The Paint Calculator converts wall, ceiling and trim dimensions into the gallons of paint a job needs, after deducting doors and windows and applying the number of coats specified. Coverage rates are set by surface texture, since a textured wall and a smooth drywall wall consume paint at different rates from the same can. Results show exact gallons alongside the number of cans to buy at quart, gallon and five-gallon sizes.
Calculate paint for walls, ceilings and trim
Paintable area is the surface being covered, not the room's floor plan, and walls, ceilings and trim each use a different area formula because each is a different shape relative to the room.
A room's four walls form a rectangle unrolled flat; a ceiling is the same footprint as the floor; trim is a long, narrow strip measured by linear length rather than by width and height.
Four walls = 2 × (Length + Width) × Height
Ceiling = Length × Width
Trim = Perimeter × Trim height (baseboard, crown moulding, casing)A bedroom, a hallway and a garage all use the four-wall formula for their vertical surfaces. Baseboard, crown moulding and door or window casing are entered separately from the walls, since trim is usually painted in a different sheen and sometimes a different colour, and mixing its area into the wall total would understate both.
Deduct doors and windows from the area
Doors and windows do not get painted with wall paint, so their area comes off the gross wall calculation before coverage is applied. The calculator deducts each opening by a fixed standard area rather than requiring the exact door and window dimensions to be measured and entered individually.
Net wall area = Gross wall area − (Doors × 21 ft²) − (Windows × 15 ft²)A standard interior door, close to 3 ft by 7 ft, is deducted at 21 ft² per door. A standard window is deducted at 15 ft² per window. These are stated assumptions rather than hidden ones: a room with an oversized picture window or an unusually large door should have that opening measured directly rather than relying on the standard figure, since the standard deduction can understate the true opening by a meaningful margin on non-standard sizes.
Apply the paint coverage formula
Coverage per gallon is the number that turns an area into a paint quantity, and it depends on the surface being painted, not on the paint alone. The formula multiplies area by the number of coats, then divides by coverage, since two coats consume twice the paint of one over the identical surface.
Gallons = (Net area × Coats) ÷ Coverage per gallonA can label's stated coverage, typically 350 to 400 ft² per gallon for interior paint on drywall, assumes a reasonably smooth, properly primed surface. Rough or porous surfaces absorb more paint per coat and need a lower coverage figure than the can label states, covered in full in the surface texture table below.
Estimate paint for a 12' × 14' room
A room measuring 12 ft by 14 ft, with an 8 ft ceiling, two doors, two windows, and two coats on smooth drywall, needs 1.72 gallons of paint for the walls. Every step below carries the units through so the total can be checked.
1. Find the gross wall area. 2 × (12 + 14) × 8 = 2 × 26 × 8 = 416 ft²
2. Deduct the doors and windows. 2 doors × 21 ft² = 42 ft² 2 windows × 15 ft² = 30 ft² Total deduction = 72 ft²
3. Find the net area. 416 − 72 = 344 ft²
4. Multiply by the number of coats. 344 ft² × 2 coats = 688 ft²-coats
5. Divide by coverage. 688 ÷ 400 ft² per gallon = 1.72 gallons
A single gallon covers most of this job, but 1.72 gallons cannot be bought as a fraction, so the practical order is two gallons, or one gallon plus a quart if the supplier sells that size. Painting the same room with only one coat instead of two would need just 0.86 gallons, under a single can, which is why the number of coats belongs on the calculator alongside the room dimensions rather than being assumed.
Match coverage to surface texture
Surface texture changes how much paint a given area absorbs, and using the smooth-drywall coverage figure on a rough or porous surface understates the paint needed by a wide margin. The calculator sets coverage from the selected surface and leaves the figure editable for a specific product's label value.
| Surface | Coverage |
|---|---|
| Smooth drywall | 400 ft²/gal |
| Textured | 350 ft²/gal |
| Wood siding | 275 ft²/gal |
| Brick | 200 ft²/gal |
| Stucco | 175 ft²/gal |
Smooth drywall is the most paint-efficient surface on the table, and every rougher surface below it needs proportionally more paint for the same area. Stucco, at the bottom of the table, needs more than double the paint per square foot that smooth drywall does, since its texture creates far more actual surface area than its flat footprint suggests. Porosity, not just visible texture, drives this figure: an unsealed masonry surface can absorb paint into the material itself well beyond what a simple texture comparison would predict.
Decide how many coats to apply
Coat count is set by the contrast between the old and new colour and by whether the surface is bare or previously painted, not by a fixed rule that applies to every job. Getting this number right before ordering avoids a second trip to the store mid-project.
