Paint Calculator

How to use this calculator

Measure each wall's width and multiply by the ceiling height, then add the walls together for the total wall area. Enter the area of doors and windows you will not paint — a standard interior door is about 20 ft² and a typical window 15 ft². Coats default to two, which is what most manufacturers recommend for a color change; coverage defaults to 350 ft² per gallon, a mid-range figure for interior latex on smooth drywall.

The result shows a decimal number of gallons and a rounded shopping list of gallons plus quarts. Working backwards is useful too: enter the amount of paint you already have to see how much wall it will cover, or how many coats it allows.

The formula

A_net = A − A_open

P = A_net × n / c

A: total wall area
A_open: doors & windows area
A_net: paintable area
n: number of coats
c: coverage per gallon
P: paint needed

Worked example

A 12 × 14 ft bedroom with 8 ft ceilings has 416 ft² of wall; call it 400 ft² after trim. One door and two windows take away about 40 ft². You plan two coats with a paint rated at 350 ft² per gallon.

Paintable area = 400 − 40 = 360 ft².
Paint = 360 × 2 coats ÷ 350 ft²/gal = 2.06 gal.
Rounded up to whole quarts: 2 gallons + 1 quart.

Buy two gallons and one quart, or three gallons if you want spare paint for touch-ups.

Typical coverage rates

Surface / product	ft² per gallon
Interior latex, smooth primed drywall	350–400
Interior latex, light texture	300–350
Primer on new drywall	200–300
Exterior paint on wood siding	250–350
Exterior paint on stucco or brick	150–200

Always check the label of the product you buy; coverage varies with sheen and brand.

Assumptions and limits

Coverage is the manufacturer’s figure for a single coat on a sealed surface. Porous or textured surfaces can need 30–50% more.
Ceilings and trim are not included — measure them separately and use a ceiling or trim paint coverage figure.

Frequently asked questions

How much paint does a 12 × 12 room need?

With 8 ft ceilings the walls total 384 ft². Minus about 40 ft² for a door and window and with two coats at 350 ft²/gal, you need just under 2 gallons.

Do I need primer?

Use primer on new drywall, bare wood, stains, or when changing from a dark to a light color. Self-priming paints reduce but do not always eliminate the need.

Is it cheaper to buy gallons or quarts?

A gallon usually costs about the same as three quarts, so if you need more than two-thirds of a gallon extra, buy the full gallon.