Step 1: Measure the wall area
The amount of paint you need depends on surface area, not floor space, so start with the walls. The fastest way to get total wall area is to measure the perimeter of the room — the distance all the way around the floor — and multiply it by the ceiling height.
For a rectangular room, the perimeter is simply two times the length plus two times the width: perimeter = 2 × (length + width). Multiply that by the height and you have the gross wall area in square feet. You don't need to measure each wall separately; going around the perimeter captures all four at once.
Step 2: Subtract the doors and windows
You're not painting the openings, so take them out. Exact sizes vary, but these averages are accurate enough for a paint estimate:
- A standard interior door is about 21 square feet (roughly 3 ft × 7 ft).
- An average window is about 15 square feet (roughly 3 ft × 5 ft).
Count the doors and windows in the room, multiply by those figures, and subtract the total from your gross wall area. If you'd rather keep a little margin, you can skip this step entirely — leaving the openings in just builds in extra paint for touch-ups.
Step 3: Divide by coverage, then multiply by coats
One US gallon of interior paint covers roughly 350 to 400 square feet in a single coat on a smooth, sealed wall. Divide your net wall area by that coverage number to get gallons per coat. Then multiply by the number of coats — and most jobs need two for even color and full coverage.
You'll also want primer in a few situations: when you're going light over a dark color, when painting bare or freshly patched drywall, or when covering stains or glossy surfaces. Primer seals porous areas so the finish coats lay down evenly, and a good tinted primer can sometimes save you a finish coat.
gallons = (perimeter × height − openings) ÷ coverage × coats, where coverage is about 350–400 sq ft per gallon and most rooms get 2 coats. Round up to the next whole gallon (or quart for small leftovers).A worked example
Take a bedroom that's 12 ft × 14 ft with standard 8 ft ceilings, containing one door and two windows.
- Perimeter: 2 × (12 + 14) = 52 ft
- Gross wall area: 52 × 8 = 416 sq ft
- Openings: 1 door (21) + 2 windows (2 × 15 = 30) = 51 sq ft
- Net wall area: 416 − 51 = 365 sq ft
At roughly 350–400 sq ft per gallon, 365 square feet is just about one gallon per coat. For the standard two coats, that's 2 gallons of paint. You can run any room through the free paint calculator and it does this math for you in seconds.
Quick reference: room size to gallons
This chart assumes 8 ft ceilings, a rough subtraction for one door and a window or two, and two coats. Use it as a sanity check against your own measurements.
| Room size | Approx. wall area | Gallons (2 coats) |
|---|---|---|
| 8 × 10 ft | ~250 sq ft | 1.5 gallons |
| 10 × 12 ft | ~300 sq ft | 2 gallons |
| 12 × 14 ft | ~365 sq ft | 2 gallons |
| 14 × 16 ft | ~430 sq ft | 2.5 gallons |
| 16 × 20 ft | ~525 sq ft | 3 gallons |
| 20 × 24 ft | ~640 sq ft | 3.5 gallons |
Because paint sells in gallons and quarts, round up to the next size you can buy. A job that needs 2.3 gallons means buying 3, or 2 gallons plus a quart if your store sells them that way.
Tips that keep your estimate honest
Buy a little extra. A spare quart for touch-ups is cheap insurance, and matching paint later — even the same color — rarely looks identical once the wall has aged. Keeping leftover paint from the actual batch is the most reliable way to fix scuffs.
Ceilings and trim are separate. The wall estimate above does not include the ceiling, baseboards, or door and window casings. Measure the ceiling on its own (length × width) and treat trim as its own small job, usually a quart or two depending on how much there is.
Account for texture and porosity. Smooth, previously painted drywall is the easy case. Knockdown texture, stucco, raw wood, fresh joint compound, and masonry all drink far more paint — sometimes 20% or more — because there's more surface area and the material soaks it up. On rough or thirsty surfaces, plan toward the lower end of the coverage range and prime first.
The takeaway
Estimating paint comes down to one chain of steps: perimeter times height for the wall area, minus the doors and windows, divided by coverage, times the number of coats. Measure carefully, round up when you buy, and keep a little extra for the inevitable touch-ups. Do that and you'll finish the room without a mid-project store run or a closet full of unused cans. When you're ready, plug your numbers into the paint calculator to confirm.
Frequently asked questions
- How much area does a gallon of paint cover?
- One US gallon of interior wall paint covers roughly 350 to 400 square feet in a single coat on a smooth, primed surface. Rough, porous, or previously unpainted surfaces absorb more, so plan toward the lower end of that range or add extra.
- Do I really need two coats of paint?
- Most jobs do. Two coats give even color, hide the surface underneath, and last longer. You might get away with one coat when repainting a similar color over a clean, primed wall, but two is the safe default for accurate estimating.
- When do I need primer?
- Use primer on bare or patched drywall, when covering a dark color with a lighter one, over stains, or on glossy and slick surfaces. Primer seals porous areas and helps the topcoat grip, which can reduce how many finish coats you need.
- Should I subtract doors and windows from the wall area?
- Yes, for accuracy. A standard door is about 21 square feet and an average window about 15 square feet. Subtracting them prevents over-buying, though leaving small openings in is a harmless way to build in a little extra paint for touch-ups.
This guide offers general estimating guidance. Actual coverage varies by product, surface, and application — always check the coverage rate on the can and measure your own space before buying.