Paint Calculator
This paint calculator works out how many gallons of paint you need for a room from its size, the number of doors and windows, and how many coats you plan. It rounds up to whole gallons and quarts so you buy once.
Result
2 gallons of paint
| Wall area to paint | 334 ft² |
|---|---|
| Exact amount (2 coats) | 1.91 gal |
Rounded up to whole gallons and quarts. Three quarts cost more than a gallon, so we switch to a gallon.
How Much Paint Do I Need?
To estimate paint you need three numbers: how much wall area you’re painting, how many coats you’ll apply, and how far a gallon spreads. The calculator above puts them together. Here’s the same math so you can check it or do it with a tape measure and a phone.
How to Calculate Paint by Hand
1. Find the wall area
Add up the length of every wall to get the room’s perimeter, then multiply by the wall height:
Wall area (ft²) = 2 × (length + width) × height
2. Subtract doors and windows
Paintable area = wall area − (doors × 20 ft²) − (windows × 15 ft²)
Use your own sizes if you have patio doors, big picture windows or a wide opening.
3. Divide by coverage and multiply by coats
Gallons = paintable area × coats ÷ coverage per gallon
Round up. If the remainder is a small fraction you can buy a quart or two instead of another gallon. Once you’d need three quarts, a gallon is the better buy.
Worked example: a 14 × 16 ft living room
Eight foot walls, one door, two windows, two coats:
- Perimeter: 2 × (14 + 16) = 60 ft
- Wall area: 60 × 8 = 480 ft²
- Minus openings: 480 − 20 − (2 × 15) = 430 ft²
- Paint: 430 × 2 ÷ 350 = 2.46 gallons
- To buy: 2 gallons + 2 quarts
If you’re also painting the ceiling, that’s another 224 ft² (14 × 16). Two coats at 350 ft² per gallon is 1.28 gallons, so 1 gallon + 2 quarts of ceiling paint.
Tips & Common Mistakes
Coverage on the can is a best case
Manufacturers rate coverage on smooth, sealed surfaces. Knockdown or orange-peel texture, fresh drywall and bare plaster all soak up more. If your walls are textured, set coverage to 300 ft² per gallon instead of 350.
Deep and bright colors need more coats
Reds, yellows and deep accent colors often need a third coat, or a gray-tinted primer first. If you’re going from a dark color to a light one, use primer plus two coats. That usually beats three or four coats of finish paint.
Buy it all at once and mix the cans
Custom-tinted paint can vary slightly from one mixing to the next. Buy everything you need in one trip. On larger jobs, pour the gallons together into a bucket before you start (“boxing” the paint) so every wall matches.
Keep a little for touch-ups
The rounding usually leaves you some paint. Label what’s left with the room and the date. Scuffs and nail holes are much easier to fix with the original batch.
Trim and doors are a separate job
This calculator covers walls and, if you choose, the ceiling. Baseboards, casings and doors usually get a semi-gloss or gloss trim paint. A quart often covers the trim in an average bedroom, but large rooms or six-panel doors can take more.
Ceiling paint and wall paint aren’t the same
Ceiling paints are usually flat and formulated to spatter less, which helps when you’re rolling overhead. That’s why the result lists ceiling paint on its own line. If you’ll use your wall paint on the ceiling too, just add the two amounts together.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
With 8 foot walls, a 12 by 12 room has 384 square feet of wall. Take off one door and two windows and about 334 square feet is left. Two coats at 350 square feet per gallon comes to 1.9 gallons, so buy 2 gallons. Painting the ceiling as well adds about 0.8 gallon for two coats.
How many square feet does a gallon of paint cover?
Most interior wall paints cover about 350 to 400 square feet per gallon per coat on smooth, primed walls. Textured, porous or unprimed surfaces soak up more, so coverage drops. This calculator defaults to 350 to leave a margin, but the can or the product data sheet gives the real figure for your paint.
Do I need two coats of paint?
Usually, yes. Two coats give even color and sheen, especially when you are changing color, painting new drywall or using a deep or bright shade. One coat can be enough when you are refreshing walls in the same color with a good paint.
Do I need primer before painting?
Prime new drywall, patched areas, stains, bare wood and glossy surfaces, and prime before a big jump from dark to light. A gallon of primer covers less than paint, typically about 200 to 300 square feet, so turn on the primer option to add it to your list.
Should I subtract doors and windows when estimating paint?
Yes, for an accurate estimate. A standard interior door is about 20 square feet and an average window about 15 square feet, which are the calculator's defaults. Skipping small openings is fine and gives you a little extra paint.
Is it cheaper to buy paint in gallons or quarts?
Per square foot, quarts are the most expensive way to buy paint, and three quarts usually cost more than a gallon. That is why the calculator switches to a full gallon once you need three quarts or more. For jobs over about four gallons, look at 5 gallon buckets.
How much paint do I need for a ceiling?
Multiply the room's length by its width. A 12 by 12 ceiling is 144 square feet, so two coats at 350 square feet per gallon is about 0.8 gallon. Ceilings usually get a flat ceiling paint, so the calculator lists ceiling paint separately from wall paint.
Related Calculators
Sources & Assumptions
- Sherwin-Williams paint calculator — a gallon of paint covers about 350–400 ft²; a gallon of primer about 200–300 ft²
- Default door area is 20 ft² (a 3 ft × 6 ft 8 in door); default window area is 15 ft² (about 3 × 5 ft). Both are editable.
- Trim, baseboards and doors themselves are not included. Estimate them separately with a trim paint.
- Areas are rounded only for display. Gallons and quarts always round up.
Last reviewed: