🎨 Paint Calculator
Subtracts ~20 sq ft each
Subtracts ~15 sq ft each
Standard: 350-400 sq ft
For cost estimate
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— gal
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What Is a Paint Calculator?
A paint calculator is an essential tool that estimates how much paint you need for any painting project — whether you’re refreshing a single bedroom, renovating an entire house, or tackling a commercial space. By factoring in your wall area, the number of doors and windows, paint coverage per gallon, and the number of coats required, the calculator eliminates the guesswork that leads to wasted money on excess paint or frustrating mid-project trips to the hardware store.
According to the Paint Quality Institute, homeowners waste an average of 10-15% of purchased paint due to overestimation. A precise paint calculator can save you $40-$100 on an average room project. But beyond cost savings, accurate estimation helps you buy the right amount from a single batch — ensuring consistent color across your entire project.
How to Use the Paint Calculator
Our paint calculator is designed for simplicity and accuracy. Follow these steps to get a precise paint estimate for your project:
Step 1: Measure Your Walls
Calculate the total wall area by measuring the height and width of each wall and multiplying them. For a rectangular room, the formula is: (Wall 1 Height × Width) + (Wall 2 Height × Width) + (Wall 3 Height × Width) + (Wall 4 Height × Width). For example, a 12×12 room with 8-foot ceilings has approximately 384 square feet of wall area (4 walls × 12 ft × 8 ft). Enter this total into the calculator.
Step 2: Account for Doors and Windows
Doors and windows reduce the paintable surface. The calculator automatically subtracts 20 square feet per door and 15 square feet per window — standard industry estimates. A room with 1 door and 2 windows saves you about 50 square feet of painting area, which is roughly 1/7th of a gallon of paint per coat.
Step 3: Choose Your Coverage Rate
Most latex paints cover 350-400 square feet per gallon on smooth, primed surfaces. The default is 350 sq ft/gal — a conservative estimate that accounts for surface texture and application method. If you’re using premium paint on smooth drywall, you may achieve closer to 400 sq ft/gal. For rough surfaces like stucco or brick, coverage can drop to 200-300 sq ft/gal.
Step 4: Select Number of Coats
Most interior walls require 2 coats for uniform coverage — one coat rarely provides complete hide, especially with color changes. Factors that increase the coat count include: painting light over dark colors (may need 3+ coats with primer), using low-quality paint, painting over stains, or working with highly porous surfaces. The calculator multiplies your area by the number of coats.
Step 5: Enter Paint Cost (Optional)
Add the price per gallon for a total project cost estimate. Premium paints like Sherwin-Williams Emerald or Benjamin Moore Aura cost $60-90 per gallon, while budget options range from $25-40. The calculator rounds up to the nearest full gallon and multiplies by your price per gallon.
Paint Coverage Guide: How Much Area Does One Gallon Cover?
Paint coverage varies significantly based on surface type, texture, and paint quality. Use this reference table to adjust your coverage estimate:
| Surface Type | Coverage (sq ft/gal) | Gallons for 400 sq ft | Notes |
|---|---|---|---|
| Smooth drywall (primed) | 400 | 1.0 | Best-case scenario |
| Textured drywall | 300-350 | 1.1-1.3 | Orange peel or knockdown |
| Rough plaster | 200-300 | 1.3-2.0 | Older homes |
| Stucco / Brick | 150-250 | 1.6-2.7 | Highly porous |
| Wood siding | 200-300 | 1.3-2.0 | Depends on grain |
| New drywall (unprimed) | 250-300 | 1.3-1.6 | Absorbs more paint |
Factors That Affect Paint Coverage
Surface Porosity
Porous surfaces absorb paint into their structure, reducing visible coverage. New drywall, bare wood, and masonry are the most absorbent and may require a primer coat before painting. Primer seals the surface and creates a uniform base, which can reduce the number of paint coats needed by 1-2. Without primer on porous surfaces, you may need 50-100% more paint than our calculator estimates.
