Paint Calculator
Estimate paint quantity needed to cover any room.
About the Paint Calculator
The Paint Calculator estimates how many gallons of paint you need to paint a room based on its dimensions, the number of coats, whether you're painting the ceiling, and the number of doors and windows to subtract from the paintable wall area. The calculator uses the industry-standard coverage of 350 square feet per gallon for interior wall paint and accounts for typical door (21 sq ft) and window (15 sq ft) sizes.
Most DIY painters dramatically overestimate or underestimate paint needs. Buying too little means a mid-project trip to the hardware store and risking a color mismatch between batches. Buying too much wastes money ($30-50 per gallon) and creates disposal headaches since latex paint can't simply be poured down the drain. The calculator's Math.ceil rounding ensures you buy enough without massive overbuying.
The calculator also estimates primer separately at 400 sq ft/gallon (primer spreads thinner because it's absorbed into the raw surface). Primer is essential when painting over a darker color, painting new drywall, or covering stains — skipping it usually means 3+ coats of finish paint instead of 2, which costs more in the long run.
How It Works
Wall area = perimeter × height = 2(length + width) × height. The perimeter is the sum of all four walls; multiplying by ceiling height gives the total wall area. For a 14×12 room with 8-foot ceilings, that's 2 × (14 + 12) × 8 = 416 sq ft.
Subtract doors and windows. A standard interior door is 3 feet wide by 7 feet tall = 21 sq ft. A standard double-hung window is about 3×5 = 15 sq ft. These are subtracted from the wall area to avoid counting surfaces that won't be painted (the door itself is usually painted separately or not at all). For one door and two windows, subtract 21 + 2×15 = 51 sq ft, giving net wall area of 365 sq ft.
Ceiling area (if included) = length × width. For a 14×12 room, that's 168 sq ft. Total paintable area = net wall area + ceiling area. With 2 coats, the paintable area doubles (since each coat covers the same area): 2 × (365 + 168) = 1,066 sq ft.
Gallons needed = paintable area / 350 sq ft per gallon. So 1,066 / 350 = 3.05 gallons, rounded up to 4 gallons to buy. Primer coverage is slightly higher at 400 sq ft/gal because it's absorbed into the surface — for 533 sq ft of primer coverage, you need 2 gallons.
The 350 sq ft/gallon figure assumes a flat or eggshell finish on a smooth, primed surface. Glossy paints spread slightly less (300 sq ft/gal). Textured walls (knockdown, orange peel) reduce coverage to 250-300 sq ft/gal. Dark colors over light walls may require 3 coats — the calculator lets you set the coat count explicitly.
Worked Examples
Default: 14×12 room, 8-foot ceiling, 2 coats, paint ceiling, 1 door, 2 windows. Wall area = 416 sq ft, minus 51 sq ft for door+windows = 365 sq ft net. Ceiling = 168 sq ft. Total = 533 sq ft. With 2 coats = 1,066 sq ft. At 350 sq ft/gal, need 3.05 gallons, buy 4. Cost at $45/gal = $180.
Master bedroom 16×14, 9-foot ceiling, 2 coats, no ceiling, 2 doors, 3 windows. Wall area = 2×(16+14)×9 = 540 sq ft, minus 2×21 + 3×15 = 87 sq ft = 453 sq ft net. With 2 coats = 906 sq ft. Need 2.59 gallons, buy 3. Cost = $135.
Bathroom 8×5, 8-foot ceiling, 2 coats, paint ceiling (high humidity), 1 door, 1 window. Wall area = 2×(8+5)×8 = 208 sq ft, minus 21+15 = 36 sq ft = 172 sq ft. Ceiling = 40 sq ft. Total = 212 sq ft. With 2 coats = 424 sq ft. Need 1.21 gallons, buy 2. Use bathroom-specific paint with mildew resistance.
Living room 20×15, 10-foot ceiling, 3 coats (dark accent wall), no ceiling, 2 doors, 4 windows. Wall area = 2×(20+15)×10 = 700 sq ft, minus 2×21+4×15 = 102 sq ft = 598 sq ft. With 3 coats = 1,794 sq ft. Need 5.12 gallons, buy 6. Cost = $270. A single accent wall in a darker color would use much less.
When to Use This Tool
- Budgeting paint for a DIY room refresh before visiting the hardware store.
- Estimating total paint for a multi-room interior project (calculate each room and sum).
- Deciding between 1, 2, or 3 coats based on color change intensity.
- Comparing primer + 2 finish coats versus 3 finish coats without primer.
- Estimating cost for a professional painting quote to verify the contractor's materials list.
- Painting a bathroom, kitchen, or other high-humidity room with appropriate coverage assumptions.
- Planning an accent wall: calculate only that wall's area and reduce waste.
Limitations & Disclaimer
This calculator uses standard interior paint coverage of 350 sq ft/gallon, which varies in practice from 250-400 sq ft/gal based on paint quality, surface texture, roller nap, and application technique. The 21 sq ft per door and 15 sq ft per window are standard sizes but actual openings vary — measure manually for non-standard sizes. Trim paint (baseboards, crown molding, window casings) is not included and requires separate estimation. Textured walls reduce coverage 15-25%. The cost estimate uses $45/gallon as a mid-quality paint average; premium paints ($60-80/gal) and budget paints ($25-35/gal) will shift the cost significantly. Exterior paint, specialty paints (chalkboard, magnetic), and spray paint are not supported. See our disclaimer for full terms.
Frequently Asked Questions
How accurate is the 350 sq ft/gallon coverage?
It's a typical average for flat or eggshell interior latex on a primed, smooth wall. Actual coverage varies 250-400 sq ft/gal based on paint quality, surface texture, roller nap, and application technique. Textured walls (knockdown, orange peel) reduce coverage by 15-25%. Always buy slightly extra to be safe.
Should I include the ceiling?
Only if you're painting it. Ceilings are typically painted flat white and need separate calculations because they're a different surface. If your room has a popcorn ceiling, paint coverage drops to 200-250 sq ft/gal. New construction ceilings may need primer + 2 coats.
What about doors, baseboards, and trim?
Doors are subtracted from wall area in this calculator because they're typically painted separately or not at all. Trim (baseboards, crown molding, window casings) requires separate paint (usually semi-gloss) — estimate 1 quart per 100 linear feet of trim.
Do I really need primer?
If you're painting over a similar color, a quality self-priming paint may suffice with 2 coats. For darker colors over light, light over dark, fresh drywall, or stained walls, primer is essential. Skipping primer usually means 3+ coats of finish paint — which costs more than 1 coat of primer + 2 of finish.
Can I save the leftover paint?
Yes, properly sealed latex paint lasts 5-10 years. Store cans upside down (the paint seals the lid against air), in a temperature-stable location (50-80°F). Label each can with the room name and date. Future touch-ups will match exactly — impossible with newly purchased paint due to batch variation.
Why do I need to round up rather than buy the exact amount?
Purchasing paint in fractional gallons is impractical (quarts cost disproportionately). Buying slightly extra ensures you don't run out mid-project, which risks color mismatch between batches. Leftover paint is useful for future touch-ups. Overbuying by 10-15% is the standard recommendation.
Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team