Paint calculator: how much paint do I need?
Dario’s PaintingSep 30, 20267 min readLast updated: Sep 30, 2026
A gallon of paint typically covers about 350 to 400 square feet per coat, according to Sherwin-Williams, and a quart covers about 100. To find how much you need, multiply the room’s perimeter by its height, subtract the doors and windows, multiply by your coats and divide by the coverage. The calculator below does it for you: a 12 by 12 foot room with 8-foot walls, one door and two windows takes about 2 gallons for two coats on the walls. This guide covers the math, the coverage numbers and when to buy extra. Rather hand it off? See interior painting in Memphis, where the paint is included in the quote.
Paint Calculator
Enter your room’s length and width. The other boxes start at common values: 8-foot walls, one door, two windows and two coats. Tick the ceiling box to add it.
The calculator subtracts 21 sq ft for each door and 15 sq ft for each window, the sizes Sherwin-Williams uses in its own calculator: a 3 by 7 foot door and a 3 by 5 foot window. It rounds up to the nearest quart, since paint is sold in quarts as well as gallons.
How the Paint Math Works
The steps are the same whether you use the calculator or a pencil. Here is a 12 by 12 foot room with 8-foot walls, one door, two windows, two coats and 350 sq ft per gallon.
Measure each wall, then take out the doors and windows
A 12 by 12 foot room has four walls like this one: 2 × (12 + 12) × 8 = 384 sq ft before the openings.
| Step | The math | Result |
|---|---|---|
| 1. Wall area | 2 × (12 + 12) × 8 | 384 sq ft |
| 2. Subtract doors and windows | 1 door × 21 + 2 windows × 15 = 51, so 384 − 51 | 333 sq ft |
| 3. Multiply by coats | 333 × 2 | 666 sq ft to cover |
| 4. Divide by coverage | 666 ÷ 350 | 1.90 gallons |
| 5. Round up to the nearest quart | 1.90 gallons | 2 gallons |
| 6. Ceiling, if you paint it | 12 × 12 = 144, then 144 × 2 ÷ 350 = 0.82 | 1 gallon |
- 1 Perimeter × height
- 2 Minus doors and windows
- 3 × coats
- 4 ÷ coverage
- 5 Round up
Ceilings are just length times width, and they use the same coverage number. Trim is different: Behr assumes trim is about six inches wide, so multiply the total trim length by 0.5 to get square feet.
How Many Square Feet Does a Gallon Cover?
It depends on the brand, the product and the wall. These are the manufacturers’ own numbers.
Square feet one gallon covers, one coat
Sherwin-Williams
:- 350–400. Its FAQ. A quart covers about 100 sq ft; a gallon of primer, about 200–300.
S-W calculator, inside
:- 400
S-W calculator, outside
:- 350
Duration Home
:- 350–400. Sherwin-Williams interior data sheet
Duration Exterior
:- 250–300. Sherwin-Williams exterior data sheet
Behr
:- 250–400. Rough, textured or porous surfaces take more.
Glidden
:- Up to 400. On primed, smooth, non-porous surfaces.
Regal Select
:- 400–450. Benjamin Moore interior
Benjamin Moore exterior
:- 250–400. Depending on texture and porosity
The calculator starts at 350, the low end of Sherwin-Williams’ typical range, so you are less likely to run short. Pick 400 for smooth, primed walls. The 300 option is a cautious number inside Behr’s range for rough or textured walls.
When You Need More Paint
Textured or rough walls
Use the 300 setting. Behr says rough, textured or porous surfaces take more paint, and Sherwin-Williams says wall texture can mean you want a little more than a gallon on hand.
A big color change
Going from dark to light, or covering a strong color, can need primer under the two finish coats. Glidden says drastic color changes typically need a second coat.
Bare or new surfaces
Behr says a porous or previously unpainted surface generally takes more paint. Prime first. Primer covers less than paint, about 200 to 300 sq ft per gallon according to Sherwin-Williams, so run the same math with that number.
Trim and doors
The calculator covers walls and ceiling. For trim, multiply the total length by 0.5 and use about 100 sq ft per quart. For a door, multiply height by width, and count both sides.
How you apply it
Sherwin-Williams and Benjamin Moore both say the amount you need varies with the application method, so treat any calculator as a rough estimate.
Rounding
Sherwin-Williams’ calculator recommends rounding up to the nearest gallon. This one rounds up to the nearest quart.
Paint Needed by Room Size
This is what the same math gives for common rooms, with 8-foot walls, two coats and 350 sq ft per gallon. Each room has one door and two windows, except the bathroom, which has one window.
Paint for the walls, by room size
Two coats, 8-foot walls, 350 sq ft per gallon, rounded up to the nearest quart.
One can is a gallon; a part-filled can is the extra quarts.
