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How much paint do I need?

Enter length, width and ceiling height, and this free calculator says how many gallons to buy, doors and windows subtracted, second coat included. It assumes 350 sq ft per gallon, the same figure Benjamin Moore's own calculator uses, or type the coverage printed on your can. Switch to meters and liters if that's how your tape measure thinks.

The can says a gallon does 400 square feet in one coat. The wall disagrees about the coat, and usually about the 400.

Your room

Start from a room size

Your can says a different coverage

We assume 350 sq ft per gallon, the same figure Benjamin Moore's own calculator uses. Type the coverage printed on your can.

You need

2.0 gal

Buy 2 gallons.

Walls, minus 2 openings
348 sq ft · 2.0 gal

Cans are rounded up to sizes you can buy. Most people are short on the second coat and long on the first — this plans for 2 coats so the wall gets both.

Before you buy it: the quantity is the easy part. See the color on your own walls first with the free paint visualizer, and if the shade you love is in another brand, the color matcher finds its closest code.

How many square feet does a gallon of paint cover?

A gallon of interior paint covers about 350 to 400 square feet in one coat, the range Sherwin-Williams prints on its SuperPaint, Duration Home, Emerald and Captivate data sheets. Benjamin Moore claims a little more: Regal Select and ben say 400 to 450 sq ft per gallon, Aura 350 to 400, all “depending on surface texture and porosity.” Behr Premium Plus runs 250 to 400, Valspar Signature 200 to 400, and Glidden Diamond says up to 400 on primed, smooth, nonporous surfaces. Notice how the small print always points down.

Plan on the low end. Sherwin-Williams' own FAQ calls 400 “theoretical” because the figure counts neither the wall's texture nor the paint that stays on the roller, and every number above is for one coat. Textured walls and bare plaster drink more, which is why this calculator defaults to 350 and adds 15% when you check the textured box. Buying for 400 and finding out on the last wall is the expensive kind of optimism.

Coverage per gallon, per coat, as printed on each maker's own data sheet or product page and read on 2 October 2026; the lower number is the one to plan with.
PaintStated coverage per gallon, per coatThe small print
Sherwin-Williams SuperPaint, Duration Home, Emerald, Captivate350–400 sq ftAt 4 mils wet; SW calls 400 "theoretical"
Benjamin Moore Regal Select, ben400–450 sq ftDepending on surface texture and porosity
Benjamin Moore Aura350–400 sq ftDepending on surface texture and porosity
Behr Premium Plus, Ultra Scuff Defense250–400 sq ftDepending on application method and porosity
Behr DynastyUp to 400 sq ftDepending on application method and porosity
Valspar Signature200–400 sq ftTwo coats recommended
Glidden DiamondUp to 400 sq ftOn primed, smooth, nonporous surfaces

How much does a quart of paint cover?

A quart of interior paint covers about 100 square feet in one coat, the figure in Sherwin-Williams' own calculator FAQ and roughly a quarter of a gallon's 350 to 400. That's one coat on an accent wall 12 ft wide under an 8 ft ceiling. A quart is the right buy when the math lands just past a whole gallon, as the 12 × 14 ft room does at 2.17 gallons: two gallons and a quart, which is how this calculator rounds. It's the wrong buy for a whole room. Four open quarts is a gallon with extra lids.

What sizes do paint cans come in?

US interior paint comes in quarts, gallons and 5-gallon pails. A gallon is 4 quarts, 128 fl oz or 3.785 liters; a quart is 32 fl oz or 0.946 liters. Sherwin-Williams sells all three sizes and says a 5-gallon pail's actual fill depends on how much tint goes in. Benjamin Moore's color samples are 8 oz, a half pint, which paints a test patch and little else. The common UK tins are 2.5 L and 5 L, with 1 L and 10 L in some ranges. This calculator does its shopping in whichever set you switch to.

US can sizes with their metric equivalents from NIST Handbook 44, alongside the common UK tin sizes (the metric mode buys 1, 2.5 and 5 L).
SizeHoldsCovers per coat
Sample (half pint)8 fl oz, about 0.24 LA test patch
Quart32 fl oz, 0.946 LAbout 100 sq ft
Gallon128 fl oz, 3.785 LAbout 350–400 sq ft
5-gallon pailNominal 5 gal, about 18.9 L (fill depends on tint)About 1,750–2,000 sq ft
UK tins1 L, 2.5 L, 5 L, 10 LUp to 12–14 m² per liter (matt emulsions)

How do you calculate how much paint a room needs?

