Paint Calculator
Enter your room dimensions to find out how many gallons or liters of paint you need. Doors and windows are subtracted automatically, and you can add extra coats and a waste factor. Results update as you type. Need floor area instead? Try the square footage calculator.
- Paintable area (one coat)
- 0 ft²
- Total area with coats & waste
- 0 ft²
- Paint needed (exact)
- 0 gallons
- Paint to buy
- 0 gallons (0 exact)
How to measure a room for paint
Measure the length of each wall at floor level with a tape measure, and add them together to get the room's perimeter. If the room is a simple rectangle, that is twice the length plus twice the width. Then measure the wall height from floor to ceiling. Multiply the perimeter by the height to get the gross wall area, which is the number this calculator starts from before subtracting anything.
Doors and windows do not get painted the same way as drywall, so their area comes back out. A standard interior door is about 20 square feet and a standard window is about 15 square feet, which are the defaults used here; adjust the door and window counts if your room has more or fewer than one of each, or edit the numbers directly if your openings are unusually large or small.
Most rooms need two coats for even color and coverage, especially over a different existing color or bare drywall. If you are painting over bare drywall, a stark color change, or a stain, add a primer coat by increasing the coats field by one; primer and topcoat use the same wall area, so treating primer as an extra coat gives a reasonably accurate paint estimate without a separate calculation.
Ceilings are painted separately from walls in most projects, but if you want a single combined total, check "include ceiling" and the calculator adds the floor-equivalent ceiling area (length times width) to your wall total. For exterior work, measure the perimeter and average height of the exterior walls the same way, subtract exterior doors and windows, and consider a slightly higher waste factor since textured siding uses more paint than smooth drywall.
Always add a waste factor on top of the raw coverage math. A 10 percent factor is reasonable for a straightforward rectangular room painted by a careful painter; rougher walls, cut-in trim work, or a less experienced painter can use 15 to 20 percent instead. Paint is sold by the whole gallon or liter, so this calculator rounds your exact requirement up to the next whole can, since you cannot buy a fraction of a gallon at the store.
Worked example
A room measures 12 ft by 12 ft with 8 ft walls, 1 door, 1 window, 2 coats, 350 sq ft per gallon coverage, and 10% waste.
Perimeter = 2 × (12 + 12) = 48 ft.
Gross wall area = 48 × 8 = 384 sq ft.
Doors and windows to subtract = (1 × 20) + (1 × 15) = 35 sq ft.
Paintable area (one coat) = 384 − 35 = 349 sq ft.
With 2 coats: 349 × 2 = 698 sq ft.
With 10% waste: 698 × 1.10 = 767.8 sq ft total coverage needed.
Gallons needed = 767.8 ÷ 350 = 2.19 gallons exact, rounded up to 3 gallons to buy.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12 by 12 foot room with 8 foot walls, one door, and one window needs about 349 square feet of wall paint per coat. With two coats and a 10 percent waste factor that comes to roughly 767.8 square feet of coverage, which needs about 2.19 gallons at 350 square feet per gallon coverage, so you would buy 3 gallons since paint is sold in whole cans. See the worked example above for the full arithmetic.
How does a paint calculator handle gallons for two coats?
Two coats simply double the area you need to cover, since you are applying paint twice over the same wall. This calculator multiplies your net paintable area by the number of coats you enter, adds your waste factor, then divides by the coverage rate of your paint (commonly 350 to 400 square feet per gallon) and rounds up to a whole gallon.
How much paint do I need for a bedroom ceiling?
Ceiling area equals the room's length times its width, the same as its floor area. Check the "include ceiling" box above to add that area to your wall total. Ceilings often need only one coat if they are already a similar color, but flat ceiling paint can hide imperfections better with two.
Can I use this as an exterior paint calculator by square footage?
Yes. Measure the perimeter and average height of the exterior walls the same way you would indoors, enter your door and window counts, and skip the ceiling option. Exterior siding is often more textured than interior drywall, so consider a slightly lower coverage rate per gallon and a higher waste factor, such as 15 percent.
How many gallons of paint do I need for a 3 bedroom house?
Run this calculator once per room, note the gallons for each, then add them up, since wall height, doors, and window counts differ room to room. As a rough starting point, three average bedrooms plus a shared hallway commonly use 6 to 10 gallons for two coats, but always calculate room by room for an accurate number.
Does this paint calculator include primer and subtract doors and windows?
The calculator subtracts a standard door (20 square feet) and window (15 square feet) area for every door and window you enter, and you can adjust the counts. It does not add a separate primer coat automatically; if you are priming bare drywall or a major color change, add an extra coat in the coats field to represent the primer layer.