A Free Tool · Standard 350 sq ft / gallon · Two-coat default
How much paint do you actually need?
Enter a room's length, width, and wall height. This calculator works out the wall
area, subtracts a standard door and window, multiplies by two coats, and divides by
the coverage rate the major paint brands publish — about 350 square feet per
gallon. You get gallons to buy, plus the leftover expressed in quarts so you don't
overbuy.
350 sq ft per gallon, per coat·Door & window deductions·Optional ceiling
Read this first
These are estimates that assume one standard coverage rate. Primer or a bare first coat,
a deep color change (especially dark to light), and textured or porous walls all change
the number — usually upward. Treat the result as a starting point and always
check the coverage figure printed on the can of paint you actually buy.
The calculator
Estimate paint for a room
Enter the room's dimensions in feet. The defaults — 8-foot walls, one door, one window, two coats — match a typical bedroom or living room. Adjust anything that's different and the result updates as you type.
Each deducts ~21 sq ft (a 3 × 7 ft door).
Each deducts ~15 sq ft (an average window).
Two coats is standard for even color.
Paintable area
Total area to cover (× coats)
Doors & windows deducted
Paint to buy
Coverage rate used
The math, honestly
Coverage, coats, and why the number is what it is
One gallon of interior wall paint covers roughly 350 to 400 square feet
in a single coat on a smooth, primed surface — the range Sherwin-Williams,
Benjamin Moore, and Behr all publish. This calculator uses 350 as a
deliberately conservative figure, so you're more likely to have enough than to run
short mid-wall. The wall area itself is just 2 × (length + width) × height,
minus the doors and windows you won't paint.
The other half of the math is two coats. A single coat almost never
looks finished — it goes on thin, shows roller lines, and lets the old color
ghost through. Two coats is the industry-standard default for even color and full hide,
which is why the total area to cover is the paintable area times two. Divide that by
350, round up to the next whole gallon, and buy the fractional remainder as quarts
(a gallon is four quarts) instead of a second full gallon you'll barely touch.
Paint by common room size
Gallons needed for common rectangular rooms, all assuming 8-foot walls, two coats, and
one door plus one window. These come from the same formula the calculator uses, so the
table and the tool always agree.
Room size8 ft walls
Wall areasq ft
Paintableafter 1 door + 1 window
To cover× 2 coats
Gallonsrounded up
10 × 10 ft
320
284
568
2
10 × 12 ft
352
316
632
2
12 × 12 ft
384
348
696
2
12 × 16 ft
448
412
824
3
14 × 16 ft
480
444
888
3
16 × 20 ft
576
540
1,080
4
Wall area = 2 × (length + width) × 8. Paintable subtracts one door (~21 sq ft)
and one window (~15 sq ft). Gallons = the area to cover, divided by 350 sq ft/gallon,
rounded up to the next whole gallon. Several of these round up generously — a
12 × 16 room needs 824 sq ft covered, which is 2.4 gallons, so you'd buy 2 gallons
plus 2 quarts rather than a full third gallon. Your own walls may need more if they're
textured, porous, or changing color dramatically.
Why your real number may differ
The 350 sq ft/gallon figure assumes a near-ideal wall. Most walls aren't ideal. Here are
the four things that most often push the estimate up.
Texture and porosity soak up paint
A smooth, sealed wall hits the high end of the coverage range. Knockdown, orange-peel, or popcorn texture has far more surface area, and bare or chalky surfaces drink paint. On a heavily textured or porous wall, a gallon can cover closer to 250–300 square feet rather than 350–400, so round up and consider buying an extra quart.
Bare and patched walls need a first coat that's really primer
New drywall, spackled patches, and water stains all need sealing before color. If you skip primer, your first coat of finish paint does the priming job and won't cover well, effectively adding a coat. Primer is cheaper than finish paint, seals the surface evenly, and often lets two finish coats do what three would otherwise take.
Dark-to-light changes can need a third coat
Going from a bold or dark color to a lighter one is the classic case where two coats isn't enough. Deep reds, blues, and near-blacks bleed through pale topcoats. Tinted primer plus two finish coats usually wins; if you skip the primer, plan on three coats and add that into the calculator's coats field.
Trim, doors, and ceilings are counted separately
This calculator deducts doors and windows from the wall total because trim and casing are usually painted in a different, more durable product bought by the quart. Ceilings are a separate flat paint as well — use the ceiling checkbox to fold the ceiling area into the wall estimate, but buy trim paint on its own.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These reflect common paint-industry
usage — they're background, not a how-to-paint guide.
