Paint Coverage Calculator
Last updated: October 3, 2026
Calculate gallons for a 12x12 room or any other size, including doors, windows, and coats.
Enter room dimensions, openings, and paint specs. Returns total gallons needed and rounded-up purchase quantity.
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Room dimensions
Subtract openings
Paint
How wall area and gallons are calculated
Paint for the walls of a rectangular room starts from the perimeter, not from the floor. Perimeter = 2 × (length + width). Gross wall area = perimeter × ceiling height. A 12 ft by 14 ft room with 9 ft ceilings has a perimeter of 52 ft and a gross wall area of 468 square feet. The floor of that room is only 12 × 14 = 168 square feet, and using the floor number as the paint quantity is the mistake that leaves you a gallon short.
Openings are subtracted with fixed averages built into the form. Each door counts as 21 square feet, a stand-in for a 3 ft by 7 ft door. Each window counts as 15 square feet, a stand-in for a rough 3 ft by 5 ft window. Paintable area is the gross wall minus those openings, and it will not go below zero. Coverage needed = paintable area × number of coats. Gallons = coverage needed ÷ the square feet one gallon covers. The result shows the decimal gallons and the whole gallons to buy, which is the decimal rounded up.
The coverage field defaults to 350 square feet per gallon, a common figure for interior latex on smooth, already sealed walls. Change it when the can or the surface says otherwise. Two coats is the default because a color change and most new work need two finish coats. The menu also offers one coat and three. Those defaults, run for 10×10, 10×12, 12×12, and 12×14, are tabulated in how many gallons of paint for a 12x12 room.
Worked examples
Example 1: A typical bedroom, two coats
Length 12, width 14, height 9, two doors, two windows, two coats, 350 square feet per gallon. Gross wall = 468. Openings = 2 × 21 + 2 × 15 = 72. Paintable = 396 square feet. Two coats need 792 square feet of coverage. Gallons = 792 ÷ 350 = 2.26, so the buy line rounds up to 3 gallons. The extra paint is the margin for touch-ups and for the optimism printed on the can.
Example 2: One coat on the same walls
Paintable area stays 396. One coat at 350 square feet per gallon is 396 ÷ 350 = 1.13 gallons, which rounds up to 2 gallons to buy. One coat is plausible over a similar light color on a sealed wall. It is a poor plan over a dark color, bare drywall, or stains.
Example 3: Textured walls
The same 792 square feet of two-coat coverage at 250 square feet per gallon is 792 ÷ 250 = 3.17 gallons, so you buy 4. Drop the coverage number for raw drywall, brick, heavy texture, or a porous masonry wall. The can's "up to 400" number assumes a smooth, primed, ideal surface. The article how to read a paint label explains why that printed coverage is a best case.
Example 4: The ceiling is a separate surface
This tool does not paint the ceiling. Ceiling area for the example room is the floor rectangle, 12 × 14 = 168 square feet, which you can confirm with the Square Footage Calculator. One coat at 350 square feet per gallon is about 0.48 gallon. Buy ceiling paint as its own quantity. Adding it silently into the wall gallons is how the walls run out on the last corner.
Example 5: A window that is not 15 square feet
A 6 ft by 5 ft slider is 30 square feet. The form would subtract only 15 if you enter one window. Enter it as two windows, or subtract the extra 15 square feet from the paintable area in your notes before you divide by coverage. A wall of glass should be measured. The 15-square-foot average is for ordinary windows, not a sunroom.
When to use this
Use it when you are about to buy interior wall paint for a rectangular room and you know the length, width, and ceiling height. It answers "how many gallons," which is the question the store will ask. It does not pick a sheen or a color. For an L-shaped room, split the space into rectangles, run each one, and add the gallons. For price shopping after you know the quantity, the Unit Price Calculator compares cost per gallon once the can sizes are in the cart.
Common mistakes
- Painting from the floor area. Walls are perimeter times height. The floor number is the ceiling, not the walls.
- Planning one coat for a color change. Dark over light, and especially light over dark, needs more film. Budget two finish coats, and primer when the jump is large or the surface is new.
- Trusting ideal coverage on a thirsty wall. Lower the square-feet-per-gallon field for texture and bare drywall.
- Buying different dye lots. Gallons mixed at different times can differ slightly. Buy the rounded-up quantity from one batch.
- Forgetting trim and doors. Door faces you will paint, baseboard, and window casing are not in the wall gallons. A room of trim is often on the order of a quart to a gallon of a different sheen. Measure it if the trim is elaborate.
- Double-counting a closet. If the closet is inside the length and width, its walls are not fully included anyway, because the perimeter is the outer rectangle. Measure closet walls separately if you will paint them.
Limitations
Rectangular rooms, fixed opening sizes, and no ceiling, trim, or primer in the gallon total. There is no extra waste percent beyond rounding up to the next whole gallon. Coverage is whatever you type; 350 is a starting point for smooth interior walls, not a guarantee. Exterior siding, stucco, and decking need the coverage printed for that product, which is often lower, and they need a measured area rather than a room perimeter. The tool does not price paint and does not choose primer. Openings larger or smaller than the 21 and 15 square foot stand-ins should be adjusted by changing the count, as in the slider example.
FAQ
Why does the form start at 350 square feet per gallon?
Many interior latex labels quote about 350 to 400 square feet per gallon on smooth, sealed surfaces. Porous or textured walls are often nearer 250 to 300. Replace 350 with the number from your can, then lower it again if the wall is thirsty.
Do I need primer?
New drywall, stains, a glossy surface that was not deglossed, and a large color jump usually do. Primer coverage is often lower than finish paint, sometimes around 200 to 300 square feet per gallon. Buy it as a separate line. This calculator will not add it unless you run a one-coat pass with the primer's own coverage number and treat that result as primer only.
Can I return the extra gallon?
Many stores take unopened gallons back. An opened gallon of the exact color is worth keeping for touch-ups. Sealed and stored away from freezing, leftover latex is useful for years, though the safe plan is to stir and test a patch before you rely on an old can.
How do I paint an L-shaped room?
Divide it into two rectangles, calculate each, and add the gallons. Shared doorways should be subtracted once, not in both rooms. The square footage calculator can check each floor rectangle if you are also doing the ceiling.
What about the outside of the house?
Measure each wall's area, subtract the real doors and windows, and use the exterior product's coverage. Weather, texture, and color depth all push consumption up. The room form is a shortcut for interior rectangles, not a siding takeoff.
Practical tips
Measure ceiling height in the room. An "8-foot" assumption in a 9-foot or 10-foot room underbuys paint by that same proportion. For a move from a dark color to a light one, plan on primer plus two finish coats and accept that the buy line will look generous until the first coat goes on. If the decimal gallons is just over a whole number, such as 3.05, the extra gallon the round-up suggests is your touch-up can, not waste.
Stir cans from the same batch together in a larger bucket when you can, so a slight can-to-can difference never shows up as a stripe. Keep a labeled jar of the finish color. The next scuff will cost a brush, not a new custom mix that only almost matches.
Compare the price per gallon after you know whether you need quarts or gallons. A "deal" on a gallon you cannot return is more expensive than a quart you will finish, once the unused portion is in the math. Read the label for recoating time so the second coat goes on when the first is ready, not when the calendar looks free.