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Paint Calculator · How Many Gallons Do You Need?

Buying the right amount of paint saves a second trip to the store and a color mismatch between batches. The number you need depends on four things: the paintable area, how many coats you will apply, how far a gallon stretches and what you leave out, such as doors and windows. This calculator takes the size of your room, works out the wall area, subtracts the openings and tells you how many gallons to buy.
Enter the room length, width and ceiling height in feet, the number of doors and windows, and whether you also paint the ceiling. If you already know the area to paint, switch to square feet and type it in. Choose one, two or three coats and the coverage printed on the can, which is usually 350 to 400 square feet per gallon on a smooth, previously painted wall and less on rough, textured or bare surfaces. You can add one coat of primer, and a price per gallon gives you the cost.
The result is rounded up to whole gallons, because paint is sold that way, and shows how many five-gallon buckets that is when the job is large. Baseboards, trim, closets and cabinets are not counted, so buy a small can of trim paint separately. If your home was built before 1978, test for lead paint before you sand or scrape.
Standard: 14
Standard: 12
Most US homes have 8 foot ceilings
Each door counts as 20 sq ft that does not get painted
Each window counts as 15 sq ft that does not get painted
On the can. Most paints cover 350 to 400 sq ft; rough or porous walls take less
Example price only. Check the can at your store
Room size:
Surface:
Coats:

Detailed Breakdown

6 metrics
PRIMARY RESULT
3 gallons
Gallons to Buy (2 Coats)
Gallons Needed Before Rounding Up
2.09 gallons
Paintable Area
366 square feet
Wall Area
416 square feet
Doors and Windows Subtracted
50 square feet
Paint Cost (3 × $40.00)
$120.00

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exploreReal-World Scenarios

When to Use This Calculator

Everyday financial situations where this tool gives you fast, accurate clarity:

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Repaint a Bedroom or Living Room

Get the gallons for the walls, with doors and windows already subtracted.

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Add the Ceiling

Switch on the ceiling to see how many extra gallons it needs.

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Plan Coats and Primer

Compare one, two and three coats and add a coat of primer for new or stained walls.

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Budget the Job

Enter a price per gallon to see the paint cost before you shop.

Calculation Formula

Standard Method

Wall area = 2 × (Length + Width) × Ceiling height Ceiling area = Length × Width (only if you paint the ceiling) Openings = Doors × 20 sq ft + Windows × 15 sq ft Paintable area = Wall area + Ceiling area − Openings Gallons needed = Paintable area × Coats ÷ Coverage per gallon Gallons to buy = Gallons needed, rounded up to a whole gallon Primer gallons = Paintable area × 1 coat ÷ 250 sq ft per gallon Cost = Gallons to buy × Price per gallon A door of about 3 × 6.7 feet is 20 sq ft and a window of about 3 × 5 feet is 15 sq ft. Coverage is the claim printed on the can for a smooth, previously painted surface. Primer coverage of 250 sq ft per gallon is a planning figure, so check the primer label.

Benchmark Example:
A 14 ft by 12 ft room with 8 ft ceilings, one door and two windows, walls only, two coats at 350 sq ft per gallon: • Wall area: 2 × (14 + 12) × 8 = 416 sq ft • Openings: 1 × 20 + 2 × 15 = 50 sq ft • Paintable area: 416 − 50 = 366 sq ft • Gallons needed: 366 × 2 ÷ 350 = 2.09 gallons, so buy 3 gallons • Cost at $40 a gallon: 3 × $40 = $120 If you also paint the ceiling (168 sq ft), the paintable area becomes 534 sq ft and you need 3.05 gallons, so 4 gallons. With one coat of primer on the walls only, add 366 ÷ 250 = 1.46 gallons, so 2 gallons of primer. Because 2.09 gallons rounds up to 3, you will have almost a gallon left for touch-ups. When the number lands just above a whole gallon, such as 2.05, you can sometimes get by with a gallon and a quart.

Frequently Asked Questions

8 answers

Applicable Laws & Regulatory Standards

U.S. Environmental Protection Agency & Consumer Product Safety Commission

Official Legal Reference
40 CFR Part 745, Subpart E — Residential Property Renovation (Lead Renovation, Repair and Painting Rule)

Requires certified firms and lead-safe work practices when renovating or painting homes and child-occupied facilities built before 1978. This calculator estimates paint quantity only, and old paint should be tested before you sand or scrape it.

16 CFR Part 1303 — Ban of Lead-Containing Paint and Certain Consumer Products Bearing Lead-Containing Paint

Bans consumer paint with more than 0.009 percent lead by weight, which is why paint bought today is lead-free while paint on older walls may not be.

40 CFR Part 59, Subpart D — National Volatile Organic Compound Emission Standards for Architectural Coatings

Limits the volatile organic compounds in paints and primers sold in the United States, the reason many paints advertise low-VOC formulas. Coverage per gallon on the label is based on these products.

Legal citations and statutes are provided for consumer transparency, analytical validation, and educational reference under U.S. consumer financial regulations. This tool is not a substitute for formal legal, accounting, or tax counsel.

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