How to Calculate Exactly How Much Paint You Need for a Room

Buying paint for a room is always a guessing game.

You buy too little, and you have to make a second trip to the hardware store. You buy too much, and you're stuck with half a gallon drying out in your garage.

Let's break down the exact math to buy the right amount the first time.

Step 1: Calculate the Wall Area

Measure the length and height of each wall in feet.

Width × Height = Total Square Feet

For example, a wall that is 12 feet wide and 8 feet tall is 96 square feet. If you have four identical walls, you multiply by 4 (96 × 4 = 384 sq ft).

Step 2: Subtract Doors and Windows

You don't need to paint the glass or the door.

• A standard door is about 21 square feet (3 ft × 7 ft).
• A standard window is about 15 square feet (3 ft × 5 ft).

Let's say your room has one door and two windows:
21 + 15 + 15 = 51 square feet

Subtract this from your total wall area:
384 - 51 = 333 square feet

Step 3: Divide by the Coverage Rate

A standard gallon of interior paint covers roughly 350 to 400 square feet per coat. Always check the label on your specific paint can, but we'll use 350 for this example.

333 ÷ 350 = 0.95 gallons

Step 4: Factor in the Number of Coats

If you are painting a light color over a dark color, or if you are using a primer, you will need two coats.

0.95 × 2 = 1.9 gallons

Since paint is usually sold in whole gallons, you would buy 2 gallons for this room.

Common Gotchas

• Ceilings: If you are painting the ceiling, you need to calculate the floor area (Length × Width) and add that to your total.
• Trim: Baseboards and window trim usually require a separate calculation and are painted with a different type of paint (often semi-gloss).
• Texture: Rough or porous surfaces (like unpainted drywall or stucco) absorb more paint. You should always buy 10-15% extra.

The Visual Way

If you are painting a whole house or multiple rooms, the math gets complicated fast. You have to add up four walls per room, subtract multiple doors and windows, and figure out the ceiling and trim.

If you want to calculate the exact gallons or liters of interior paint needed based on wall length, height, coats, and deductions for doors and windows, I built a free Interior Room Paint Calculator.

Try it here

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Have you ever underestimated how much paint you needed? Let me know in the comments.