Last Updated on September 20, 2026 by teamobn
There are two ways to paint a room. The first is to eyeball it, drive to the hardware store, buy “probably enough,” come home to discover it’s not, and drive back mid-project. The second is to spend two minutes with a calculator and buy exactly what you need the first time.
The math isn’t hard (350 square feet per gallon per coat is the industry rule of thumb) but nobody actually does it standing in the paint aisle. Which is why paint stores make so much money on quarts and touch-ups.
How Much Paint Do You Actually Need?
Free 60-second calculator · gallons, supplies, cost · printable shopping list
The math (so you know what the calculator is doing)
Wall area for a rectangular room = 2 × (length + width) × ceiling height. Subtract roughly 20 sq ft for each window and door. Divide the total by 350 (sq ft per gallon per coat), multiply by the number of coats, and round up to the next whole gallon. Ceilings, trim, and doors each get their own separate calculation with their own coverage.
Cheap paint is usually more expensive
Budget paint ($20–30 per gallon) often needs three coats to cover what mid-range paint ($40–50) does in two. Do the arithmetic: three gallons of budget vs two gallons of mid-range plus an extra afternoon of your life. The mid-range wins on both counts. Premium paint ($60–80) is worth it in high-traffic or high-humidity rooms (kitchens, bathrooms, kids’ rooms) where scrubbability actually matters.
The supplies you’ll actually use
A single-room paint job needs more than paint. The list: a 9-inch roller frame with 2–3 covers, a 2-inch angled sash brush for cutting in, a roller tray with liners, a canvas drop cloth (worth it. Plastic slides), painter’s tape (blue, not masking), and an extension pole if your ceiling is anywhere above 8 feet. Realistic supply cost for a single room is $35–50; you’ll reuse most of it forever.
Get your shopping list
Enter room dimensions, ceiling height, which surfaces you’re painting, coats, number of openings, and paint tier. The calculator returns exact gallons, a full supply list with quantities, total cost, and money-saving tips based on your choices.
Frequently asked questions
How much paint do I need for a 12×12 room?
For walls only, with 9-foot ceilings and 2 coats: about 2.5 gallons, round up to 3. Add roughly half a gallon for ceilings and a quart each for trim and doors.
Do I really need two coats?
Yes, in almost all cases. One coat is only reliable when you’re going over a very similar color with premium paint-and-primer. Dark-to-light or vibrant color changes may need three.
What paint sheen should I use where?
Flat or matte on ceilings; eggshell in bedrooms and living rooms; satin in hallways, kids’ rooms, and kitchens; semi-gloss on trim, doors, and bathrooms. The more traffic and moisture, the higher the sheen.
How much paint do you actually need? Let’s get started…