A colour change, especially a dark colour going over a light one or the reverse, typically needs two coats regardless of paint quality, since a single coat rarely achieves full, even opacity across a colour shift that large. Bare drywall needs a coat of primer before colour paint goes on at all, since unprimed drywall absorbs paint unevenly and can telegraph the seams and patched areas beneath it. A colour applied over a very similar existing colour, with the surface already primed from a previous paint job, can sometimes get by with a single coat, though two remains the standard recommendation for a durable, even finish.
Buy paint by can size
Can size affects both cost per gallon and batch consistency, since larger cans are mixed in bigger batches and a job spanning multiple smaller cans risks a visible seam where one batch ends and the next begins. Matching can size to job size avoids both waste and a colour mismatch.
Quarts suit small touch-up jobs and trim work where a full gallon would go mostly unused. Gallons are the standard unit for room-sized jobs and are what the coverage figures on this page are built around. A five-gallon bucket is more economical per gallon for a whole-house repaint and reduces the number of times paint has to be mixed, but every can used on a single continuous surface should be boxed, poured together and stirred into one batch, before application, since even the same product code can vary slightly in tint between manufacturing batches.
Frequently asked questions
How much paint do I need for a 12x14 room?
About 1.72 gallons for two coats on smooth drywall, with an 8 ft ceiling, two doors and two windows deducted from the wall area. Round up to two gallons or a gallon plus a quart, since paint cannot be purchased in a fractional gallon amount.
How many gallons of paint do I need per square foot?
On smooth drywall, one gallon covers about 400 ft² per coat, or 0.0025 gallons per square foot per coat. Rougher or more porous surfaces need more paint for the same area; stucco, for example, covers only about 175 ft² per gallon.
Do I need to subtract doors and windows from the paint area?
Yes. A standard door is deducted at 21 ft² and a standard window at 15 ft², since neither surface receives wall paint. Skipping this deduction overstates the paint needed, sometimes by more than a full gallon on a room with several openings.
How many coats of paint should I apply?
Two coats is standard for most jobs, and effectively required for any significant colour change, particularly a dark colour over a light one. A single coat can be enough when the new colour closely matches an already-primed, similarly coloured surface, but two remains the safer default for even coverage.
What is the difference in coverage between smooth and textured surfaces?
Smooth drywall covers about 400 ft² per gallon, while a textured wall covers about 350 ft², a rougher surface such as wood siding about 275 ft², brick about 200 ft², and stucco about 175 ft². The more texture or porosity a surface has, the more paint it absorbs per square foot of visible area.
How much paint do I need for a ceiling?
Use the ceiling's length times width as the area, with no door or window deduction since ceilings do not have openings, then apply the same coats-divided-by-coverage formula as a wall. A 12 ft by 14 ft ceiling is 168 ft², needing 168 × 2 ÷ 400 = 0.84 gallons for two coats on smooth drywall.
Should I buy paint in gallons or a five-gallon bucket?
A five-gallon bucket is more economical per gallon and reduces the number of times paint is mixed, worthwhile once a job needs three gallons or more. Smaller jobs are better served by individual gallons or quarts, since a five-gallon bucket left partly full is harder to store and more prone to skinning over between uses.
What is boxing paint, and why does it matter?
Boxing paint means combining multiple cans of the same colour into one larger container and stirring them together before application. It matters because paint tint can vary slightly between manufacturing batches, and boxing evens out that variation across a whole job instead of leaving a visible seam where one can ends and the next begins on a continuous wall or ceiling.
Does primer count as one of the coats?
No. Primer is a separate product and a separate coat from the colour coats, applied first to seal bare drywall, block stains, or help a dramatic colour change cover more predictably. The gallons calculated for colour paint on this page do not include primer; a bare-drywall job needs a primer coat estimated the same way, using the primer's own coverage rate.
How much paint do I need for trim and baseboard?
Measure trim by its linear length rather than by area in the same way as a wall, since trim is a long, narrow strip: multiply the total linear footage of baseboard, crown moulding or casing by its height or width to get an area, then apply the same coats-and-coverage formula. Trim paint is usually a different sheen than wall paint and should be calculated and purchased separately.
Summary
The Paint Calculator turns wall, ceiling and trim dimensions into gallons, deducting 21 ft² per door and 15 ft² per window before applying coats and a surface-specific coverage rate. Smooth drywall covers about 400 ft² per gallon; rougher and more porous surfaces such as stucco cover less than half that.
Two coats is the standard default, especially across a significant colour change, and cans from the same job should be boxed together to avoid a visible batch seam. Round the calculated gallons up to the nearest can size available, since paint is not sold in fractional units.