How much paint do I need? (area × coats ÷ coverage)

By Francesco Zinghinì

The one formula

Paint quantity is deterministic. Take the net paintable area (walls minus openings, plus the ceiling if you are painting it), multiply by the number of coats, divide by the paint’s coverage (the square feet a gallon covers in one coat), and round up to whole gallons:

gallons = ceil( area × coats ÷ coverage )

That is the whole thing. The how-much-paint calculator runs it for you, but knowing it means you can sanity-check any answer on the back of an envelope.

The three inputs, and where planners go wrong

• Area. Measure it — do not eyeball it. Perimeter × height for the walls, minus doors and windows; length × width for the ceiling. See how to measure a room for paint.
• Coats. Two is the safe default for most repaints and all new or primed drywall. A like-for-like refresh on a sound wall can be one; a dark-to-light change is a tinted primer plus two. The coats-by-scenario table lays it out.
• Coverage. Smooth, previously painted drywall runs about 350–400 sq ft/gal. Rough, porous or masonry surfaces drop sharply — stucco toward 150–250, unpainted brick 100–200. The can’s stated spread rate beats any rule of thumb; the coverage-by-surface reference gives the planning ranges.

Worked example: a bedroom

A 12 × 15 ft room, 8 ft walls, one door and two windows: 54 ft perimeter × 8 = 432 sq ft gross, minus 51 for the openings = 381 sq ft. Two coats at 350 sq ft/gal: ceil(381 × 2 ÷ 350) = ceil(2.18) = 3 gallons for the walls. Painting the ceiling too? 12 × 15 = 180 sq ft at one coat is ceil(180 ÷ 350) = 1 gallon. If the drywall is bare, add primer — ceil(381 ÷ 250) = 2 gallons.

Worked example: a whole house interior

Say the whole interior comes to 4,000 sq ft of paintable wall and ceiling. At two coats and 350 sq ft/gal that is ceil(4,000 × 2 ÷ 350) = ceil(22.9) = 23 gallons — buy in 5-gallon pails to save money and keep the color consistent from one batch. A common shorthand, “a 1,500 sq ft house needs 9–11 gallons,” is just this formula on a smaller paintable area.

Gallons, quarts, and buying the right containers

Once you have the number, buy in the container size that fits it. Trim, a small accent wall or a single door is often a quart job — four quarts to a gallon, so do not buy a whole gallon for 90 linear feet of baseboard. A room or two is gallons. Anything from roughly five gallons up is cheaper, and more color-consistent, in 5-gallon pails, and worth having boxed (all the paint mixed together in one container, then poured back) so the last wall matches the first. Match the container to the count and you neither overpay for half-used gallons nor make three trips for quarts.

The overage nobody budgets for

The formula gives the theoretical minimum. Real jobs use more, so a meticulous planner adds a margin before rounding:

• Texture and porosity. Knockdown walls, bare wood and masonry drink paint — drop the coverage number rather than trusting the smooth-wall figure.
• Color change. Going lighter over a dark wall can need an extra coat or a tinted primer; build that into the coats.
• Cut-in and waste. Roller loading, tray residue and cutting-in eat 5–10%. Rounding up usually absorbs it; on a big job, add a gallon of margin deliberately.
• Touch-ups. One sealed gallon of the exact batch color is cheap insurance against a scuff two years from now.

Plan the whole house as one order

If you are painting several rooms, do not price them one at a time and make five store trips. Total the paintable area room by room, group by color, and place a single order — it is cheaper (5-gallon pails beat singles), it keeps each color on one batch, and it front-loads the measuring so nothing stalls mid-project. Keep a simple list: room, paintable square feet, color, sheen, coats, gallons. Rooms sharing a ceiling white can pool into one ceiling order; trim in one enamel across the house pools into another. The room paint calculator gives each room’s number, and adding them up turns a dozen guesses into one confident buy. This is also when you decide container sizes — quarts for the trim and doors, gallons or pails for the walls — so the order matches the job instead of leaving you with seven half-used cans.

Buy from one batch

Paint tinted in different batches can vary slightly in shade. Once you know the gallon count, buy it all at once and, if it is more than a few gallons, have it boxed so every wall matches. Round up, allow for texture and waste, confirm the coverage on the can you actually buy, and the “how much paint” question is answered for good. Reach for the paint-coverage calculator to turn a known gallon count back into the area it covers, and the primer calculator for the sealer coat.