Exterior Paint Calculator

You're standing in a paint aisle holding one gallon, wondering: "Will this even cover the whole room, or will I be back here in two hours?"

Buying too little paint forces an extra trip and risks shade mismatch between batches. Buying too much wastes money. A simple paint calculator eliminates the anxiety-tell it your wall dimensions, and it delivers the exact number of gallons needed, including an allowance for waste and uneven coverage.

What This Calculator Does

This tool estimates paint coverage by converting your room dimensions into wall area, accounting for windows and doors you won't paint, then dividing by typical paint coverage (roughly 350–400 square feet per gallon for standard interior paint, single coat). You tell us the room's length, width, ceiling height, number and size of windows and doors, and whether you're doing one or two coats. We calculate the total gallons you'll need and round up to whole gallons (since paint is sold in full-gallon containers).

How to Use This Calculator

Step 1: Measure the length and width of the room in feet, and note the ceiling height. Standard ceilings are 8 feet; newer homes sometimes go to 9 or 10 feet.

Step 2: Count your windows and doors. Estimate their size-a standard entry door is about 3 feet wide by 7 feet tall; a typical window is roughly 3 feet wide by 4 feet tall. Add up the total square footage of these openings.

Step 3: Input whether you're painting one coat or two. Two coats provide better coverage and hide darker walls or stains more effectively. Most interior paints need one coat if you're covering the same color, but two coats if you're going lighter or changing color dramatically.

Step 4: Specify whether you're painting walls only, or walls plus ceiling. Ceilings require more careful prep and often get a specialized ceiling paint, so we calculate them separately.

Step 5: The calculator shows total wall area (subtracting openings), divides by coverage, and tells you the exact gallon count you need.

The Formula Behind the Math

Here's the core calculation:

Total Wall Area = (Perimeter × Ceiling Height) − Opening Areas

Gallons Needed = Wall Area ÷ Coverage per Gallon

Let's work through a real example: a 14-foot × 12-foot bedroom with 8-foot ceilings, two 3×4 windows, and one 3×7 door. You're doing two coats.

Our calculator does this instantly, accounting for typical paint coverage and rounding up so you never run short.

Living Room with Vaulted Ceiling

A vaulted or cathedral ceiling dramatically increases wall area. A 16-foot × 14-foot room with an 8-foot wall that vaults to 14 feet in the center is noticeably taller. You can estimate vaulted walls as the average height (roughly 11 feet) times the perimeter: (16 + 14) × 2 × 11 = 660 square feet. Factor in windows and doors, divide by coverage, and you're looking at 2–3 gallons for one coat-more than a standard 8-foot ceiling room.

Bathroom or Powder Room (High Moisture Area)

A typical powder room is 5 feet × 7 feet with 8-foot ceilings and minimal openings. Wall area is roughly (5 + 7) × 2 × 8 = 192 square feet minus a small door = about 171 square feet. That's 0.5 gallons for one coat. Most people buy one gallon anyway (can't buy half). Bathroom paint should be semi-gloss or satin finish to resist moisture and allow easy wiping. That finish may have slightly lower coverage than standard flat/matte paint, so confirm on the can.

Exterior House Painting

Exterior calculations follow the same logic but account for higher coverage needs due to rough surfaces (wood siding, stucco) and weather exposure. Exterior paint typically covers 300–350 square feet per gallon-less than interior because the surface is more porous. A house that's 30 feet wide, 25 feet tall (wall height only, not roof), with four sides needs: 30 × 25 × 4 = 3,000 square feet, minus openings (maybe 400 sq ft of windows/doors) = 2,600 square feet. At 325 sq ft per gallon, that's 8 gallons for one coat, or 16 for two coats (which is standard for exterior work). Plus primer for any new surfaces.

Open-Concept Living/Kitchen/Dining

Open-plan spaces are taller and have fewer wall sections than traditional compartmentalized homes. A 25-foot × 20-foot open area with 10-foot ceilings and multiple large openings (patio door, island cutouts, pass-throughs) requires careful measurement. Perimeter is (25 + 20) × 2 = 90 linear feet; wall area is 90 × 10 = 900 square feet. Subtract windows and openings (say 200 sq ft), leaving 700 sq ft. At 375 per gallon, that's about 1.9 gallons for one coat-round up to 2. For two coats, 4 gallons.

