Paint Calculator: Estimate Gallons for Interior & Exterior Walls

Why I always calculate paint before I shop

I've made both mistakes, and neither one feels good. The first time I painted a bedroom, I guessed low, ran out three-quarters of the way through the second coat, and had to make an evening run to the store with a half-dry roller sitting in a bag so it wouldn't crust over. The next project, determined not to repeat that, I way overbought — and ended up with two nearly-full gallons taking up shelf space in the garage for years.

Paint isn't cheap, and color batches can vary slightly from can to can, so buying exactly what you need — with a little cushion, not a lot — makes the whole project smoother. This calculator does the math I now do for every room and every house exterior: wall area, minus doors and windows, times your number of coats, divided by your paint's actual coverage rate.

How much paint do I need?

The formula is the same one professional painters use, and it only takes four steps.

The calculator above runs all four steps for you — just switch between the Interior and Exterior tabs depending on your project.

A worked example

Let's say I'm painting a 12×14 ft bedroom with 8-foot ceilings, one door, and two windows, and I want two coats for a full color change.

That's exactly what most paint stores will tell you for a room this size — a single gallon usually isn't quite enough once you account for two coats, which is why the "1 gallon per average bedroom" advice you sometimes see only holds for a single coat.

Painting the outside of your house

What if you're tackling the whole exterior instead of one room? The math is the same, just at a bigger scale — and the surface matters more than it does indoors. Smooth siding or previously painted wood covers close to 350 sq ft per gallon, but textured surfaces like stucco, brick, or rough-sawn wood can drop to 250 sq ft per gallon or less, because the extra surface area soaks up more paint.

When you actually need primer

What if the walls you're painting aren't a clean, similar-colored surface to start with? That's when primer earns its place in the project. I always prime over bare or new drywall, when I'm making a dramatic color change (light to dark or the reverse), and over any water stains, since paint alone won't reliably block those from bleeding through.

If you're repainting a wall in a similar shade and the existing surface is clean and in good condition, you can usually skip the extra step — regular paint alone will do the job in one or two coats. Primer covers less per gallon than paint, typically 200–300 sq ft, so budget accordingly if your project needs it.

Common mistakes to avoid

• ✕Using floor area instead of wall area. A 1,000 sq ft home doesn't have 1,000 sq ft of paintable wall — always calculate from perimeter × height, not the square footage of the floor plan.
• ✕Skipping door and window deductions. On a small room, forgetting these can mean buying half a gallon more than you actually need.
• ✕Using interior coverage rates for exterior paint. Rough exterior surfaces cover far less per gallon than smooth interior drywall — using the wrong number is one of the most common ways people run short mid-project.
• ✕Buying separate gallons for a large job. If you need 4 or more gallons of the same color, a 5-gallon bucket is usually cheaper per gallon and keeps the color consistent across the whole job — mixing (boxing) it together beforehand evens out any subtle can-to-can differences.