Paint Calculator

Decide how many coats you are doing before you buy a single gallon. A one-coat touch-up and a two-coat color change are two different paint budgets, and the wall area itself is only the middle of the math. Pick the sheen, decide on the ceiling, and then let the perimeter numbers tell you how much paint the room actually wants.

A gallon of interior paint spreads to about 350 square feet per coat on smooth drywall. That is the coverage figure the whole estimate hangs on, and it is close enough for most rooms that you will not run short by a whole gallon if your doors and windows are ordinary.

Measure the walls first

Wrap a tape measure around the room and add up all four walls. For a typical 12 by 14 foot bedroom that comes out to 52 feet of perimeter. Multiply that by the wall height, which defaults to 8 feet in most homes, and you have the gross wall area before any openings come out.

You do not need to measure every wall down to the inch. Closet bump-outs and half-walls only shift the number by a few square feet, and the waste from the roller tray will absorb that. The numbers that matter are the perimeter, the ceiling height, and whether the walls are flat or heavily textured.

Deduct doors and windows

Doors and windows are openings you will not paint, so pull them out of the gross area rather than measuring each one by hand. Budget 20 square feet for a standard interior door and 15 square feet for an average window. A double door runs closer to 40, and a large picture window can hit 25.

The 52 foot perimeter room with 8 foot walls starts around 416 square feet. Pull out one door at 20 and two windows at 30, and you are painting closer to 366 square feet. That adjustment sounds small, but it is the difference between buying exactly two gallons and realizing you need a third on the second coat.

One coat versus two coats

This is the decision that doubles your paint. A single fresh coat over a nearly identical existing color can sometimes stop at one. Almost everything else wants two. Bare drywall soaks up the first coat like a sponge, so the second coat is the one that actually looks finished. Going from a dark red to a pale gray is never a one-coat job, and the patch over a repaired hole will show through a single coat every time.

For most rooms, plan on two coats and buy the paint to match. If the coverage estimate lands at 1.6 gallons, round up to two whole gallons rather than chasing 0.6 of a gallon in quarts. A third unused gallon stores on a shelf and becomes the touch-up stock for years.

Picking the sheen

Sheen is not just a finish choice, it is a maintenance choice. Flat and matte hide roller marks and wall bumps, which is why they dominate living rooms and bedrooms where perfection is overrated. Eggshell throws off a faint glow and wipes clean a little better, making it the default for family rooms and hallways. Satin and semi-gloss are tough enough for bathrooms, kitchens, trim and baseboards, where splashes and fingerprints show up.

Stick to one sheen on the walls of a single room. Mixing flat walls with eggshell next to a doorway shows up under light, and the inconsistency is the first thing a visitor notices.

Ceilings, trim and primer

Ceilings get their own product, usually a flat white in a separate gallon, and a single coat is often enough unless the ceiling carries water stains. Trim and baseboards use a different sheen again, so budget about a quart to a gallon for the trim work in an average room.

Primer is the wildcard. Bare drywall, knots in fresh wood, and dramatic dark-to-light color changes all want a primer coat first. Buy it as its own gallon rather than pouring money into premium paint that ends up acting as primer.

What the numbers look like

Take that same 12 by 14 foot bedroom, 8 foot walls, one door and two windows. After deductions the walls sit around 366 square feet, and two coats double the wet coverage to roughly 732 square feet. Divided by 350, that calls for just over two gallons, so round the order up to 3 whole gallons and you will finish with a small leftover.

Keep that leftover. Homes need touch-ups after moving furniture, after nail holes get filled, and after the chair scraping the wall leaves a scuff that only the original batch can cover.

See also the paint spray calculator if you plan to spray instead of roll, and the fence calculator for exterior fence coverage.

A 52 foot perimeter room with 8 foot walls, one door and two windows needs about 3 gallons for two coats.