Room 1
Advanced options
Check your paint label — this is a planning figure.
Required on bare drywall, wood or masonry.
Counted separately in the breakdown.
Standard interior door is about 20 ft².
Average window. Measure large ones.
PCA recommends 10%.
This paint calculator works out how many gallons you need for walls, ceilings and primer — deducting doors and windows, and telling you whether to buy gallons, quarts or a mix of both.
The short answer
Wall area is perimeter × height. Deduct 20 ft² per door and 15 ft² per window, multiply by the number of coats, then divide by 375 ft² per gallon.
A 14 × 12 ft room with 8 ft walls, one door and two windows needs 2.25 gallons for two coats — buy 2 gallons plus a quart rather than rounding up to 3.
On this page
How to use this paint calculator
Enter your room and the calculator returns gallons for paint and primer, with the openings already deducted. Four modes:
- Room — length, width, wall height, plus how many doors and windows
- Single wall — one surface, an accent wall or a stairwell
- Ceiling only — just the overhead area
- Known area — you’ve already measured the square footage
Painting a whole house? Use Add another room for each one and the calculator totals them, which matters because paint should be bought in a single order — more on that below.
Under Advanced options: surface texture, coat count, primer, whether to include the ceiling, opening sizes, waste and prices.
The paint formula
1. Wall area = Perimeter × Wall height
2. Paintable area = Wall area − doors − windows
3. Gallons = (Paintable area × Coats) ÷ Coverage per gallon
Perimeter = 2 × (Length + Width)
Coverage is typically 375 ft² per gallon on smooth walls
Worked example — 14 × 12 ft room, 8 ft walls, 1 door, 2 windows
Coverage by surface
The 350–400 ft² figure applies to smooth, primed drywall. Anything rougher or more absorbent takes more paint, because texture increases the actual surface area and porous substrates drink the first coat.
| Surface | ft² per gallon | Notes |
|---|---|---|
| Smooth drywall, primed | 350–400 | The standard planning figure |
| Lightly textured — orange peel | 300–350 | Common in newer US homes |
| Heavy texture — knockdown, popcorn | 260–300 | Considerably more surface than it looks |
| Bare drywall, first coat | 250–300 | Prime it instead — cheaper per gallon |
| Masonry, brick, stucco | 230–260 | Very porous; consider a masonry primer |
| Rough-sawn or weathered wood | 200–250 | Absorbs heavily on the first coat |
| Primer, any surface | 200–300 | Always less than topcoat |
Manufacturers publish coverage on the can, and it varies meaningfully between products — a premium acrylic and a budget contractor paint can differ by 50 ft² per gallon. Application method matters too: spraying wastes more than rolling, and back-rolling a sprayed coat uses more still. Use the calculator to plan, then check the can before you buy.
Deducting doors and windows
| Opening | Deduct |
|---|---|
| Standard interior door (3 × 6’8″) | 20 ft² |
| Average window | 15 ft² |
| Sliding glass door | 40 ft² |
| Large picture window | Measure it |
| Fireplace or built-in you’re not painting | Measure it |
There’s a legitimate argument for skipping this step. Coverage figures are generous, waste is real, and having a little extra for touch-ups is genuinely useful. If a room has one door and one small window, the deduction is about 35 ft² — under a tenth of a gallon, comfortably inside your waste allowance.
Deduct properly when openings are large or numerous: a room with sliding doors and three big windows can easily have 100 ft² of openings, which is a third of a gallon and worth accounting for.
How many coats?
One coat
Refresh only
Same colour, sound previously painted surface. Anything else shows lap marks and patchy depth.
Two coats
The standard
Even colour, consistent sheen, proper film build. This is what manufacturers assume and what warranties are based on.
Three coats
Dramatic changes
Dark to light, strong reds and yellows, or bare drywall without primer. A tinted primer usually saves the third coat.
Reds, oranges and bright yellows are the difficult ones. Their pigments have poor hiding power, and three or even four coats is common. A grey-tinted primer underneath does more for coverage than an extra topcoat and costs less per gallon.
When do you actually need primer?
| Situation | Primer? | Why |
|---|---|---|
| Bare drywall | Yes | Porous — a PVA primer seals it so topcoat covers evenly |
| Bare or new wood | Yes | Seals grain and blocks tannin bleed |
| Masonry, brick, concrete | Yes | Highly absorbent, often alkaline |
| Water stains, smoke, marker | Yes | Stain-blocking primer — topcoat won’t hide these |
| Dark to light colour change | Yes | Tinted primer is cheaper than a third topcoat |
| Glossy surface being repainted | Yes | Bonding primer, after scuff sanding |
| Painted wall, similar colour, good condition | No | Two topcoats is enough |
| Patched or skim-coated areas | Spot prime | Compound is more absorbent than surrounding paint |
Self-priming paints are thicker topcoats with better adhesion. They’re fine over previously painted, sound surfaces — which is the situation where you didn’t need primer anyway. On bare drywall, bare wood, masonry or stains, use a dedicated primer. It costs less per gallon than paint and does a job the topcoat can’t.
Gallons or quarts?
Most calculators round up to the next whole gallon. If you need 2.15 gallons, that means buying 3 — a whole gallon more than the job takes, at full price.
A quart is a quarter gallon and covers about 94 ft² at 375 ft² per gallon. Two gallons plus a quart gets you 2.25 gallons: enough for the job, with a sensible touch-up reserve left over.
| You need | Buy | You get |
|---|---|---|
| 0.7 gal | 3 quarts | 0.75 gal |
| 1.6 gal | 1 gallon + 3 quarts | 1.75 gal |
| 2.15 gal | 2 gallons + 1 quart | 2.25 gal |
| 2.6 gal | 2 gallons + 3 quarts | 2.75 gal |
| 4.6 gal | 5 gallon pail | 5 gal |
The exception is the 5 gallon pail, which is usually cheaper per gallon than five separate gallons and comes from a single batch. Above about 4 gallons, buy the pail.
Paint is tinted in batches. Two gallons of the same colour mixed a week apart can differ enough to show as a band on a long wall. Buy the whole quantity at once, and for anything over two gallons, ask the store to box it — combine the cans in a larger pail and stir them together so every wall gets identical paint.
Paint needed by room size
8 ft walls, one door and two windows, two coats at 375 ft² per gallon, 10% waste, walls only.
| Room | Wall area | Paintable | Gallons | Buy |
|---|---|---|---|---|
| 10 × 10 ft | 320 ft² | 270 ft² | 1.58 | 1 gal + 3 qt |
| 10 × 12 ft | 352 ft² | 302 ft² | 1.77 | 2 gallons |
| 12 × 12 ft | 384 ft² | 334 ft² | 1.96 | 2 gallons |
| 12 × 14 ft | 416 ft² | 366 ft² | 2.15 | 2 gal + 1 qt |
| 14 × 16 ft | 480 ft² | 430 ft² | 2.52 | 2 gal + 3 qt |
| 16 × 20 ft | 576 ft² | 526 ft² | 3.09 | 3 gal + 1 qt |
| 20 × 20 ft | 640 ft² | 590 ft² | 3.46 | 3 gal + 2 qt |
Add the ceiling and these rise noticeably — a 12 × 14 room adds 168 ft², taking it from 2.15 to about 3.1 gallons.
Ceilings and trim
Ceilings are simply length × width. They’re usually painted flat white in a dedicated ceiling paint, which is formulated to be thicker and drip less — worth using, since you’re working overhead. Textured or popcorn ceilings drink paint; expect 250–300 ft² per gallon rather than 375, and never roll an unpainted popcorn ceiling with water-based paint without testing, because it can loosen and fall.
Trim is measured in linear feet times the width of the trim. Baseboard at 4 inches over 52 linear feet is about 17 ft² — barely a quart, and a quart is almost always the right purchase. Door and window casings add a few square feet each.
Trim is usually painted in a different sheen from walls — semi-gloss or satin against an eggshell or matt wall — so it’s a separate product and a separate purchase regardless of quantity.
Exterior painting
The method is the same, but three things change:
- Coverage drops. Siding, stucco and weathered wood are rougher and more absorbent — expect 250–350 ft² per gallon.
- Two coats is a minimum, not a target. Exterior paint is the building’s weather protection, and film thickness is what delivers it.
- Weather governs. Most exterior paints need surface temperatures above about 50°F and no rain for 24 hours. Painting in direct sun causes the film to skin before it levels.
For siding, measure each wall’s height by width and subtract openings as usual. Gables are triangles — half base times height. Don’t forget soffits, fascia and trim, which are separate products and often a different colour.
Paint estimating mistakes to avoid
- Calculating one coat. Two is standard; one is a refresh only.
- Ignoring texture. Knockdown walls take about a third more paint than smooth.
