Paint Calculator
Find out how many gallons of paint you need to buy for a room.
Room dimensions
Results
For a 12 ft × 10 ft room with 8 ft walls, 1 door, and 2 windows, painted with 2 coats, you'll need about 2 gallons of paint.
Paint needed
2 gallons
That's a typical amount for a standard bedroom, living room, or similarly sized space.
Buy it all in one trip rather than going back for more later — cans from the same production batch match color exactly, while a second trip risks a subtle shade difference on the wall.
Wall area: 304 sq ft
Raw computed amount: 1.74 gallons
How this works
Wall area is calculated as the room's perimeter times wall height, minus a standard opening size for each door and window you enter. That area is multiplied by your number of coats, then divided by the coverage rate to get raw gallons needed — rounded up to the nearest whole gallon, since paint is sold in whole cans. Standard opening sizes (about 17 sq ft per door, 15.5 sq ft per window) are used instead of asking you to measure each opening individually, which keeps the tool fast to use at the cost of some precision — if your room has an unusually large picture window or a double-wide door, the calculator will slightly overestimate the paint needed, which is the safer direction to be wrong in. For irregular rooms (L-shaped, alcoves, or rooms with a chimney breast), the most accurate approach is to measure each wall segment separately and sum the areas, rather than using a single length × width estimate — this calculator's single-rectangle model is a simplification that works well for typical rectangular rooms and slightly overestimates for rooms with jogs or offsets (since it can't account for area removed by an alcove without you explicitly excluding it). The unit toggle above switches every input and result between imperial (feet, gallons) and metric (metres, litres) — internally, every calculation always runs in the same canonical units regardless of which one you're viewing, so switching back and forth never introduces rounding drift into your entered values. Coverage rate default (350 sq ft/gallon): This Old House, citing Lowe's.
Worked example
A 12 ft × 10 ft bedroom with 8 ft ceilings, one door, and two windows, painted with two coats at the default 350 sq ft/gallon coverage rate: wall area works out to about 304 sq ft (352 sq ft of gross wall, minus roughly 48 sq ft for the door and windows). At two coats, that's about 1.7 gallons of raw paint needed — rounded up to 2 gallons, since paint is sold in whole cans.
A second example: a larger open-plan room
A 20 ft × 15 ft open-plan living area with 9 ft ceilings, two doors, and four windows, painted with two coats at the same 350 sq ft/gallon default: gross wall area is 2 × (20 + 15) × 9 = 630 sq ft. Subtracting two doors (2 × 17 = 34 sq ft) and four windows (4 × 15.5 = 62 sq ft) leaves about 534 sq ft of paintable wall. At two coats, that's 1,068 ÷ 350 ≈ 3.05 gallons — rounded up to 4 gallons. Notice the jump from 2 to 4 gallons for a room only moderately larger: purchasable-unit rounding means it's often worth remeasuring precisely before a large job, since crossing a whole-can threshold changes the total meaningfully.
Common mistakes
Forgetting the second coat
Forgetting that a second coat roughly doubles the paint needed — not just a small top-up. Most interior color changes need two full coats for even coverage, and this calculator defaults to two for that reason. If you're painting over a similar color, one coat may be enough; set coats to 1 and the estimate updates immediately.
Assuming all paint covers the same area
Coverage rate varies by paint line, sheen, and color — a deep, saturated color often needs an extra coat that a flat white wouldn't, regardless of what the can's coverage figure says, because pigment concentration affects hide. If you're making a dramatic color change (light to dark, or the reverse), budget for a third coat even if the calculator's default assumes two.
Skipping a test patch on unusual surfaces
Paint-and-primer-in-one products and specialty finishes (eggshell, satin, high-gloss) can behave differently on porous patched drywall than on the smooth section around it, sometimes showing a visible sheen difference ("flashing") over repairs. A small test patch before committing the full wall catches this before it's a whole-room problem.
Buying exact-to-the-drop instead of a small buffer
Roller absorption, edging with a brush, and inevitable spills mean the raw calculated amount is a floor, not a ceiling. This calculator already rounds up to the next whole can, which builds in a small buffer — but for rooms with heavy trim work or multiple color-cutting edges, consider sizing up one more can rather than risking a mid-project store run with wet paint on the wall.
Not stirring the paint thoroughly (or often enough)
Pigment and additives settle over time, even in a freshly opened can. A quick shake at the store isn't a substitute for a full stir with a stir stick before you start, and for a multi-can job, boxing the paint — combining all cans into one larger container and mixing — prevents subtle color variation between individual cans of the same product line.
Painting in the wrong temperature or humidity
Most latex paints specify an application range on the can, typically 50–85°F with moderate humidity. Painting outside that range — a cold garage in winter, a humid bathroom mid-shower-season — slows drying, can affect how the finish cures, and increases the odds of visible lap marks between sections painted at different times.
When this calculator's model stops applying
- Vaulted or sloped ceilings: this tool assumes a flat wall height entered once. For a room with a cathedral ceiling on one wall, measure that wall's average height and expect the estimate to run slightly low — the peak area needs proportionally more paint than a flat-ceiling room of the same footprint.
