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Tool: Paint Calculator — Gallons, Liters & Coats | Printed on:
Paint Calculator — Gallons, Liters & Coats
Calculate gallons and liters of paint for interior walls, ceilings, and trim.
Project Specifications
Paint Coverage Formula
Transparent mathematical methodology explained in plain English.
Gallons = [ (Perimeter × Height) – Door/Window Openings ] × Coats / Coverage per Gal Multiply room perimeter by ceiling height, subtract openings (21 sq ft per door, 15 sq ft per window), multiply by number of coats, and divide by 350 sq ft/gal.
Variables Explained:
- Coverage: Standard interior latex paint covers 350 to 400 sq ft per gallon.
12 ft × 15 ft Room with 8 ft Ceilings (2 Coats)
Calculating paint for a standard bedroom with 1 door and 2 windows.
- Perimeter: 2 × (12 + 15) = 54 ft.
- Gross wall area: 54 × 8 = 432 sq ft.
- Deductions: 1 door (21 sq ft) + 2 windows (30 sq ft) = 51 sq ft.
- Net wall area: 432 - 51 = 381 sq ft.
- Two coats: 381 × 2 = 762 sq ft total coverage.
- Gallons needed (at 350 sq ft/gal): 762 ÷ 350 = 2.18 gallons.
Frequently Asked Questions: Paint Calculator
Click any question below to expand quick answers, formulas, and expert tips.
How many square feet does one gallon of paint cover?
One gallon of quality interior paint covers between 350 and 400 square feet with a single coat. On unprimed porous drywall or rough masonry, coverage drops to approximately 250 to 300 square feet.
How do you calculate how many gallons of paint to buy for a room?
Measure room perimeter (2 × length + 2 × width) and multiply by ceiling height to get gross wall area. Subtract 21 sq ft per standard door and 15 sq ft per window. Multiply net area by the number of coats (usually 2) and divide by 350.
Do I always need two coats of interior wall paint?
Yes. Even "paint and primer in one" formulas require two coats for rich color depth, consistent sheen, and durable scrub resistance. When painting light colors over dark walls, three coats or a tinted primer may be required.
How much paint is subtracted for doors and standard windows?
Standard estimation rules subtract 21 square feet for each standard interior door (3 ft × 7 ft) and 15 square feet for each standard residential window (3 ft × 5 ft).
How much paint do I need for a 12x12 bedroom?
A 12 ft × 12 ft room with 8 ft ceilings has approx. 384 sq ft of gross wall area. Deducting 1 door and 2 windows leaves ~333 sq ft. For two coats (666 sq ft), you need 1.9 gallons — buy two 1-gallon cans.
What is the difference between paint finishes (Flat, Eggshell, Satin, Semi-Gloss)?
Flat/Matte hides wall flaws and is best for ceilings and low-traffic bedrooms. Eggshell and Satin offer a soft sheen that is easily cleaned, ideal for living rooms and hallways. Semi-Gloss provides high moisture resistance, ideal for kitchens, bathrooms, and baseboards.
Do I need primer before applying regular wall paint?
Primer is essential over fresh unpainted drywall, bare wood, repaired spackle patches, water stains, smoke residue, or when making drastic color shifts (e.g. dark navy to off-white).
How do I calculate paint needed for ceilings and baseboards?
For ceilings, multiply room length by width (e.g. 12 × 15 = 180 sq ft ÷ 350 = 0.51 gal, purchase 1 gallon). For baseboards, multiply perimeter by height in feet (e.g. 54 ft × 0.33 ft = 18 sq ft, purchase 1 quart).