How to Calculate the Amount of Paint You Need

Not sure how many cans of paint to buy? You’re not alone. Buying too much wastes money, but running out mid-project can stall your momentum and make it tough to match colours later. The good news is—it’s easy to estimate what you need with just a few quick measurements.

What You’ll Need:

• Measuring tape
• Calculator (or phone)
• A rough idea of how many coats you plan to apply

Step 1:

Measure Your Walls

Measure the height and width of each wall. Multiply those two numbers to get the square footage. Do this for each wall you plan to paint.

Example:
A wall that’s 10 ft high by 12 ft wide = 120 sq. ft.

Add all the wall areas together to get the total surface area.

Step 2:

Subtract Windows and Doors

You can subtract about 15 sq. ft. per window and 20 sq. ft. per door from your total.

Pro tip: If you’re painting over a dark colour or a textured surface, plan for an extra coat or two for full coverage.

Step 3:

Do the Math

Most paint cans will list their coverage per litre or gallon—usually around 350–400 sq. ft. per gallon (or 32–37 m²). Divide your total square footage by the coverage amount to estimate how much paint you’ll need.

Bonus Tips:

• Add 10% extra for touch-ups or miscalculations.
• If you’re painting a ceiling or trim, calculate those areas separately using the same formula.

Need Help with the Numbers?

Bring in your room dimensions or a rough sketch and the Autohome team will help you calculate the right amount—so you’re not making a second trip halfway through.