Paint Calculator

A paint calculator turns wall sizes into cans: gallons = wall area × coats ÷ coverage per gallon. A 12 × 14 ft bedroom with 8 ft walls has 416 sq ft of wall; minus one door and two windows that leaves 372 sq ft. Two coats at 350 sq ft per gallon need 744 ÷ 350 ≈ 2.13 gallons, so buy 2 gallons and 1 quart.

How to use the paint calculator

1. Add a row for each room (“Room (4 walls)”) or list walls one by one with “Wall” and “Gable”. Sizes take feet and inches, or meters and centimeters.
2. Subtract doors and windows with the − Door and − Window buttons, then type their real sizes. Use Count for identical openings.
3. Enter the number of coats and the coverage per gallon printed on your can or its data sheet.
4. Tick “Also paint the ceilings” or “Prime first” if you need them. Open the options to set ceiling and primer coverage and to type prices.
5. Read the shopping list. Open “Show steps” to see every area and every rounding decision.

The paint formula

Every paint estimate comes down to three steps.

• Wall area = perimeter × wall height − doors − windows. For a rectangular room the perimeter is 2 × (length + width).
• Paint needed = wall area × number of coats ÷ coverage per gallon. Coverage is the area one gallon covers in one coat, in square feet per gallon.
• Cans to buy = round the gallons up to whole gallons plus quarts. A quart is ¼ gallon, so the leftover part of a gallon is rounded up to the next quarter.

A gable end is a triangle, so its area is base × height ÷ 2. A ceiling is length × width. In metric the same formula gives liters: area in m² × coats ÷ coverage in m² per liter. One US gallon is 231 cubic inches, about 3.785 liters.

How much area does a gallon of paint cover?

Coverage depends on the paint and the surface. Manufacturer data sheets state it as a range per gallon for one coat:

So 250 to 450 sq ft per gallon is a typical span for interior wall paint, and the calculator starts at 350. That is an example, not a promise: smooth, previously painted walls sit at the top of the range, while rough plaster, textured ceilings, bare drywall, patched areas and sprayed paint sit at the bottom. If your can says 400, type 400.

Worked examples

Example 1: a bedroom, two coats

The room is 12 ft by 14 ft with 8 ft walls. Perimeter = 2 × (12 + 14) = 52 ft, so the walls are 52 × 8 = 416 sq ft. The door is 3 ft by 6 ft 8 in. Since 8 in is 8 ÷ 12 ≈ 0.6667 ft, the door is 3 × 6.6667 = 20 sq ft. Two 3 × 4 ft windows are 24 sq ft. Wall area = 416 − 20 − 24 = 372 sq ft.

Two coats at 350 sq ft per gallon: 372 × 2 ÷ 350 ≈ 2.13 gallons. Two gallons fall short by about 0.13 gallon, which is about half a quart, so round up to 1 quart. Buy 2 gallons and 1 quart (2.25 gallons), with about 0.12 gallon left for touch-ups.

Example 2: adding the ceiling

The same ceiling is 12 × 14 = 168 sq ft. Two coats at 350 sq ft per gallon need 336 ÷ 350 = 0.96 gallon. Four quarts would be a full gallon, so buy 1 gallon of ceiling paint. Ceiling paint gets its own line in the shopping list.

Example 3: priming new drywall

New drywall soaks up the first coat, so many painters prime it. One coat of primer on the 372 sq ft of wall at 300 sq ft per gallon is 372 ÷ 300 = 1.24 gallons: buy 1 gallon and 1 quart. With the ceiling included the primer area is 372 + 168 = 540 sq ft, and 540 ÷ 300 = 1.8 gallons rounds to 2 gallons.

Example 4: gallons or quarts?

A quart often costs a good share of a gallon. With prices filled in, the calculator compares the mixes. Say a gallon costs $45 and a quart $18 (use your store’s prices). For 2.13 gallons, 2 gallons + 1 quart costs 2 × 45 + 18 = $108, while 3 gallons cost $135, so the quart wins. If a quart cost $25 instead, two quarts ($50) would already cost more than a gallon ($45), so for a job that needs half a gallon more the tool buys the extra gallon.

Quick reference: paint for a room

Four walls, 8 ft high, no doors or windows subtracted, two coats at 350 sq ft per gallon, rounded to whole gallons plus up to 2 quarts:

Subtracting a door and two windows usually saves a few quarts at most, so the table is a safe upper estimate for a quick shop.

Common mistakes

• Using floor area instead of wall area. A 12 × 14 room has 168 sq ft of floor but 416 sq ft of wall at 8 ft high.
• Forgetting the second coat. Most data sheets assume two coats for full color, so double the area before dividing.
• Writing 6 ft 8 in as 6.8 ft. Eight inches is 8 ÷ 12 ≈ 0.67 ft, so 6 ft 8 in is about 6.67 ft.
• Trusting the top of the coverage range on rough walls. Texture and porosity can cut coverage a lot. When in doubt, use a lower number.
• Rounding down. 2.13 gallons is not “about 2”. Running short mid-wall means a second trip and a possible color mismatch between batches.

Real-life uses

Plan an accent wall, a whole floor of rooms (one Room row each, or Count for identical rooms) or an exterior gable end. Painters can type a client’s price list to compare a 5-gallon pail against single gallons. Leftover paint is easiest to manage when you buy close to the amount you need; PaintCare, the paint-stewardship nonprofit, suggests measuring carefully and checking the coverage of the paint you are buying.

Results are estimates for planning. Real coverage depends on the surface, the color change, the tools and the person holding the roller, so check the can and ask at the paint counter if you are unsure.

Sources

• Benjamin Moore Regal Select Interior, technical data sheet N548 (spread rate 400–450 sq ft per gallon; checked November 2026)
• BEHR PREMIUM PLUS Interior Flat, product data (250–400 sq ft per gallon; checked November 2026)
• KILZ 2 All-Purpose primer, product data (300–400 sq ft per gallon; checked November 2026)
• PaintCare: paint calculators and buying the right amount (checked November 2026)
• NIST Handbook 44, Appendix C: general tables of units of measurement (1 US gallon = 231 cubic inches exactly ≈ 3.785 L; 4 quarts = 1 gallon; checked November 2026)

Paint quantity practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

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

With 8 ft walls the four walls total 2 × (12 + 12) × 8 = 384 sq ft. Two coats at 350 sq ft per gallon need 768 ÷ 350 ≈ 2.19 gallons, so 2 gallons and 1 quart. Subtract the door and windows for a closer figure.

How many square feet does a gallon of paint cover?

Data sheets for common interior paints list roughly 250 to 450 sq ft per gallon for one coat. The exact figure is printed on the can; rough or porous surfaces get less.

Do I need two coats of paint?

Usually. Paint makers such as Benjamin Moore recommend two coats for full color and performance, and big color changes may need more. A fresh coat in the same color over sound paint can sometimes be one coat.

Should I subtract doors and windows?

Yes, if you want an accurate number. A standard door and two windows remove about 44 sq ft from a bedroom, roughly a quarter of a gallon for two coats. If you will paint the door, leave it in.

Is it cheaper to buy quarts or a gallon?

It depends on your store’s prices. Type the gallon and quart prices into the calculator and it picks the cheaper mix that still covers the job. Without prices it rounds three quarts up to a gallon.

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• Square footage calculator for floors and odd-shaped rooms
• Area and perimeter calculator
• Unit converter for gallons, liters and square meters
• Sales tax calculator for the final bill