Paint Quality
Premium paints contain higher solids content (more pigment and binder relative to solvent), which provides better hide per gallon. Budget paints often require an extra coat to achieve the same coverage, negating the initial cost savings. A $65 premium gallon that covers in 2 coats may be cheaper overall than a $30 budget gallon that requires 3-4 coats. Consumer Reports testing shows that top-rated paints can cover 400+ sq ft/gal while bottom-rated paints may cover only 200-250 sq ft/gal.
Color Changes: Light to Dark and Dark to Light
Light Over Dark
Painting a light color over a dark one typically requires 2-3 coats minimum, often with a tinted primer in-between. For example, covering a deep navy blue wall with off-white may need a gray-tinted primer plus 2-3 finish coats. The calculator’s default of 2 coats won’t suffice for dramatic light-over-dark transitions — increase to 3-4 coats in these cases.
Dark Over Light
Dark colors over light surfaces generally cover better — 2 coats are usually sufficient. However, very saturated colors (deep reds, rich blues, forest greens) may still need 3 coats because dark pigments are more transparent than you’d expect. Using a gray-tinted primer under deep colors can reduce the finish coats needed from 3 to 2, saving up to 33% on paint.
How Much Paint Do I Need? Room-by-Room Guide
Quick reference for common room sizes (walls only, standard 8-ft ceilings, 1 door + 2 windows, 2 coats, 350 sq ft/gal coverage):
| Room Size | Wall Area (sq ft) | Paint Needed | Estimated Cost* |
|---|---|---|---|
| Small bathroom (5×8) | 208 | 1 gallon | $35-70 |
| Bedroom (10×12) | 352 | 2 gallons | $70-140 |
| Master bedroom (14×16) | 480 | 3 gallons | $105-210 |
| Living room (16×20) | 576 | 3-4 gallons | $105-280 |
| Open-plan area (20×30) | 800 | 5 gallons | $175-350 |
*Cost estimate based on $35-70/gallon paint. Premium paints can double this.
The calculator above provides a more precise estimate by factoring in your specific door/window counts, coverage rate, and preferred number of coats. For whole-house projects, calculate each room separately and sum the totals — different rooms often require different paint types (e.g., semi-gloss for bathrooms/kitchens, flat or eggshell for bedrooms).
Pro Tips for an Accurate Paint Estimate
Always Measure, Never Guess
Use a laser measure for accuracy — a 25-foot tape measure works but introduces 1-2% error per measurement. For irregular rooms, break the space into rectangles, calculate each separately, and sum them. Don’t forget to measure closet walls if you’re painting inside closets — they add surprising square footage (a 6×2 ft closet with 8-ft ceiling adds 50 sq ft per coat).
Buy from the Same Batch
Paint mixed at different times can have slight color variations even with the same formula. When you need multiple gallons, ask the store to mix them together in a 5-gallon bucket (“boxing”) or ensure all cans come from the same tint batch. The calculator’s rounding-up feature already accounts for this — buy all gallons at once rather than returning for a “touch-up” can.
Add 10% Contingency
Our calculator provides a precise estimate, but professionals recommend adding 10% extra for touch-ups, spills, and uneven application. If the calculator says you need 3.2 gallons (buy 4), the extra 0.8 gallons becomes your touch-up reserve. Label and store leftover paint properly — it lasts 2-5 years when sealed tightly and stored at room temperature.
Don’t Forget the Ceiling
If you’re painting the ceiling, calculate its area separately (length × width) and add it to your total. Ceilings typically need 1-2 coats and are often done in flat white ceiling paint (which may have different coverage than wall paint). A 12×12 bedroom ceiling adds 144 sq ft per coat — roughly 0.4 gallons per coat at 350 sq ft/gal.
Consider the Application Method
Rollers use 10-15% more paint than spraying due to absorption into the roller cover. Sprayers waste 20-40% through overspray unless you’re experienced. For roller application, decrease your coverage estimate by 10% (use 315 sq ft/gal instead of 350). For spray application by a professional, use the standard 350-400 sq ft/gal rate.
