Question 1

How do I calculate how much paint I need?

Accepted Answer

Multiply the perimeter of the room by the wall height to get total wall area. Subtract the area of doors (about 21 sq ft each) and windows (about 15 sq ft each). Divide by the paint coverage rate (typically 350 sq ft per gallon for standard paint). Multiply by the number of coats. Add 10 percent for waste. This calculator does all steps automatically.

Question 2

How many square feet does a gallon of paint cover?

Accepted Answer

A standard gallon of interior paint typically covers 350 to 400 square feet per coat on smooth, primed surfaces. On rough or porous surfaces like bare drywall, unprimed masonry, or textured walls, coverage drops to 250 to 300 square feet per gallon. Premium paints and primers may differ. Always check the coverage rate listed on the paint can label.

Question 3

Do I need to add extra paint for waste?

Accepted Answer

Yes. Professional painters typically add 10 percent to the calculated paint quantity to account for waste from roller loading, brush drips, touch-ups, and the residual paint left in cans and trays. For complex rooms with many corners or architectural features, add 15 percent. This calculator includes a 10 percent waste buffer by default.

Question 4

How much paint do I need for two coats?

Accepted Answer

Multiply your single-coat paint quantity by two. Most interior painting projects require two coats for complete, even coverage, especially when changing colors dramatically or painting over dark colors. Some high-hide paints claim one-coat coverage, but two coats typically produce better durability and a more professional result.

Question 5

How do I calculate paint for a ceiling?

Accepted Answer

Ceiling paint quantity equals room length multiplied by room width, divided by the coverage rate. For example, a 12 by 15 foot room has a ceiling area of 180 square feet. At 400 sq ft per gallon, you need 180 divided by 400 = 0.45 gallons per coat. For two coats, you need 0.9 gallons. Round up to the nearest quart or gallon.

Question 6

What is paint coverage rate?

Accepted Answer

Paint coverage rate is how many square feet (or square metres) one gallon (or litre) of paint covers in a single coat. Standard interior latex paint covers approximately 350 to 400 square feet per gallon. High-gloss and semi-gloss paints spread more thinly, covering 250 to 350 sq ft per gallon. Primers typically cover 200 to 300 sq ft per gallon depending on surface porosity.

Question 7

Should I subtract doors and windows from my paint calculation?

Accepted Answer

Yes. Subtract the area of each door (typically 21 square feet for a standard 3 by 7 foot door) and each window (typically 12 to 15 square feet). However, if there are fewer than two doors and two windows in a room, many painters simply skip subtracting them and use the extra paint for touch-ups. Larger windows like bay windows or patio doors should always be subtracted.

Question 8

How much paint do I need for a 12x12 room?

Accepted Answer

A 12 by 12 foot room with 9-foot ceilings has a wall perimeter of 48 feet. Total wall area = 48 times 9 = 432 square feet. Subtract two doors (42 sq ft) and two windows (30 sq ft) = 360 square feet net. At 350 sq ft per gallon, you need about 1 gallon per coat. For two coats plus 10 percent waste, you need approximately 2.2 gallons total.

Question 9

What is the difference between interior and exterior paint?

Accepted Answer

Interior paint is formulated for indoor use with lower VOC (volatile organic compound) emissions, easy washability, and scrubbability. Exterior paint contains UV-resistant pigments, mildewcides, and flexible binders to withstand weather, temperature changes, and moisture. Coverage rates are similar, but exterior paint typically requires two coats due to weathering exposure. Never use interior paint outdoors as it will fail quickly.

Question 10

How many gallons of paint do I need for a 1500 sq ft house exterior?

Accepted Answer

A 1,500 sq ft house with average wall height typically has about 1,500 to 2,000 sq ft of exterior wall area (accounting for gable ends and trim). At 300 to 350 sq ft per gallon for exterior paint, you need approximately 5 to 7 gallons per coat. For two coats plus 10 percent waste, budget 11 to 15 gallons total. Trim and accents require an additional 1 to 2 gallons.

Question 11

How much does a gallon of paint cost?

Accepted Answer

As of 2026, interior latex paint ranges from about $25 to $35 per gallon for builder-grade paint, $45 to $65 for mid-range brands like Behr or Valspar, and $70 to $100 or more for premium brands like Benjamin Moore or Sherwin-Williams. Exterior paint costs $40 to $80 per gallon. A quart (quarter gallon) costs roughly one-third to one-half the price of a full gallon.

Question 12

What size roller should I use to paint a wall?

Accepted Answer

For smooth walls, a 3/8 inch nap roller gives the best finish. For textured or slightly rough walls, use a 1/2 inch nap. For rough masonry, stucco, or brick, use a 3/4 to 1 inch nap to push paint into the texture. A 9-inch roller width is standard for walls and ceilings. Use a 4-inch mini roller or brush for trim, corners, and cutting in.

Question 13

How long does it take to paint a room?

Accepted Answer

An experienced DIYer can typically paint a standard 12 by 12 foot room including cutting in, rolling walls, and a second coat in 4 to 6 hours spread over two days (allowing drying time between coats). First-time painters should budget 6 to 8 hours per coat. Professional painters can complete the same room in 2 to 3 hours due to experience and equipment.

Question 14

How do I convert square metres to gallons of paint?

Accepted Answer

First convert square metres to square feet by multiplying by 10.764. Then divide by your paint coverage rate in sq ft per gallon. Alternatively, use coverage in litres per square metre: standard paint covers about 10 to 12 square metres per litre. Divide total square metres by 10.5 for an approximate litre quantity per coat, then multiply by number of coats and add 10 percent.

Question 15

Should I prime before painting?

Accepted Answer

Prime bare drywall, raw wood, masonry, stain-prone surfaces, or when making a dramatic color change from dark to light. Primer improves paint adhesion, provides a uniform base, and can reduce the number of topcoats needed. A dedicated primer coat typically requires 1 gallon per 200 to 300 sq ft. Many modern paints include primer (paint-and-primer-in-one) which reduces the need for a separate primer step.

Question 16

What is the formula for calculating paint quantity?

Accepted Answer

Paint gallons = (Total paintable area in sq ft / Coverage rate in sq ft per gallon) x Number of coats x 1.10 (waste factor). Total paintable area = (Room perimeter x Wall height) - (Door area x Number of doors) - (Window area x Number of windows) + Ceiling area if painting. This calculator applies this formula automatically when you enter room dimensions.

Surface Type	Coverage (sq ft/gal)	Coverage (sq m/L)	Notes
Smooth / primed walls	380–400	11–12	Best coverage, minimal absorption
Standard interior (unpainted)	350–380	9–11	Most common scenario
Lightly textured / eggshell	300–350	8–10	Orange peel, skip trowel
Heavily textured	250–300	7–8.5	Popcorn, stomp, heavy knockdown
Bare drywall / primer	200–250	5.5–7	High absorption — always prime first
Masonry / brick	150–200	4–5.5	Very porous — may need masonry primer

Element	Standard Size (sq ft)	Standard Size (sq m)
Standard interior door (3×7 ft)	21 sq ft	1.95 sq m
Standard window	15 sq ft	1.39 sq m
Large bay window	30 sq ft	2.79 sq m
Sliding patio door	35 sq ft	3.25 sq m

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