Paint Calculator
Buying too much paint wastes money, while running short mid-project risks mismatched colors between batches. Our paint calculator estimates exactly how many gallons you need based on your room dimensions, paint type, and number of coats. We account for doors and windows to give you an accurate paintable area. Whether you're using flat, eggshell, or high-gloss paint, get the right amount for a professional finish without excess waste or stressful mid-project store runs.
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- Updated 2026-02-04
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Average length of walls to paint
Most projects need 2 coats
Standard door ~20 sq ft, window ~12 sq ft
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How It's Calculated
1. Total Wall Area = Wall Length × Height × Number of Walls 2. Paintable Area = Total Area - Openings 3. Area with Waste = Paintable Area × (1 + Waste Factor) 4. Coverage Needed = Area with Waste × Number of Coats 5. Gallons = Coverage Needed ÷ Coverage per Gallon 6. Gallons (rounded up) = Ceiling of gallons 7. Cost = Gallons × Price per Gallon Coverage per gallon: • Flat/Eggshell: 400 sq ft • Satin/Semi-Gloss: 350 sq ft • High-Gloss/Primer: 300 sq ft
Worked Examples
A 12×12 room with 8 ft ceilings: Total area = 384 sq ft. Minus 45 sq ft openings = 339 sq ft. With 10% waste = 373 sq ft. Two coats = 746 sq ft. At 400 sq ft/gal = 2 gallons. Cost ~$80.
A 16×16 room with 9 ft ceilings: Total area = 576 sq ft. Minus 80 sq ft openings = 496 sq ft. With waste = 546 sq ft. Two coats = 1,092 sq ft. At 350 sq ft/gal = 4 gallons. Cost ~$180.
Changing from dark to light may need 3 coats. Area = 392 sq ft - 55 = 337 sq ft. With waste = 371 sq ft. Three coats = 1,113 sq ft. Need 3 gallons. Consider tinted primer to reduce coats.
Assumptions & Waste Factor
- Coverage rates are for smooth surfaces
- Textured surfaces require 15-25% more paint
- Dark-to-light color changes may need extra coats
- Primer recommended for new drywall or color changes
- Professional painters often achieve better coverage
- Gallon covers 350-400 sq ft per coat (varies by sheen)
Standard 10% waste accounts for trim, touch-ups, and minor spills. Textured surfaces may need 15-20% more. Smooth new drywall with primer may use as little as 5% waste.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12×12 room with 8-foot ceilings has about 384 sq ft of wall space. Subtracting for a door and window (~40 sq ft), you need about 2 gallons for two coats. Add another gallon if painting the ceiling.
How many coats of paint do I need?
Most projects need two coats for even coverage and durability. One coat may suffice for same-color touch-ups. Dark-to-light changes may need three coats. Using primer first reduces finish coats needed.
Should I use primer before painting?
Use primer on new drywall, over stains or dark colors, on glossy surfaces, or when switching paint types (latex over oil). Modern paint-and-primer combos work for basic same-color or light-to-dark changes.
What is the difference between flat and eggshell paint?
Flat paint hides imperfections but marks easily, making it best for ceilings and low-traffic areas. Eggshell has slight sheen, is easier to clean, and works for most living spaces. More sheen = more durable but shows imperfections.
How long does paint take to dry?
Paint is dry to touch in 1-2 hours and ready for second coat in 2-4 hours. Full cure takes 2-4 weeks, so avoid washing or heavy contact during this time. Latex dries faster than oil-based paint.
What coverage should I expect per gallon?
Quality paint covers 350-400 sq ft per gallon on smooth surfaces. Textured surfaces, porous materials, or poor-hiding colors reduce coverage by 15-25%. Always check the can for manufacturer specifications.
Does paint expire?
Unopened latex paint lasts 10+ years; opened paint lasts 2-5 years if sealed properly. Frozen or separated paint is unusable. Stir old paint thoroughly and if it blends smoothly, it's usually fine to use.
How do I estimate paint for trim and doors?
A standard door needs about 1 quart; window trim needs 1/4 pint each. For baseboards and crown molding, calculate linear feet and use 1 quart per 75-100 linear feet per coat.
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Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements may vary based on site conditions, installation methods, and other factors. Always consult with a qualified professional before making purchasing decisions.