Buy too little paint and you’re back at the store halfway through the second coat, hoping the new can matches. Buy too much and it sits in the garage for years. Five minutes with a tape measure gets it right.

Measure the Walls

Add up the width of every wall, then multiply by the ceiling height. A 12-by-12-foot room with 8-foot ceilings has 48 feet of wall, so 48 × 8 = 384 square feet.

Take Out Doors and Windows

Subtract about 20 square feet for each door and 10 for each window. With one door and two windows, that 384 becomes 344. If you’d rather have a little extra on hand, skip this step.

Do the Math

A gallon of wall paint covers roughly 350 to 400 square feet per coat. Check the label for your paint’s number. Most rooms need two coats, so double the wall area, then divide by the coverage. For the example room, 344 × 2 = 688, and 688 ÷ 350 comes to just under 2. Buy 2 gallons.

Textured and rough walls soak up more paint, so allow a little extra for them.

Don’t Forget the Ceiling and Trim

For the ceiling, multiply the room’s length by its width. That same room has a 144-square-foot ceiling, so one gallon of ceiling paint covers two coats with some to spare. Trim takes far less. In most rooms, a quart or two covers the baseboards, door, and window frames.

Do You Need Primer?

Usually not, if you’re painting over a clean, sound wall in a similar color. Prime bare drywall, patched spots, stains, glossy surfaces, and big color changes, such as light over dark. Primer covers less than paint, about 200 to 300 square feet per gallon.

Buy It All at Once

Color can vary slightly from one batch to the next. Buy every can in one trip, and if you have more than one can of the same color, pour them together in a bucket before you start so every wall matches. Save the leftover for touch-ups, with the room and color written on the lid.

My guide walks you through painting a room, step by step.

Get the full guide: How to Paint a Room by Hank Palmer, on Amazon