How Much Paint Do I Need?
A simple, accurate method for estimating how much paint to buy for any room — with a worked example, a quick reference table, and the exact formula painters use.
To find how much paint you need, calculate the wall area with 2 × (length + width) × wall height, subtract about 21 sq ft for each door and 15 sq ft for each window, multiply by the number of coats (usually two), then divide by the paint's coverage — roughly 350 square feet per gallon. A typical 12 × 14 ft room with 8 ft walls and two coats needs about 3 gallons.
Painting a room is one of the highest-impact, lowest-cost upgrades you can make — but buying the wrong amount of paint turns a weekend project into two trips to the store or a shelf of half-used cans. The good news: estimating paint is simple arithmetic once you know the four numbers that matter. This guide walks through the method painters actually use, then gives you a reference table and a worked example you can copy.
If you’d rather skip straight to an answer, the paint calculator does every step below for you — but understanding the math makes it easy to sanity-check any estimate.
The four numbers that decide everything
Every paint estimate comes down to four inputs:
- Wall area — the total square footage you’re covering.
- Openings — doors and windows you don’t paint.
- Coats — how many passes you’ll apply (usually two).
- Coverage — how far a gallon stretches (about 350 sq ft).
Get those four right and the rest is multiplication and division.
Step 1: Measure the wall area
For a whole room, add the length and width, double it (that’s the perimeter), then multiply by the wall height:
Wall area = 2 × (length + width) × wall height
For a 12 ft × 14 ft room with 8 ft ceilings, that’s 2 × (12 + 14) × 8 = 416 sq ft.
Painting just one wall? Use wall length × wall height. Doing a ceiling? It’s simply length × width. You never need to type a square footage into the paint calculator — it derives the area from your dimensions.
Step 2: Subtract doors and windows
You don’t paint the glass or the door slab, so deduct them. Reasonable averages are 21 sq ft per standard door and 15 sq ft per window. In our example room with one door and two windows:
416 − 21 − (2 × 15) = 365 sq ft paintable
Don’t bother deducting outlets or small trim gaps — they’re rounding error, and a little extra paint is a feature, not a bug.
Step 3: Multiply by coats
Almost every repaint needs two coats for even color and full hiding. One coat rarely looks finished, especially over a color change. So multiply your paintable area by two:
365 × 2 = 730 sq ft of coverage needed
Wondering whether you can get away with one coat — or whether you need three? See How Many Coats of Paint Should You Apply?.
Step 4: Divide by coverage
A gallon of typical interior latex covers about 350 sq ft on a smooth, previously painted wall. Divide:
730 ÷ 350 ≈ 2.1 gallons
Round up to whole gallons: buy 3 gallons (or 2 gallons plus a quart to cut the waste). Coverage isn’t a fixed number, though — rough, porous, or brand-new surfaces drink far more. Paint Coverage Explained covers where the 350 figure comes from, and Paint Coverage by Surface shows how to adjust it.
Quick reference: paint for common rooms
These assume two coats over smooth, previously painted walls with standard 8 ft ceilings. Treat them as a starting point, then confirm with the paint calculator.
| Room | Approx. size | Wall area | Paint (2 coats) |
|---|---|---|---|
| Bathroom | 8 × 10 ft | ~288 sq ft | 2 gallons |
| Bedroom | 12 × 14 ft | ~416 sq ft | 3 gallons |
| Living room | 16 × 20 ft | ~576 sq ft | 4 gallons |
| Kitchen | 10 × 12 ft | ~352 sq ft | 2–3 gallons |
When to buy the bigger container
Above roughly four gallons, a single 5-gallon bucket is usually cheaper per gallon than four individual cans — and it guarantees perfectly consistent color, since it’s all one batch. Below that, gallons and a quart give you more flexibility for touch-ups.
Don’t forget primer
Primer is a separate product from your finish paint, and it covers a bit less — about 200–300 sq ft per gallon. You need it on new drywall, bare wood or masonry, stained surfaces, and dramatic dark-to-light changes. Because it’s bought separately, size it on its own. The paint calculator lists primer on its own line when you turn it on.
Where estimates go wrong
The most common paint-buying mistakes are underbuying (forgetting the second coat), ignoring how thirsty a rough or new surface is, and skipping primer. See Common Painting Mistakes for the full list — and how to avoid each one.
Once you’ve got your number, choosing the right paint matters too. If any of the walls are outdoors, read Interior vs. Exterior Paint before you buy.