How Many Gallons of Paint to Paint a House — 5 Tips: Practical rules, quick math, and design ideas to estimate paint needed for interior and exterior projects

I once agreed to paint a client’s tiny studio for a fixed price and wildly underestimated the gallons because I forgot to count a vaulted ceiling—lesson learned the hard way. Small mistakes like that taught me to measure smarter, and to remember how small spaces can actually spark big creativity. If you sketch a plan before you buy paint, you’ll save money and regret; for visual helpers I often point people to space planning examples to check proportions and paint zones.

1. The basic coverage rule: 350–400 sq ft per gallon

Most interior paints cover roughly 350–400 square feet per gallon for one coat. That means you calculate the total wall area (length × height for each wall), subtract windows and doors, then divide by coverage to get gallons per coat. It’s simple and reliable, but expect variation by paint type and wall texture—rough surfaces can eat paint.

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2. Two coats are usually required — double your math

If your walls need two coats (very common for color changes or fresh drywall), multiply the one-coat gallons by two. Primer can reduce finish coats, but primer itself has coverage similar to paint, so factor it in if you’re priming large areas. Budget-wise, two coats increase cost but produce the durable, even finish worth the extra gallon purchases.

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3. Don’t forget trim, doors, and ceilings

Trim, doors, and ceilings are calculated separately: trim coverage is about 350–450 sq ft per gallon depending on brush/roller use; doors take about a half to one quart each. For ceilings, multiply ceiling area (floor area) by coverage—ceilings often need more paint if textured. Planning these separately avoids last-minute trips to the store and keeps color choices consistent.

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4. Exterior painting needs its own rules

Exterior paint coverage is similar per gallon but the total paintable surface includes siding, gables, and trim, and is affected by surface porosity and weathered wood. For typical siding, a rough rule is 1 gallon per 250–350 sq ft for one coat—again, factor in two coats, primer, and the condition of the existing surface. It’s a bit messier than interiors, but a careful measurement and a good primer save repaint cycles.

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5. Smart purchasing and design choices to reduce gallons

Choose higher-quality, higher-hide paints when possible: they often require fewer coats. Use accent walls, durable high-traffic washable paints in zones, and coordinate trims to limit the number of different finishes. If you’re reworking a kitchen, for example, targeted repainting plus good planning with your kitchen layout ideas can reduce wasted gallons and give the biggest visual return.

Quick practical checklist: measure wall areas carefully, subtract openings, use 350–400 sq ft/gallon as a baseline, double for two coats, add 10% extra for touch-ups, and consider primer needs. I also keep a small reserve quart for future patching—saved me on a late-night scuff fix more than once.

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FAQ

Q: How do I calculate total wall area?

A: Measure each wall’s length and multiply by ceiling height, then add all walls together. Subtract the area of doors and windows to get paintable wall area.

Q: How many gallons for a 10×12 room with 8ft ceilings?

A: Wall area = (10+10+12+12) × 8 = 352 sq ft. At 350–400 sq ft per gallon, you’ll need about 1 gallon for one coat, 2 gallons for two coats, plus touch-up allowance.

Q: Does primer count as one of the coats?

A: Primer is separate: it seals the surface and can reduce the number of finish coats needed, but you should still count its gallons when estimating total product needed.

Q: What coverage number should I use for rough plaster or textured walls?

A: Use the lower end—around 250–300 sq ft per gallon—because texture increases surface area and paint absorption. Always test a small area if unsure.

Q: How much paint for exterior siding on a 2,000 sq ft house?

A: Exterior paint needs vary a lot; a rough estimate is 1 gallon per 250–350 sq ft for one coat of siding. Factor in trim, porosity, and two coats when budgeting.

Q: Should I buy exactly the calculated gallons or more?

A: Buy about 10% extra for touch-ups and cutting in, and keep a labeled quart for future repairs. Color batches can vary, so having extra from the same batch avoids mismatch.

Q: Where can I find a paint calculator or official coverage info?

A: Many manufacturers and retailers offer online calculators; for manufacturer guidance, Sherwin-Williams notes that a gallon typically covers roughly 350–400 sq ft, depending on product and surface (see https://www.sherwin-williams.com for details).

Q: Can smart design choices reduce how much paint I buy?

A: Yes—using accent walls, coordinating trim colors, and choosing paints with better hide reduces the number of coats and total gallons. Tools that visualize color placement can help you pick strategies that maximize impact with minimal paint; I sometimes use AI design inspiration to preview options.

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