Paint Calculator
Estimate the gallons of paint and number of cans you need for any room.
This calculator runs entirely in your browser. Nothing you enter is uploaded.
How to use this tool
Measure your room and enter the length, width, and wall height in feet. Then tell the calculator how many doors and windows are on the walls so it can subtract that area, since you are not painting those openings. Choose how many coats you plan to apply (two is common for an even finish) and how many square feet a gallon of your paint covers. Press Calculate Paint to see the gallons needed and how many wall paint cans to buy.
The formula and how it works
Paintable area = Wall area - (doors x 21) - (windows x 15)
Gallons = (Paintable area x coats) / coverage per gallon
Cans = round up of (Gallons / can size)
The walls of a rectangular room form the perimeter, which is two times the length plus the width. Multiplying the perimeter by the wall height gives the total wall surface. From that we subtract a standard door opening of about 21 square feet and a standard window of about 15 square feet. Multiplying by the number of coats and dividing by the paint coverage (often around 350 square feet per gallon) gives the gallons of paint. Because you can only buy whole cans, the can count is always rounded up.
A real example
Imagine a 12 by 10 foot room with 8 foot walls, one door, two windows, and two coats of paint. The wall area is 2 x (12 + 10) x 8 = 352 square feet. Subtracting one door (21) and two windows (30) leaves 301 square feet of paintable wall. With two coats that is 602 square feet. At 350 square feet per gallon you need about 1.72 gallons, which rounds up to one 5 gallon can or two 1 gallon cans depending on the size you choose.
Common questions
How many gallons of paint do I need for a room?
It depends on the wall area and the number of coats. Most interior paints cover about 350 square feet per gallon, so a typical bedroom with two coats often needs one to two gallons. Enter your measurements above for an exact paint calculator gallons estimate.
Why does the tool subtract doors and windows?
You do not paint the glass or the door slab, so that area should not count toward your wall paint cans needed. The calculator uses roughly 21 square feet per door and 15 square feet per window as standard sizes.
How many coats should I plan for?
Two coats is the standard choice for an even color and good coverage, especially over a different color or fresh drywall. A single coat may work for a small refresh over the same color. The room painting estimate adjusts automatically when you change the coats.
Can I change the coverage per gallon?
Yes. Coverage varies by paint type and surface texture, so check the label on your can. Smooth walls cover closer to the high end while rough or textured walls use more paint. Update the coverage field to match your product.
Does this include the ceiling?
No, this tool estimates wall paint only. Ceilings are usually painted separately and can be measured as length times width as their own surface. Add that area to your plan if you intend to paint the ceiling too.
This is an estimate for educational planning only. Always buy a little extra for touch-ups and confirm coverage on your paint can before purchasing.