Coverage rate
How much surface a given amount of paint will cover, stated as square feet per gallon for one coat. Major brands publish roughly 350–400 sq ft per gallon for interior wall paint on a primed, smooth surface. This site uses 350 as a conservative default. The figure on your actual can is the one that matters.
Square footage
The area of a surface, in square feet. For walls in a rectangular room it's the perimeter times the height: 2 × (length + width) × height. For a ceiling it's length times width. Paint is estimated against square footage, not the room's volume or floor area alone.
Coat
One full application of paint over a surface. Two coats is the standard for even color and full hide; the calculator defaults to two. A "coat" assumes the previous layer has dried before the next goes on.
Primer
A preparatory undercoat that seals porous or bare surfaces, blocks stains, and improves how the finish paint adheres and covers. Used on new drywall, patches, glossy surfaces, and big color changes. Often lets two finish coats do the work of three. Primer is estimated and bought separately from finish paint.
Sheen / finish
How much light the dried paint reflects — from flat (no shine) through eggshell and satin to semi-gloss and gloss. Flatter sheens hide wall imperfections; glossier sheens are more washable and are common on trim and doors. Sheen doesn't change the coverage math, but it does change which product you buy for walls versus trim.
Cut in
To brush a clean band of paint along edges, corners, and around trim and ceilings before rolling the open wall. Cutting in is the slow part of painting but doesn't materially change how much paint you need — it's the same square footage, applied with a brush instead of a roller.
Spread rate
Another name for coverage rate — the area one gallon spreads over at the recommended thickness. Lower spread rate (e.g. on textured or porous walls) means you need more paint for the same wall. Manufacturers list it on the can, usually as a range.
Frequently asked
For a standard 12 by 12 foot room with 8-foot walls, two coats, and one door and one window, you need about 2 gallons. The walls measure 2 × (12 + 12) × 8 = 384 square feet; subtracting one door (about 21 sq ft) and one window (about 15 sq ft) leaves 348 square feet of paintable wall. Two coats means 696 square feet to cover, and at 350 sq ft per gallon that's 1.99 gallons — which rounds up to 2 gallons. Run your exact numbers in the calculator above.
Often, yes. Primer is recommended on bare drywall, fresh patches or repairs, stained or water-damaged areas, glossy surfaces, and when making a big color change such as covering a dark wall with a light one. Primer seals the surface so the topcoat goes on evenly and may save you a coat of finish paint. If you're repainting a similar color over a sound, previously painted wall, you can usually skip a separate primer, especially with a paint-and-primer-in-one product.
Two coats is the standard and the default in this calculator. A single coat rarely covers evenly, can look patchy, and shows roller marks — especially over a different color. Use two coats for most repaints; you may need three when going from a dark color to a light one, painting over bold colors, or covering a porous or heavily patched surface. If you're applying a true matching color over the same color, one careful coat is sometimes enough. Change the coats field above to match your situation.
It can, under ideal conditions. Major brands such as Sherwin-Williams, Benjamin Moore, and Behr typically state coverage of roughly 350 to 400 square feet per gallon for one coat on a smooth, primed, sealed wall. This calculator uses 350 as a conservative default so you're less likely to come up short. Textured, porous, or unprimed surfaces and dark-to-light color changes absorb more paint and reduce real-world coverage, so always check the coverage figure printed on your specific paint can.
For a rectangular room, measure the length and width of the floor and the height of the walls. Add the length and width, double the result to get the perimeter, then multiply by the height: wall area = 2 × (length + width) × height. For example, a 12 by 12 foot room with 8-foot walls has 2 × (12 + 12) × 8 = 384 square feet of wall. Then subtract the area of doors and windows you won't paint — the calculator does this for you.
Usually yes. Ceiling paint is typically a flat, ultra-low-sheen formula designed to hide imperfections, minimize spatter, and reduce glare overhead, and it's often sold as a dedicated product. Wall paint comes in a range of sheens such as eggshell or satin that are more washable. If you paint the ceiling, treat it as a separate purchase and add its area (length × width) to your calculation. This calculator can include the ceiling area when you check the ceiling option above.
Trim, baseboards, and doors are usually painted in a different product (often a durable semi-gloss) and are estimated separately from the walls. As a rough rule, a quart of trim paint covers a typical room's baseboards and door and window casings, and each door takes roughly a quarter of a quart per coat. Because trim paint is bought by the quart rather than the gallon, this wall calculator deducts door and window areas from the wall total rather than adding trim paint to it.