How Much Paint Do I Need? Walls, Openings, Coats and Coverage
Running out of paint with one wall left is frustrating; so is a closet full of half-used gallons. Getting the quantity right comes down to three measurements and two numbers from the paint can. This guide explains each one and the judgment calls that move the estimate.
The formula
For a 14 × 12 ft room with 8 ft walls: the perimeter is 52 ft, so the walls are 416 sq ft. Subtract two 3 × 4 ft windows (24 sq ft) and one 3 × 7 ft door (21 sq ft), and you have 371 sq ft to paint. Two coats at 350 sq ft per gallon is 371 × 2 ÷ 350 ≈ 2.1 gallons.
Measuring the room
- Length and width: measure along the floor, wall to wall.
- Wall height: floor to ceiling. Standard is often 8 ft, but many homes have 9 ft or taller walls, and that difference adds up fast.
- Windows and doors: measure the opening including the trim if you are not painting the trim with wall paint.
Should you subtract small windows? For a single small window, the difference is small and leaving it in builds in a margin. For rooms with lots of glass, sliding doors or wide openings, subtract them; they can be a big share of the wall.
Reading the coverage on the can
Most interior wall paints list coverage as a range, commonly somewhere in the 250 to 400 sq ft per gallon territory. The range exists because surfaces differ:
| Surface | Where to plan in the range |
|---|---|
| Smooth, previously painted, similar color | High end |
| Light texture (orange peel, knockdown) | Middle to low |
| Heavy texture, brick, block | Low end or below |
| New drywall (unprimed) | Low; prime first |
| Big color change, especially dark to light | Low, and possibly an extra coat |
Use the number from the exact product you plan to buy. The calculator defaults to 350 as a starting point only.
One coat or two?
Two coats is the safe plan for most repaints. It gives even color and sheen, and it hides thin spots and roller marks. One coat can work when you are repainting the same color on a clean, sound surface with a paint that is designed for high hide. Some “paint and primer” products advertise one-coat coverage under specific conditions; whether that holds on your walls depends on the color change and surface.
When to use primer
- New drywall or fresh patches, which absorb paint unevenly.
- Stains from water, smoke or marker that can bleed through.
- Glossy surfaces that new paint won’t grip.
- Large color changes, where a tinted primer can save a coat of finish paint.
Primer has its own coverage rate, usually listed on its label. The calculator lets you add one coat of primer to walls, or walls and ceiling.
Ceilings
The ceiling area is simply length × width. Ceiling paint is usually a different product (flat, often formulated to reduce spatter), so the calculator estimates it separately. One coat is often enough when refreshing a ceiling in the same color.
Gallons, quarts and buying smart
Paint is usually sold in gallons, quarts and 5-gallon buckets. A quart is a quarter of a gallon but often costs a large share of a gallon’s price, so topping up with quarts only makes sense for small remainders. The calculator buys whole gallons and covers a remainder of half a gallon or less with quarts when you enter a quart price.
Keep a little leftover. A labeled, sealed can makes touch-ups match. If you need several gallons of a custom color, ask the store to mix them together, or blend them yourself in a larger bucket, so the color is consistent wall to wall.
Run your room through the Interior Paint Calculator to get the purchase plan and cost, then see the interior painting budget guide for everything else you’ll spend money on.
This guide offers general planning information, not professional, structural, safety or building-code advice. Product instructions and local codes take priority. See our terms for details.