- Rounding up to whole gallons. Quarts exist and cover 94 ft² each.
- Buying in two trips. Different batches can show as a band on a long wall.
- Skipping primer on bare drywall. Self-priming paint isn’t primer.
- Forgetting the ceiling. A 12 × 14 room adds 168 ft² — most of a gallon.
- Using wall paint on trim. Different sheen, different formulation, different product.
Frequently asked questions
How much paint do I need?
Multiply the room perimeter by wall height, subtract 20 ft² per door and 15 ft² per window, multiply by the number of coats, then divide by 375 ft² per gallon. A 12 × 14 ft room with 8 ft walls needs about 2.15 gallons for two coats.
How much does a gallon of paint cover?
350 to 400 square feet per coat on smooth, primed drywall. That drops to 300–350 on lightly textured walls, 260–300 on heavy texture, and 230–260 on masonry. Primer covers 200–300 regardless of surface.
How much paint for a 12×12 room?
With 8 ft walls, one door and two windows, that’s 334 ft² of paintable wall — about 2 gallons for two coats including waste. Add the ceiling and it rises to roughly 2.8 gallons.
Do I subtract doors and windows?
For a room with one door and one window, the 35 ft² difference sits inside your waste allowance and can reasonably be skipped. Deduct properly when openings are large or numerous — sliding doors and multiple big windows can total 100 ft², which is a third of a gallon.
How many coats of paint do I need?
Two is standard and what manufacturers assume. One coat only works when refreshing the same colour on a sound surface. Three is needed for dramatic colour changes or strong reds and yellows, though a tinted primer usually saves the third coat.
How much does a quart of paint cover?
About 94 square feet on smooth walls — a quarter of a gallon’s coverage. Quarts are useful for trim, touch-ups, and topping up an order rather than rounding all the way to the next gallon.
Do I need primer?
Yes on bare drywall, bare wood, masonry, stains, glossy surfaces being repainted, and dramatic colour changes. No on previously painted walls in good condition going to a similar colour. “Paint and primer in one” products are thicker topcoats, not a substitute for dedicated primer on bare substrates.
How much primer do I need?
Primer covers 200–300 square feet per gallon and needs only one coat, so roughly 1.5 times as much per coat as topcoat. For a 366 ft² room that’s about 1.6 gallons, and primer costs less per gallon than finish paint.
Should I buy gallons or a 5 gallon pail?
Above about 4 gallons, buy the pail. It’s usually cheaper per gallon and comes from a single batch, so there’s no colour variation between cans. Below that, buy gallons plus quarts to match your requirement rather than rounding up.
Why does my paint look different on one wall?
Usually batch variation — cans tinted at different times can differ slightly. Buy the whole quantity at once and box it, meaning combine the cans in a larger pail and stir them together before starting. Sheen differences and uneven film thickness from a thin second coat cause the same effect.
How much paint for a ceiling?
Ceiling area is simply length × width, so a 12 × 14 room is 168 ft² — about 1 gallon for two coats. Textured or popcorn ceilings take considerably more, around 250–300 ft² per gallon rather than 375.
Can I use this paint calculator in metric?
Yes. Switch the header toggle from Feet to Metres and enter room dimensions in metres. Coverage rates and results are given in square feet and US gallons; multiply gallons by 3.785 for litres.
Sources and method
- Painting Contractors Association coverage standards — 350–400 ft²/gal for smooth surfaces, 200–300 ft²/gal for primers, 10% material overage
- Manufacturer technical data sheets for coverage by surface type and sheen
- Standard opening deductions of 20 ft² per interior door and 15 ft² per average window
- Coverage on any specific product should be taken from the container label, which varies by formulation
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How this paint calculator works out its numbers
Wall area is perimeter times height, less the openings you enter at the areas set under Advanced options. Ceiling area is length times width and is tracked separately so the breakdown shows each. Gallons are paintable area times coats, divided by the coverage rate, plus your waste allowance — then rounded up to the nearest quarter gallon, since a quart is the smallest practical purchase.
The buying recommendation converts that figure into whole gallons plus quarts rather than rounding to the next gallon, which on a typical room saves most of a gallon. Primer is calculated as a single coat at its own coverage rate, since primer covers less than topcoat and is rarely applied twice.
Coverage rates are planning figures from Painting Contractors Association standards. The figure printed on your paint container is more accurate than any calculator, because coverage varies meaningfully between products and with application method — spraying uses more than rolling, and back-rolling a sprayed coat more still.