color.
How Our Paint Calculator Works
Four steps from blank walls to a confident trip to the hardware store.
Measure Your Walls
Measure room length, width, and wall height in feet. No need to measure each wall separately — we use perimeter × height.
Enter Room Details
Add doors and windows for accurate deductions. Set number of coats, coverage rate, and price per gallon. Toggle ceiling or trim.
Review Your Estimate
Instantly see total paintable area, gallons required, a 10% waste buffer, and total estimated cost — all in one view.
Buy With Confidence
Head to the store with exact numbers. No over-buying, no mid-project shortages, no return trips.
The Paint Calculation Formula
Deductions = (Doors × 20) + (Windows × 15)
Paintable Area = Wall Area − Deductions + Ceiling (optional)
Gallons Needed = (Paintable Area × Coats) ÷ Coverage Rate
Total to Buy = Gallons Needed × 1.10 (10% buffer)
Standard coverage rate: 350–400 sq ft per gallon (interior smooth walls). Exterior: 250–350 sq ft/gal. Always round up to the nearest whole gallon.
Paint Coverage Guide
Coverage varies widely by surface type. Use this table to choose the right coverage rate in your calculation.
One gallon of standard interior latex paint covers 350 to 400 square feet on smooth walls per coat. For two coats, plan 175–200 sq ft per gallon. Textured, porous, or exterior surfaces reduce coverage by 20–40%.
| Surface Type | Coverage per Gallon | Extra Paint Needed | Notes |
|---|---|---|---|
| Smooth Interior Walls | 350–400 sq ft | — | Drywall, plaster, primed surfaces |
| Light Texture (Orange Peel) | 300–350 sq ft | +10–15% | Common in modern construction |
| Medium Texture (Knockdown) | 250–300 sq ft | +20–30% | Adds surface area to cover |
| Heavy Texture / Popcorn | 150–200 sq ft | +40–50% | Calculate coverage at lower end |
| Exterior Siding (Smooth) | 300–350 sq ft | +10% | Vinyl, fiber cement, smooth wood |
| Exterior Siding (Rough Wood) | 200–300 sq ft | +25–35% | Cedar, rough-sawn, weathered wood |
| Brick or Concrete Block | 150–200 sq ft | +40–50% | Highly porous; absorbs heavily |
| Stucco (Exterior) | 150–200 sq ft | +50% | Very rough; needs masonry paint |
| Primer (Interior) | 200–300 sq ft | Calculate separately | Always budget primer as a separate line item |
Interior vs Exterior Paint: Coverage Differences
Interior Paint
- Covers 350–400 sq ft per gallon on smooth walls
- 2 coats recommended for full coverage
- Finishes: flat, eggshell, satin, semi-gloss
- Dry time: 2–4 hours between coats
- Cost range: $30–$80 per gallon
- Not UV-resistant — do not use outdoors
Exterior Paint
- Covers 250–350 sq ft per gallon on siding
- Always apply 2 coats for weather protection
- Formulated for UV resistance and weatherproofing
- Dry time: 4–6 hours; avoid direct sun or humidity
- Cost range: $35–$90 per gallon
- Rough surfaces (brick, stucco) need 50% more
6 Common Paint Estimation Mistakes
Even experienced DIYers make these mistakes. Knowing them upfront saves money and extra store trips.
Not Subtracting Doors & Windows
Calculating full wall area without deducting openings leads to over-buying by 0.5–1 gallon per room. Subtract 20 sq ft per door and 15 sq ft per window.
Forgetting the Second Coat
One coat almost never provides full, even coverage over a different color. Always multiply your coverage calculation by your number of coats (usually 2) before buying.
Using the Wrong Coverage Rate
Coverage printed on paint cans assumes ideal smooth surfaces. Real-world coverage is often 10–15% less. Textured or porous walls need significantly more paint.
Skipping the 10% Waste Buffer
Paint is lost to roller nap absorption, drips, and touch-ups. Running out before finishing a wall creates a visible dry seam. Always add 10% to your total estimate.
Not Calculating Primer Separately
Primer covers only 200–300 sq ft per gallon — much less than topcoat. New drywall, dark color changes, and stain coverage always require primer as a separate line item.
Using Interior Paint Outdoors
Interior and exterior paints are chemically different. Using interior paint outside causes peeling within months. Exterior paint also covers fewer sq ft due to rougher surfaces — account for this.
Paint Calculator FAQ
Answers to the most common questions about estimating paint for home projects.
Multiply your room perimeter (2 × length + width) by wall height to get total wall area. Subtract 20 sq ft per door and 15 sq ft per window. Divide the remaining area by your paint's coverage rate (typically 350 sq ft/gallon). Multiply by the number of coats. Add a 10% waste buffer. Round up to the nearest full gallon.
One gallon of interior latex paint covers 350 to 400 square feet on smooth walls with a single coat. Exterior and textured surfaces typically yield 250–350 sq ft per gallon. Primer covers less — around 200 to 300 sq ft per gallon — and should always be calculated separately.
A 10×10 room with 8-foot ceilings has about 280 sq ft of gross wall area. After deducting one door (20 sq ft) and two windows (30 sq ft), the paintable area is approximately 230 sq ft. For two coats at 350 sq ft/gallon coverage, you need roughly 1.5 gallons — so purchase 2 gallons to include a safety buffer.
Only if you are painting the ceiling. If you use ceiling paint (which is typically a different product), add the ceiling area (room length × width) as a separate calculation. Our calculator includes an optional ceiling toggle for convenience. Most standard ceiling paint also covers 350–400 sq ft per gallon on smooth surfaces.
Yes. The standard approach is to subtract 20 sq ft per door and 15 sq ft per window from your gross wall area. This gives you a more accurate paintable area and prevents you from buying more paint than you actually need. Our calculator handles this deduction automatically.
For 1,500 sq ft of exterior smooth siding at 300 sq ft/gallon coverage with two coats, you need approximately 10 gallons. Add a 10% waste buffer and buy 11 gallons. For rough siding, wood, or brick, coverage drops significantly — plan for 13–16 gallons. Always factor in surface texture before purchasing.
One coat is rarely sufficient for a complete paint job. It's acceptable only for minor touch-ups over an identical existing color using the same paint. For any color change, new surfaces, or full repaints, two coats are the minimum for even coverage, proper color depth, and long-term durability.
Paint material costs for an average 12×12 bedroom (two coats) typically range from $50 to $130 depending on paint brand and quality. Hiring a professional painter adds $300–$600 per room in labor. Use the price-per-gallon field in our paint calculator to get an instant materials cost estimate for your specific project.
Interior paint covers 350–400 sq ft per gallon on smooth surfaces. Exterior paint typically covers 250–350 sq ft per gallon due to rougher textures and greater absorption. Rough exterior surfaces like brick and stucco may only yield 150–200 sq ft per gallon. Always set the coverage rate in our calculator based on your specific surface type.
For vaulted ceilings, use the average wall height. If one wall spans from 8 to 14 feet, the average height is 11 feet. For irregular shapes, measure each wall individually, calculate their area separately, and then add the totals together. Our calculator also accepts manual area input for rooms that don't follow a standard rectangular shape.
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