How this paint calculator works
Paint is an area problem with two multipliers — coats and coverage — and two deductions that most people forget:
gallons = area × coats ÷ coverage, rounded up · 350 ft²/gal typical
At the defaults — a 12 × 12 × 8 room, one door and two windows, two coats — that’s 2 gallons, about $70; the full chain is in the coverage math section below.
Paint, primer and sheen
What you buy is three decisions — how many coats of finish, whether to prime, and which sheen — and only the first two change the gallon count:
Finish paint and coats
Two coats is the honest default: one coat flashes against patches and never quite evens out on a color change. “Paint and primer” products can one-coat a same-color refresh, but a real color change still wants two.
Primer
A bonding or stain-blocking undercoat over bare drywall, patches, stains, glossy surfaces, or a big color jump. The primer checkbox adds one coat at the same coverage — far cheaper than a third coat of finish paint chasing coverage.
Sheen
Flat and matte hide flaws but scrub poorly; eggshell and satin are the washable living-space standard; semi-gloss and gloss go on trim, doors and wet rooms. Sheen changes the look and the durability, not the square feet a gallon covers.
Paint type and quality
Water-based (latex/acrylic) paint is the interior standard — low odor, fast dry, soap cleanup — and better-quality lines hide in fewer coats because they carry more solids per gallon. Oil-based (alkyd) enamel still shows up on trim and doors for its hard, level finish, though water-based enamels have largely caught up. A pricier one-coat-capable paint can beat a cheap one that needs three; the coverage field lets you enter the real number off the can.
Sundries
Tape, roller covers and sleeves, tray liners, drop cloths, spackle and caulk — the $20–40 of small stuff a room needs beyond the paint itself. Match the roller nap to the surface: a short 3/8 in nap on smooth drywall, a thicker 1/2–3/4 in for texture and masonry.
How many gallons of paint by room size?
Table 1 runs common rooms through the engine — walls only, two coats, with a door and two windows deducted.
| Room | Wall area | Gallons |
|---|---|---|
| 10 × 10 × 8 | 269 ft² | 2 |
| 12 × 12 × 8 | 333 ft² | 2 |
| 14 × 16 × 8 | 429 ft² | 3 |
| 16 × 20 × 9 | 597 ft² | 4 |
How to measure a room for paint
Measure the walls you’re painting and subtract the holes — the two deductions are what separate a right order from a leftover-gallon one.
- Measure length, width and ceiling height — the calculator builds the wall perimeter and multiplies by height.
- Count doors and windows. The engine deducts 21 ft² a door and 15 ft² a window; a wall of glass changes the answer.
- Decide the ceiling — tick it on to add its area at the same coverage; ceilings usually take flat paint.
- Set the coats and coverage — two coats default, and drop the coverage to 250–300 for bare, porous or textured walls.
- Add primer where the surface calls for it; it’s one coat at the same rate.
- Enter the gallon price — and round your real order to the way paint sells (gallons and quarts), buying the whole color in one tint run.
Worked example: paint for a 12 × 12 room
The default room, worked line by line — 12 × 12 × 8 ft, one door and two windows, two coats at 350 ft²/gal:
openings = 21 + (2 × 15) = 51 ft²
paintable = 384 − 51 = 333 ft²
coverage needed = 333 × 2 coats = 666 ft²
gallons = 666 ÷ 350 = 1.9 → 2 gallons ≈ $70
The 51 ft² of openings is the line people skip — and on a small room with a door and a couple of windows it’s a real chunk of a gallon. The math also lands at 1.9, so the second gallon is mostly touch-up and future patching; keep the leftover labeled for the inevitable scuff.
What this paint estimate doesn’t include
- Trim and doors. Baseboard, casing and doors take their own (usually semi-gloss) paint, figured separately.
- Ceiling paint unless you tick the box — often a dedicated flat ceiling product.
- Sundries. Tape, roller covers, trays, drop cloths, spackle and caulk.
- Surface prep. Patching, sanding, degreasing and washing before a brush touches the wall.
- Specialty coatings. Bathroom-grade, masonry or cabinet paint priced apart from wall paint.
- Labor. Cutting in and rolling — most of a hired-out bill.
Paint FAQ
How much paint do I need for a 12 × 12 room?
Two gallons for two coats on the walls: 384 ft² of wall minus a door (21 ft²) and two windows (30 ft²) leaves 333 ft², times two coats at 350 ft² per gallon = 1.9 gallons. Painting the ceiling too takes it to three gallons.
How much paint do I need for a whole house interior?
Total the paintable wall (and ceiling) area room by room, multiply by coats, and divide by 350 ft² per gallon — or run each room here and add the gallons. A rough planning figure is one gallon per 350–400 ft² of surface per coat; the room-by-room takeoff, with openings subtracted, is the accurate one.
How much does a gallon of paint cover?
About 350 ft² per coat on smooth, previously painted walls — the number printed on most cans is 350–400. Textured, porous or bare surfaces drink more; coverage of 250–300 is realistic there, and the coverage field accepts whatever your can says.
Do I need two coats of paint?
Almost always yes for a color change, and usually for a same-color refresh — one coat covers unevenly as it flashes against patched spots. Quality paint-and-primer products can one-coat over similar colors; drastic changes may need a tinted primer plus two.
When do I need primer?
Bare drywall, patched areas, stains, glossy surfaces, and going light over dark. The primer checkbox adds one coat of coverage at the same rate — cheap insurance compared to a third coat of finish paint.
What paint sheen should I use in each room?
Flat and matte hide wall flaws but scrub poorly — good for ceilings and low-traffic bedrooms. Eggshell and satin are the living-room and hallway standard: washable, low glare. Semi-gloss goes on trim, doors, kitchens and baths where moisture and hands hit; high-gloss is for accent trim. Sheen does not change the coverage, so the gallons stay the same.
How much does it cost to paint a room?
The default room is about $70 in paint at $35 a gallon, plus roughly $20–40 in sundries (tape, roller covers, patching). Hiring it out, interior painting typically runs $2–6 per square foot of floor area in 2026 — labor is most of the bill.
Sources and standards
- Manufacturer coverage figures: the 350–400 ft² per gallon per coat printed on most interior-paint cans.
- MPI (Master Painters Institute) guidance: the two-coat-plus-primer practice behind the defaults.
- Standard opening allowances: 21 ft² per door and 15 ft² per window, the estimating deductions the engine uses.
- The can: coverage varies with sheen, color and surface — the coverage field takes your product’s printed number.
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