- Heavily textured surfaces (stucco, popcorn ceilings, rough brick): coverage can drop 20–35% below the smooth-surface default. Always enter a manually reduced coverage rate for these surfaces rather than trusting the default.
- Very large open-concept spaces (over roughly 500 sq ft of continuous wall): at this scale, small per-wall measurement errors compound. Measure and sum individual wall segments rather than one length/width estimate for the whole space, and treat this calculator's single-room model as a starting point, not a final order quantity.
- Exterior walls: different coverage rates, different dry-time constraints, and siding-specific prep (see the FAQ above) mean this interior-focused model should be treated as directional only for exterior work.
- Multi-room or whole-house projects bought as one combined order: buying all cans from the same production batch matters more at this scale, since a subtle color shift between batches is far more visible across an open floor plan than within a single small room — ask your retailer to confirm matching batch/lot numbers on a large order.
- Rooms with mixed sheens or colors on different walls (an accent wall, for example): calculate the accent wall and the remaining walls as two separate areas with their own coat counts, since an accent color often needs a different number of coats than the surrounding walls to achieve full, even coverage.
FAQ
Does this calculate primer separately?
No — this calculator estimates topcoat paint only. Primer typically covers less area per gallon (roughly 200–300 sq ft) than finish paint, so budget a separate can if your walls need priming. Use the linked Primer Calculator for that estimate.
Why does the calculator subtract doors and windows?
Doors and windows aren't painted, so their area doesn't need paint. This calculator subtracts a standard-size opening for each door and window you enter, which is more accurate than estimating from wall perimeter alone. A standard interior door is treated as about 17 sq ft (0.8m × 2.0m) and a standard window as about 15.5 sq ft (1.2m × 1.2m).
What if my walls are textured or unprimed?
Textured, porous, or unprimed surfaces absorb more paint and cover less area per gallon than the 350 sq ft/gallon default. Use the coverage rate field to enter a lower figure (check your paint can's label, which usually states the manufacturer's own coverage estimate for that specific product).
Should I round up or down if the result is right between two amounts?
Always round up. This calculator already rounds every result to a whole purchasable can, but if you're adjusting the coverage rate or comparing your own estimate against ours, treat any fractional result as the next whole can up — running short mid-wall costs more time (waiting for a store trip) than a partial extra can costs in money.
Does the number of coats affect anything besides the paint amount?
Yes — drying time between coats. Most latex paints need 2–4 hours minimum before a second coat, and longer in humid conditions. If you're painting a room in one day, factor coat count into your schedule, not just your shopping list.
Why is my result different from the paint store's estimate?
In-store estimators often use a flat 350–400 sq ft/gallon assumption regardless of your specific paint product, surface condition, or number of coats — this calculator lets you adjust the coverage rate to match what's actually printed on your chosen paint's label, which is the more accurate figure when it's available.
Can I use this for exterior walls?
The wall-area math works the same way, but exterior coverage rates, weather-driven dry times, and surface prep (siding type, existing paint condition) differ enough from interior painting that we'd recommend treating this as a starting estimate only — see the edge-case guidance below.
Which sheen should I choose, and does it change the amount I need?
Sheen doesn't meaningfully change the volume you need, but it does affect where you'd use it: flat/matte hides wall imperfections best and suits low-traffic bedrooms and ceilings; eggshell and satin are the common living-area choice, wiping clean without much sheen glare; semi-gloss and gloss are more washable and standard for kitchens, bathrooms, trim, and doors because they resist moisture and repeated cleaning better than flat finishes.
How do I estimate paint for a whole house rather than one room?
Run this calculator once per room and add the results — don't try to sum total square footage across rooms and divide once, because door/window counts and coverage rates often differ room to room (a kitchen might use semi-gloss at a different coverage rate than a bedroom's flat paint), and per-room whole-can rounding means summing individual results is more accurate than rounding once at the end.
Can I switch between imperial and metric partway through?
Yes — the unit toggle at the top of the calculator converts every field you've already entered, so you won't lose your measurements. Internally the tool always computes from the same canonical values regardless of which unit is currently displayed, so switching back and forth doesn't introduce any rounding error into your result.
Do ceilings need a separate calculation?
Yes — this calculator estimates wall paint only, not ceilings. Ceiling paint is usually a different, flatter-sheen product (and often a different color, typically white), so run a separate estimate: ceiling area is simply room length times width, with no openings to subtract, divided by the same coverage-rate logic at whatever coat count you're planning.
How much extra should I buy for touch-ups later?
Beyond the whole-can rounding this calculator already applies, it's common practice to keep a small labeled leftover amount (a quart or partial can, clearly marked with the room and date) for future touch-ups, since matching a discontinued color or faded batch years later can be difficult or impossible. This is a storage decision, not something to add to your initial purchase quantity.
Related tools
Primer Calculatorif your walls are bare drywall, patched, stained, or you're making a big color change, work out how much primer you need before buying paint.More painting tools (paint cost estimator) are planned but not yet published.