Measure the walls. Add the four wall lengths together and multiply by the ceiling height. A 12 × 12 ft room has a 48 ft perimeter, and under an 8 ft ceiling that's 384 square feet of wall. Metric mode runs the same sum in meters.

Take off the doors and windows. Each door counts as 3 × 7 ft, 21 square feet, and each window as 3 × 5 ft, 15 square feet, the same allowances Sherwin-Williams uses in its own quick calculator. One of each brings the 12 × 12 room to 348 square feet. Check the ceiling box and its 144 square feet go back on.

Multiply by coats. Two coats is the default, which is what Benjamin Moore and Valspar recommend, so 348 becomes 696 square feet of painting. Textured or bare plaster walls get 15% added. A big color change adds one coat of primer, planned on its own at 300 sq ft per gallon, because primer spreads thinner than paint.

Divide by coverage and round up to cans. 696 square feet at 350 sq ft per gallon is 1.99 gallons, so you buy 2. The calculator rounds up to cans you can actually buy and spells out the mix of 5-gallon pails, gallons and quarts; metric mode starts from 11 m² per liter and buys 5 L, 2.5 L and 1 L tins. Type the coverage printed on your own can and the whole sum reruns.

Questions people ask before they buy paint

How much paint do I need for a 12 × 12 room?
Two gallons covers the walls of a 12 × 12 ft room with an 8 ft ceiling, one door, one window and two coats. That's 348 square feet of wall, 696 over two coats, or 1.99 gallons at 350 sq ft per gallon. Add the ceiling and it's 2.81 gallons, so buy 3. In metric, a 3.7 × 3.7 m room with a 2.4 m ceiling has 32.2 m² of wall and needs 5.9 liters at 11 m² per liter: a 5 L tin and a 1 L tin.
How many gallons of paint do I need for a bedroom?
Two gallons covers a bedroom from 10 × 10 to 12 × 12 ft with two coats under an 8 ft ceiling; bigger rooms or taller ceilings need a quart or a third gallon. At the defaults here a 10 × 10 ft bedroom needs 1.62 gallons, a 12 × 12 needs 1.99, and a 12 × 14 needs 2.17, so 2 gallons plus a quart. Raise the 12 × 12 ceiling to 9 ft and it's 2.26 gallons, so you buy 3. Ceilings, primer and textured walls are extra.
Is one coat ever enough?
One coat is rarely enough, and it isn't when the color changes much. Benjamin Moore and Valspar recommend two coats, Benjamin Moore's own calculator assumes two, and so does this one. One coat can hold up when you're repainting the same color in the same sheen over a sound wall, and it's worth trying on one wall before you commit the room to it. Sherwin-Williams' quick calculator does assume a single coat, so its totals run lower than ours. The wall settles the argument, usually in the afternoon light.
How much primer do I need?
Plan primer at 300 sq ft per gallon, so the 12 × 12 ft room needs 1.16 gallons, a gallon plus a quart. Primer spreads thinner than paint, though makers disagree by how much: Sherwin-Williams puts a gallon at 200 to 300, KILZ Original and KILZ 2 say 300 to 400, and Zinsser Bulls Eye 1-2-3 about 400 to 450, 350 on porous surfaces. Flag a big color change and this calculator adds one primer coat at 300, the top of Sherwin-Williams' range and the bottom of KILZ's.
Should I subtract doors and windows?
Yes, subtract them, and this calculator already does: 21 square feet per door (3 × 7 ft) and 15 per window (3 × 5 ft), the same allowances Sherwin-Williams builds into its own quick calculator. In the 12 × 12 ft room, one door and one window take 384 square feet down to 348. Count a wall of glass or a double doorway as two; leave a small window in, because the leftover is the touch-up can you'll want next spring.
Do I need different paint for the ceiling?
No, you don't have to, though ceiling paints are made flat to hide roller marks. Sherwin-Williams ProMar Ceiling is rated 350 to 400 sq ft per gallon, Benjamin Moore Waterborne Ceiling 400 to 450, Behr Premium Plus Ceiling 250 to 400. Check the ceiling box and the calculator adds its area, 144 square feet in the 12 × 12 room, lifting the buy from 2 to 3 gallons. It gives one total, so if the ceiling gets a different product or color, buy that as its own can.

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The quantity was the easy part

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Coverage figures read from the makers' own data sheets and calculator pages on 2 October 2026; they change without notice, so if your can disagrees with this page, trust the can and tell us.