Most people buy the wrong amount of paint — and end up either short halfway up a wall or with three half-used cans in the garage. The math isn't hard, but the number on the can ("350–400 sq ft per gallon") is an ideal, not a promise. This guide walks you through the real calculation: measuring your space, coats, primer, surface, and sheen — with a room-size cheat sheet at the end.
If you just want the answer, the calculator does all of this in a few taps.
Step 1 — Measure Your Walls
Paint covers area, so everything starts with the square footage of the surfaces you're painting.
For a rectangular room, add the lengths of all the walls (the perimeter) and multiply by the ceiling height:
Wall area = (length + width) × 2 × ceiling height
Then subtract the openings you won't paint — roughly 20 sq ft per door and 15 sq ft per window (adjust if yours are larger). If you're painting the ceiling too, add length × width.
A worked example for a 12 × 14 ft room with 8 ft ceilings: (12 + 14) × 2 × 8 = 416 sq ft of wall. Subtract one door and two windows (about 50 sq ft) and you're at roughly 366 sq ft to cover — per coat.
Step 2 — Know What a Gallon Actually Covers
On a smooth, primed wall, one gallon covers about 350–400 sq ft per coat. That's the figure manufacturers like Sherwin-Williams and Behr print, and it holds up well on walls that are in good shape.
But two things change it fast:
- Coats. Most projects need two, which effectively halves your coverage to about 175–200 sq ft per gallon in practice.
- Primer. Primer soaks in rather than sitting on top, so a gallon only covers about 200–300 sq ft — always buy a little more primer than paint.
Step 3 — One Coat or Two?
One coat can work when you're painting over a similar color with a premium paint. Plan on two coats when you're:
- making a big color change (especially light over dark, or any bold color),
- covering bare or repaired drywall, stains, or a damaged surface, or
- after maximum durability and an even finish.
When in doubt, budget for two. It's the difference between a wall that looks finished and one that looks patchy in raking light.
Step 4 — Surface and Texture
The number on the can assumes a smooth, sealed wall. Real surfaces drink more:
- Textured walls (stucco, knockdown, popcorn ceilings, brick) have more actual surface area — buy 20–50% more paint.
- Bare or fresh drywall is very porous and soaks paint up; prime it first (often two coats of primer) so your finish paint goes further.
- Glossy or slick existing surfaces may need de-glossing or a bonding primer for the paint to grip.
Step 5 — Sheen Changes the Math Too
Finish isn't just a look — it affects how many coats you'll need.
- Flat and matte paints are thicker, hide flaws, and tend to cover in fewer coats.
- Satin and eggshell sit in the middle — durable and the most popular for living spaces.
- Semi-gloss and gloss are thinner and more reflective; they show every imperfection and often need an extra coat, especially on trim and doors.
The Room-Size Cheat Sheet
Quick estimates for two coats, 8 ft ceilings, standard doors/windows, ~375 sq ft/gallon. These are starting points — your surface and color can push them up.
| Room size | Wall area (1 coat) | Paint (2 coats) |
|---|---|---|
| 10 × 10 ft | ~285 sq ft | ~2 gallons |
| 12 × 12 ft | ~350 sq ft | ~2 gallons |
| 12 × 14 ft | ~365 sq ft | ~2 gallons |
| 14 × 16 ft | ~415 sq ft | ~3 gallons |
Don't Forget the Waste Allowance
Always add about 10% extra for touch-ups, spills, and a second pass on thin spots — and try to keep your paint from the same batch/can run so the color matches exactly. A leftover quart from the right batch is worth far more than a perfect-on-paper estimate that leaves you one wall short.
Pro Tips
- Buy one gallon at a time for big rooms if you're unsure — most stores take back unopened cans, and you avoid over-buying.
- Sample first. Paint a test patch and view it in daylight and at night before committing to gallons.
- Tinted primer in a shade near your final color makes dramatic changes cover in fewer coats.
- Cutting in (edges/corners with a brush) uses more paint than you'd think — don't shave your estimate too close.
Frequently Asked Questions
How much paint do I need for a room? Measure the wall area — (length + width) × 2 × ceiling height, minus doors and windows — then divide by about 350–400 sq ft per gallon and multiply by your number of coats. Most average rooms need about 2 gallons for two coats. The calculator does this instantly.
How many square feet does a gallon of paint cover? About 350–400 sq ft per coat on a smooth, primed wall. Primer covers less, around 200–300 sq ft per gallon. A quart covers roughly 100 sq ft.
How do I calculate paint for my walls? Add up the wall lengths (the perimeter), multiply by ceiling height for total wall area, subtract about 20 sq ft per door and 15 per window, multiply by coats, then divide by ~375 sq ft per gallon.
Do I need one coat or two? Two is standard. One coat can work over a similar color with premium paint, but big color changes, bare drywall, and durability all call for two.
How much paint for a 12×12 room? About 2 gallons for two coats on 8 ft walls with standard openings. Add more for textured walls or a dramatic color change.
Does primer count separately from paint? Yes. Primer is its own product with lower coverage (~200–300 sq ft per gallon) and is essential on bare drywall, stains, or big color changes. Estimate it separately from your topcoat.
How much extra paint should I buy? About 10% over your estimate for touch-ups and waste — and keep it from the same batch so the color matches.
Does paint sheen change how much I need? It can. Flat and matte tend to cover in fewer coats; semi-gloss and gloss are thinner and reflective, so they often need an extra coat, especially on trim.
Estimate yours in seconds
Sources
- Sherwin-Williams; Behr; KILZ; Lowe's — gallon coverage (350–400 sq ft paint; 200–300 sq ft primer), coats, and surface guidance.
- This Old House / Lowe's — wall-area method and per-opening subtractions (≈20 sq ft door, ≈10–15 sq ft window).
- Manufacturer guidance — textured surfaces +20–50%, +10% waste allowance.