10 × 10 ft. 269 sq ft of wall · ceiling 3 quarts
: 1 gallon + 3 quarts10 × 12 ft. 301 sq ft of wall · ceiling 3 quarts
: 1 gallon + 3 quarts12 × 12 ft. 333 sq ft of wall · ceiling 1 gallon
: 2 gallons12 × 14 ft. 365 sq ft of wall · ceiling 1 gallon
: 2 gallons + 1 quart14 × 16 ft. 429 sq ft of wall · ceiling 1 gallon + 2 quarts
: 2 gallons + 2 quarts5 × 8 ft bath. 172 sq ft of wall · ceiling 1 quart
: 1 gallon
Taller walls add up fast. The 12 by 12 foot room needs 2 gallons + 1 quart for the walls with 9-foot ceilings and 2 gallons + 2 quarts with 10-foot ceilings.
Bigger Jobs and Hiring a Painter
More than one room in the same color? Benjamin Moore says to add up the total square footage. Run each room through the calculator, add the wall areas, then divide by the coverage once at the end.
For an exterior, Behr’s method is to multiply the house’s perimeter by its height, subtract the doors and windows, add each gable (width × height ÷ 2), then add trim and doors separately. Sherwin-Williams’ exterior calculator assumes 350 sq ft per gallon.
If Dario paints your rooms in Memphis or Shelby County, you skip the math. He measures the job at the free walkthrough, the paint is included in the quote, and the estimate names the paint for each surface. Want to use paint you already have? Say so and it is priced that way. See interior painting, what a job costs in our house painting cost guide, or DIY vs hiring a painter. Doing it yourself? Our guide on how to paint a room covers the prep and technique.
Got questions?
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350 to 400 square feet, a quart about 100 and a gallon of primer about 200 to 300. Behr gives 250 to 400 for one coat, Glidden says up to 400 on primed, smooth, non-porous surfaces, and Benjamin Moore’s interior Regal Select data sheets list 400 to 450. Rough or textured walls take more paint, so use the lower numbers for them.
How much paint do I need for a room?
A 12 by 12 foot room with 8-foot walls, one door, two windows and two coats needs about 2 gallons for the walls and 1 gallon for the ceiling at 350 sq ft per gallon. Enter your own room in the calculator above, or check the room-size table.
How much paint and primer do I need?
Work out the paint with the steps above, then run the same math for primer with its own coverage, about 200 to 300 sq ft per gallon according to Sherwin-Williams, for one coat. For the 333 sq ft of wall in the example, that is about 1.1 to 1.7 gallons of primer. Dario’s quotes include primer where the surface needs it.
Do I subtract windows and doors?
Yes, it gives a closer number. Sherwin-Williams’ calculator uses a 3 by 7 foot door (21 sq ft) and a 3 by 5 foot window (15 sq ft), and Behr says an average door is 20 sq ft and an average window 15. For a sliding glass door or a big picture window, measure it and count it as more than one.
How many coats of paint do I need?
Two is the standard. Benjamin Moore’s calculator assumes two coats, its Regal Select data sheets recommend two for full color and performance, and Dario’s quotes are priced on two coats over primer where the surface needs it. Glidden adds that drastic color changes typically need a second coat.
Can I use this calculator for a house exterior?
Not directly, because it is built for rooms. Use Behr’s method above for the body, gables and trim, and expect less coverage per gallon outdoors: Sherwin-Williams’ exterior calculator assumes 350 sq ft, its Duration Exterior data sheet lists 250 to 300, and Benjamin Moore’s exterior data sheets list 250 to 400. Dario measures the whole house at the free walkthrough.
Sources
- Sherwin-Williams: How to Use Paint Calculator (FAQs) — a gallon covers about 350–400 sq ft, a quart about 100, a gallon of primer about 200–300
- Sherwin-Williams: Paint Calculator (Quick Calculation) — 3 by 7 ft doors, 3 by 5 ft windows; 400 sq ft per gallon indoors, 350 outdoors, one coat; round up
- Sherwin-Williams: Duration Home Interior Latex Satin data sheet (A97) — 350–400 sq ft per gallon
- Sherwin-Williams: Duration Exterior Acrylic Low Lustre data sheet (K30) — 250–300 sq ft per gallon
- Behr: Determine how much interior paint to buy — 250–400 sq ft per coat, perimeter × height, six-inch trim rule, rough surfaces take more paint
- Behr: How much exterior paint do I need? — average door 20 sq ft, window 15 sq ft; gables, trim and doors
- Benjamin Moore: Paint Calculator — assumes two coats and no ceiling; a rough estimate; add up rooms of the same color
- Benjamin Moore: Regal Select Interior Flat N547 data sheet — 400–450 sq ft per gallon; two coats recommended
- Benjamin Moore: Regal Select Exterior Low Lustre 262 data sheet — 300–400 sq ft per gallon
- Benjamin Moore: Aura Exterior Low Lustre 634 data sheet — 250–350 sq ft per gallon
- Glidden: How Much Paint Do I Need? — up to 400 sq ft on primed, smooth, non-porous surfaces; drastic color changes need a second coat
Checked September 2026. Prices and product details change; your written estimate has the numbers for your house.