Frequently Asked Questions
How many gallons of paint do I need for a 12×12 room?
A 12×12 room with 8-foot ceilings has approximately 384 square feet of wall area (4 walls × 12 ft × 8 ft). Subtract 1 door (20 sq ft) and 2 windows (30 sq ft) for a net of 334 sq ft. With 2 coats at 350 sq ft/gal coverage, you need 1.9 gallons — so buy 2 gallons. If you’re painting the ceiling too, add another gallon (144 sq ft ceiling × 2 coats = 0.8 gal, buy 1 gallon of ceiling paint).
Do I need primer before painting?
Primer is recommended in these situations: (1) new drywall — unprimed drywall absorbs paint unevenly and may show joint compound “flashing”; (2) dramatic color changes — especially light over dark; (3) stained surfaces — water stains, smoke, or tannin bleed-through; (4) glossy surfaces — primer improves adhesion. If none of these apply and you’re using quality paint, you can often skip primer. Using a paint-and-primer-in-one product on previously painted walls in good condition works well for 90% of repainting projects.
How much does it cost to paint a room?
DIY painting a 12×12 room costs approximately $100-200 for materials: 2 gallons of quality paint ($70-140), plus supplies — roller covers ($10), painter’s tape ($8), drop cloths ($15), brushes ($15), and a roller frame/tray ($20). Professional painting of the same room costs $350-800 including labor. Per square foot, DIY painting costs $0.25-0.50/sq ft for materials, while professional painting runs $1.50-3.50/sq ft all-inclusive.
How many coats of paint do I need?
Most projects require 2 coats for uniform coverage. Exceptions: (1) 1 coat may suffice when repainting the same color with quality paint on a well-maintained wall; (2) 3-4 coats are needed for light-over-dark color changes or when using budget paint on a porous surface. A tinted primer — primer mixed to a shade close to your finish color — can reduce the number of finish coats from 3 to 2, saving both time and money.
What is the standard coverage of one gallon of paint?
The industry standard is 350-400 square feet per gallon on a smooth, primed surface with one coat. This is the manufacturer’s stated coverage printed on every paint can. However, real-world coverage is often lower — Consumer Reports testing found actual coverage averages 300-350 sq ft/gal for most major brands. Using 350 sq ft/gal in our calculator provides a conservative, realistic estimate that accounts for typical application conditions.
How do I calculate paint for multiple rooms?
Calculate each room separately using our calculator, then sum the gallon requirements across all rooms. Don’t average them — a 12×12 bedroom and a 5×8 bathroom have very different paint needs per square foot due to door/window ratios. For whole-house projects, create a simple spreadsheet: room name, wall area, doors, windows, net area, coats, gallons. Most 2,000 sq ft homes require 12-15 gallons of wall paint for a complete interior repaint (walls only, excluding ceilings and trim).
Conclusion: Stop Guessing, Start Painting
An accurate paint estimate is the foundation of a successful painting project. Overbuying means wasting money on paint that sits in your garage for years. Underbuying means a frantic mid-project hardware store run, potentially ending up with a mismatched batch that ruins your finish. Our Paint Calculator eliminates both problems by giving you a precise, project-specific estimate based on your exact measurements, surface conditions, and paint preferences.
Whether you’re refreshing a powder room with a single gallon or repainting an entire house, the same principles apply: measure accurately, account for obstructions, choose the right coverage rate for your surface, and always buy a little extra for touch-ups. Use the calculator above to get your estimate in seconds — then head to the paint store with confidence, knowing you’re buying exactly what you need.
Disclaimer: This calculator provides estimates for planning purposes only. Actual paint requirements may vary based on surface texture, application method, paint brand, color choice, and environmental conditions. Always consult the paint manufacturer’s label for product-specific coverage data. Prices shown are estimates based on national averages and may not reflect local market conditions or specialty paint products.