How to Calculate How Much Paint You Need

Buying too little paint means an extra trip to the store mid-project — and risking a color batch mismatch. Buying too much wastes money and leaves you with cans to store or dispose of. Getting the quantity right from the start is one of the most practical things you can do before a painting project. Here's exactly how to calculate how much paint you need for any room.

The Basic Formula

The standard rule of thumb is that one gallon of paint covers approximately 350–400 square feet with one coat on a smooth, primed surface. On rough or unprimed surfaces, coverage drops to around 250–300 square feet per gallon. Here's the step-by-step calculation:

1. Measure the perimeter of the room (add up the width of all four walls).
2. Multiply by the ceiling height to get the total wall area in square feet.
3. Subtract doors and windows — a standard door is about 20 sq ft, a standard window is about 15 sq ft.
4. Divide by 350 to get the number of gallons needed per coat.
5. Multiply by the number of coats you plan to apply (usually 2).

Tools You'll Need

• Paint can pour spout — for accurate, drip-free pouring when dividing paint between trays
• Airtight paint storage container — for storing any leftover paint after the project
• Floetrol paint conditioner — improves coverage and reduces the number of coats needed

Example Calculation

Let's say you're painting a 12x14 foot bedroom with 9-foot ceilings, two windows, and one door:

• Perimeter: (12 + 14 + 12 + 14) = 52 feet
• Wall area: 52 x 9 = 468 sq ft
• Subtract openings: 468 – 20 (door) – 30 (2 windows) = 418 sq ft
• Gallons per coat: 418 ÷ 350 = 1.2 gallons
• For 2 coats: 1.2 x 2 = 2.4 gallons
• Buy 3 gallons to have a small buffer for touch-ups.

Adjustments for Special Situations

• Textured walls (orange peel, knockdown): Reduce coverage estimate to 300 sq ft per gallon — texture absorbs more paint.
• Unprimed drywall or bare wood: Reduce to 250 sq ft per gallon for the first coat.
• Dark to light color change: Plan for 3 coats and calculate accordingly.
• Using Floetrol: Adding Floetrol improves flow and leveling, which can slightly improve coverage and reduce the number of coats needed on smooth surfaces.

Calculating Paint for Trim and Ceilings

Ceiling: Multiply room length x width. A 12x14 room has a 168 sq ft ceiling — one gallon is usually enough for two coats.

Trim and baseboards: Measure the linear feet of trim and divide by 150 for a quart estimate. Most rooms need one quart of trim paint for two coats.

Always Buy a Little Extra

Round up to the next full gallon and keep the extra for touch-ups. Store leftover paint in an airtight storage container with the color name and room written on the lid. Use a pour spout when transferring paint to avoid spills and waste.

Online Paint Calculators

Most major paint brands (Sherwin-Williams, Benjamin Moore, Behr) have free online paint calculators on their websites. Enter your room dimensions and they'll calculate the quantity for you. These are useful for quick estimates, but the manual calculation above gives you more control over adjustments for texture, coats, and coverage rate.

A few minutes of measuring and calculating before you buy paint saves you time, money, and frustration. Measure twice, buy once — and always keep a little extra for touch-ups.