Tips and Things to Watch Out For

Primer is its own purchase. If you're painting over a darker color, glossy surface, stains, or new drywall, primer is essential. It's usually cheaper than finish paint and covers similarly (roughly 300–350 sq ft per gallon). Don't skip it thinking finish paint will cover everything-you'll waste paint and end up with patchy, uneven results. Calculate primer area the same way, but buy 1-quart cans for spot priming touch-ups or stain blocking (shellac-based primer blocks stains better than latex).

Two coats is standard practice, especially for color changes. One coat rarely provides uniform coverage. Paint absorbs differently on unpainted drywall, patched areas, and previously painted surfaces. Two coats ensures a professional look and better color accuracy. The paint can label often specifies recommended coats-follow it.

Flat/matte paint hides imperfections but shows fingerprints; satin/semi-gloss is easier to clean. Flat paint has higher pigment and lower sheen, so it covers better and hides wall texture. But it marks easily. Bathrooms, kitchens, and high-touch areas (kids' rooms) benefit from satin or semi-gloss-more durable, wipeable, but you see every flaw in the wall beneath. Know your surface before you buy.

Always order 10–15% more material than your calculation to account for waste, cuts, breakage, and measurement errors. Spills, poorly sealed seams, drywall patching, and re-coating touched-up areas all consume extra paint. A 371-square-foot room should be ordered for 427 square feet, which is roughly 1.15 gallons for one coat-buy 2 gallons. Better to have leftover paint for touch-ups than run short mid-room.

Temperature and humidity affect paint application and drying. Most interior paints require 50–85°F and low humidity for proper curing. Cold or damp conditions slow drying and can cause mildew or adhesion issues. If painting in winter or a humid climate, ensure adequate ventilation and check the can for temperature recommendations.

Buy all paint from the same batch if possible. Even the same color and brand can have slight shade variations between batches. For large rooms, a single batch ensures consistency. If you must buy paint on different days, ask the store to tint several gallons at once, or accept minor shade variation.

Frequently Asked Questions

How much paint do I need for a 200 square foot room?

A 200-square-foot room (walls only, one coat) needs about 0.5–0.6 gallons. Buy 1 gallon. For two coats (which is common), buy 2 gallons. Ceilings add more square footage and require additional paint.

Should I paint the ceiling the same color as the walls?

Ceilings are typically painted white or off-white, even if walls are a bold color. White ceilings feel higher and brighter. But many designers now paint ceilings the same color as walls for a cocoon-like effect in bedrooms or intimate spaces. Either way, ceiling paint must account for its area separately-and use ceiling-specific paint (flat finish, covers well, easy application).

What's the difference between one and two coats?

One coat provides basic coverage; two coats ensures uniform color and better opacity, especially over darker previous colors. Two coats is standard in the trade and what most people expect for quality results. Check the paint can-it specifies recommended coats.

How do I paint over a dark or bold wall color?

Use primer first, even if the new color is also bold. Primer seals the old color and prevents bleed-through. Then apply two coats of finish paint. Skipping primer forces you to apply three or four coats of finish paint to achieve uniform coverage-a waste of money. Primer is cheaper and more efficient.

Can I use leftover paint from last year?

If it's been sealed properly (lid on tight, stored indoors at moderate temperature), yes. Stir it well to re-blend separated pigments. If it smells sour or has a skin on top that won't break up, toss it. Old latex paint can separate or grow mold; old oil paint can skin over and become unusable.

Does paint coverage vary by color?

Slightly. Light or neutral colors tend to cover slightly better; dark colors may require more coats. Reds, greens, and deep blues are notorious for poor coverage-some brands recommend three coats. Always check the can's coverage estimate; premium paints sometimes cover better per gallon than budget brands, making them a better long-term investment.

What if I'm painting trim, crown molding, or baseboards separately?

Treat trim as its own project. Measure the linear feet and thickness to estimate surface area. Trim paint is often high-gloss for durability and easy wiping. It covers similarly to wall paint but is applied differently (brush, not roller). Typically, trim uses far less paint than walls—1 quart might cover 200 linear feet of baseboard.

Related Calculators

Use the square footage calculator to measure wall areas precisely, and the drywall calculator if you're patching or replacing wall sections before painting. For exterior projects, check material cost estimator to budget primer, paint, and labor together.