Paint Calculator — How Many Gallons You Need
A 12 × 14 ft room with an 8 ft ceiling has 372 ft² of paintable wall after deducting one door and two windows — two coats at 350 ft² per gallon comes to 2.13 gallons. This paint coverage calculator handles surface texture, primer, ceilings and trim, and solves for the cheapest combination of can sizes actually sold. Coverage rates verified 2026-08-04.
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Prepared with MaterialMath — https://estimatormath.com. Quantities are estimates based on the dimensions entered and the published figures cited on the tool page (verified 2026-08-04). Verify against your own measurements and your supplier's actual pack sizes before ordering. Not professional engineering, construction or financial advice.
How is paint quantity calculated?
Paint estimating is one equation with one difficult input. The equation is trivial; the difficult input is coverage, because it depends on the surface rather than the paint, and it is where nearly every wrong answer comes from.
Note that the perimeter formula does not care about the shape of the room beyond its length and width, and that is fine — for a rectangular room it is exact. For an L-shaped room, measure each wall and add the lengths, then multiply the total by the ceiling height.
The last step, which most calculators skip, is turning a decimal number of gallons into cans you can actually buy. Paint comes in quarts, gallons and five-gallon buckets. This tool solves for the combination with the least surplus rather than rounding everything up to whole gallons — with one caveat covered below.
A worked example: a 12 × 14 ft bedroom
A bedroom 14 ft long and 12 ft wide with an 8 ft ceiling, one door, two windows, two coats on smooth previously painted walls. These figures are produced at build time by the same code the calculator runs.
Step by step
- Perimeter
- 2 × (14 + 12) = 52 ft
- Gross wall area
- 52 × 8 = 416 ft²
- Deduct 1 door + 2 windows
- 20 + (2 × 12) = 44 ft²
- Paintable wall
- 416 − 44 = 372 ft²
- Two coats
- 372 × 2 = 744 ft² of coverage
- At 350 ft²/gal
- 744 ÷ 350 = 2.13 gallons
- Buy
- 2 × 1 gallon
- Buy
- 1 × 1 quart
That last line is the interesting one. The arithmetic says 2.13 gallons. Rounding up gives three gallons and nearly a full gallon of surplus; two gallons plus a quart covers it with a few ounces spare and costs meaningfully less.
The catch — and this is the thing worth knowing that no coverage chart tells you — is that quart and gallon bases are different products. They use different tint formulas to hit the same colour, and on a large flat wall in raking light the difference can be visible where the two meet. On a hallway, take the saving. On a feature wall, buy the third gallon. The tool flags this whenever its answer includes a small can.
Who this calculator is for
- Homeowners painting a room who want to make one trip to the store rather than two.
- Painting contractors quoting by the room, where getting coverage right on textured or new-drywall jobs is the difference between margin and no margin.
- Landlords and property managers costing repaints across multiple identical units.
- Anyone painting over a dark colour — the colour change field is there because underestimating this is the most common way a paint job runs out of paint.
If you are painting new drywall, work out the board and compound first with the drywall calculator, since the finish level you choose there determines the coverage figure you should choose here.
What this calculator does not account for
- Spray application. Everything here assumes brush and roller. Airless spraying loses 20–40% to overspray and to what stays in the hose and pump — budget a third more paint if you are spraying, more outdoors in any wind.
- Trim, doors and casings. These are a different product in a different sheen and are not in the wall figure. As a rough guide, a quart cuts in the trim of an average room.
- Really porous or repaired surfaces. A wall with extensive patching has bare compound at every repair, and compound absorbs far more than the surrounding paint. Spot-prime the patches or budget an extra coat.
- Sheen changes. Going from gloss to flat, or flat to gloss, needs sanding or a bonding primer regardless of the coverage arithmetic.
- Temperature and humidity. Most latex paint needs above 50°F, and cold or damp conditions change how the paint lays off and how much you end up using.
Coverage by surface, for reference
| Surface | Per gallon | Per litre | Why |
|---|---|---|---|
| Smooth, previously painted | 350 ft² | 8.6 m² | Sealed surface, nothing absorbs |
| New drywall, primed | 300 ft² | 7.4 m² | Paper and compound still draw slightly |
| Light texture | 270 ft² | 6.6 m² | More actual surface than the plan area |
| Knockdown, stucco, popcorn | 200 ft² | 4.9 m² | Deep profile holds paint in every hollow |
| Rough masonry or block | 150 ft² | 3.7 m² | Open pores fill before any film builds |
Figures are per coat on a prepared surface, drawn from the 250–400 ft²/gal range published by Sherwin-Williams, Benjamin Moore and Behr. The band is wide because coverage is a property of the wall far more than of the paint.
