Buying too little paint means a mid-project trip to the store and a risk of slightly different color batches. Buying too much wastes money. A few measurements and simple math get you close to the right amount.

Step 1: Measure the walls

Measure the length of each wall and the ceiling height in feet. For a rectangular room, add the length and width, double it, and multiply by the height:

wall area = 2 × (length + width) × height
Example: 2 × (14 + 12) × 8 = 416 sq ft

Step 2: Subtract doors and windows

A standard interior door is about 20 sq ft and an average window about 15 sq ft. Measure yours if they’re unusually large. In the example, one door and two windows remove 50 sq ft, leaving 366 sq ft.

Step 3: Add the ceiling if you’re painting it

The ceiling area is simply length × width. In the example, that’s 168 sq ft. Ceilings usually use a dedicated flat ceiling paint, so many people buy it separately.

Step 4: Divide by coverage and multiply by coats

Most interior paints cover roughly 350 to 400 sq ft per gallon on smooth, primed walls. The exact figure is printed on the can. Two coats is standard.

gallons = area × coats ÷ coverage
Example walls: 366 × 2 ÷ 350 ≈ 2.1 gallons

In this example, two gallons plus a quart would cover the walls. Or skip the math and use our Paint Calculator, which tells you exactly how many gallons and quarts to buy.

When you’ll need more

• New drywall, patches or bare wood: apply primer first, or the paint will soak in unevenly.
• Dramatic color changes: dark to light often needs a third coat or a tinted primer.
• Textured or rough surfaces: more surface area means more paint.
• Spraying: sprayers often use more paint than rollers because of overspray.

Tips to avoid running short

1. Buy all the paint for one room at the same time, so it comes from the same batch.
2. For large jobs, mix the cans together in a bucket (called “boxing”) for perfectly even color.
3. Keep a little leftover, labeled with the room and date, for future touch-ups.
4. Estimate trim and doors separately; they usually take a different sheen.

Key takeaways

• Wall area = perimeter × height, minus doors and windows.
• Gallons = area × coats ÷ the coverage on the can.
• Prime new or patched surfaces and buy everything from one batch.