Common questions
How many square feet does a gallon of paint cover?
350 square feet per coat is the working figure for a smooth, previously painted wall, and it is the default here. Manufacturers publish 250–400 ft²/gal, and where you land inside that band depends almost entirely on the surface rather than the paint.
New drywall drinks roughly 300 ft²/gal even after priming. Light texture takes you to 270, knockdown or popcorn to about 200, and rough masonry or split-face block down to 150 — a block wall can consume more than twice what the same square footage of smooth drywall does.
Should I subtract doors and windows from the wall area?
Only if the openings add up to more than about 100 ft². Below that, the deduction is usually smaller than the rounding up to the next whole can, so it changes the arithmetic without changing what you buy.
A standard 3'0" × 6'8" door is 20 ft² and a 3' × 4' window is 12 ft². A typical bedroom with one door and two windows loses 44 ft² — around 10% of the wall area, which is well inside a single gallon's coverage. In a room with a wall of glass it matters a great deal.
How many coats of paint do I need?
Two, in almost every case. One coat covers reliably only when you are repainting the same colour in the same sheen on a sound surface.
Three coats, or two over a tinted primer, when you are going from a dark colour to a light one — this is the single most common cause of a job running out of paint halfway. Red and deep yellow pigments are the worst offenders and can need four. If you are covering a colour change, tint the primer towards the topcoat and you will usually save a full coat.
Does this include the ceiling?
Only if you tick the box, and it is worth being deliberate about. Ceiling paint is a different product — flat, higher-hiding, formulated not to spatter — so most jobs need a separate can rather than more of the wall paint.
Tick "price ceiling separately" and the tool splits it into its own line so you can price the two products independently.
Why does the calculator sometimes suggest a quart alongside gallons?
Because it solves for the cheapest combination of the can sizes actually sold, rather than rounding everything up to whole gallons. If you need 2.1 gallons, two gallons plus a quart is genuinely less paint and less money than three gallons.
There is a real caveat and the tool flags it: quart and gallon bases tint from different formulas, so the colour can differ very slightly between them. On a feature wall in raking light, buy the extra gallon. On a hallway, take the quart.
How much paint do I need for a 12x12 room?
A 12 × 12 ft room with an 8 ft ceiling has 384 ft² of wall. Deduct one door and one window and you have 352 ft². At two coats and 350 ft²/gal that is 2.01 gallons — so two gallons, with essentially nothing spare.
Add the ceiling (144 ft², two coats, 0.82 gal) and you want a separate gallon of ceiling paint. That is the answer for a smooth, previously painted room; on new drywall the same room needs closer to 2.4 gallons plus primer.
How much paint do I need for a 2,000 sq ft house exterior?
Exterior walls run roughly 1.0–1.2 ft² of wall per ft² of floor area for a single-storey house, so a 2,000 ft² house has very roughly 2,000–2,400 ft² of exterior wall before deducting windows and doors. At one coat and 350 ft²/gal that is about 6–7 gallons; most exteriors need two coats, so budget 12–14. Measure your actual perimeter and wall height in the room tool above for an exact figure — exteriors vary far more than interiors because of gables, dormers and trim.
Do I need primer, and does it count as a coat?
Primer is not a coat of paint — it is a separate, cheaper product that seals the surface and helps colour hold true, and it is calculated on its own coverage rate here (about 300 ft²/gal). You need it on new drywall, bare wood, patched areas, and any major colour change. On a wall you are simply repainting the same colour, you can usually skip it.
Sources for the figures used
Every rate, coverage figure and code limit in this calculator comes from a published source. Where a figure is a convention rather than a standard, it says so.
- Sherwin-Williams product technical data sheets — Published spread rates of 250–400 ft² per gallon per coat (checked 2026-08-04)
- Benjamin Moore technical data — Coverage by substrate and sheen (checked 2026-08-04)
- Paint Quality Institute — surface preparation and coverage — Effect of substrate porosity and texture on spread rate (checked 2026-08-04)
- US door and window sizing conventions — 3'0" × 6'8" door (20 ft²) and 3' × 4' window (12 ft²) used as deduction defaults; both editable in the tool (checked 2026-08-04)