Paint Calculator
Enter your room's dimensions. We'll tell you the paintable area and the exact amount of paint to buy.
Advanced options
- Wall area
- 0 sq ft
- Ceiling area
- 0 sq ft
- Total paintable
- 0 sq ft
- With waste factor
- 0 sq ft
- One-coat amount
- 0
- Estimated cost
- —
How the paint calculator works
A 12 × 12-ft room with 8-ft ceilings needs 4 gallons of paint for two coats including the ceiling. That room has 478 sq ft of paintable surface — 334 sq ft of wall (384 sq ft gross, minus 20 sq ft for one door and 30 sq ft for two windows) plus a 144 sq ft ceiling — which becomes 526 sq ft once you add a 10% waste factor. At 350 sq ft per gallon that is 1.5 gallons per coat, or just over 3 gallons for two coats, and paint is sold in whole gallons.
The formula behind that: for a rectangular room, the paintable wall area is 2 × (length + width) × height. We subtract a standard area per door and per window. If you include the ceiling, we add length × width.
That gives you the one-coat paintable area. Multiply by the number of coats, divide by coverage per container, and add a 10% waste factor for spills, roller loading, and touch-ups. We round up to whole containers because paint stores don't sell fractions.
Coverage is a property of the wall, not the paint
The "350–400 sq ft per gallon" printed on a can is a laboratory number, measured on a sealed, smooth, primed panel at the manufacturer's specified wet film thickness — typically 4 mils wet, which dries to about 1.5 mils. Every departure from that panel costs you coverage. Texture increases the actual surface area you are coating; porosity means part of your first coat soaks in instead of sitting on top. Both effects are large, and both are why people run short.
The Advanced options let you override the coverage figure. Use this table to pick a realistic number before you trust the gallon count:
| Surface | Coverage per gallon | Metric | Why it differs |
|---|---|---|---|
| Previously painted, smooth drywall | 350–400 sq ft | 8.5–10 m²/L | The reference condition — sealed and flat |
| New drywall, primed with PVA | 350 sq ft | 8.5 m²/L | Primer does the sealing so the topcoat behaves normally |
| New drywall, unprimed | 200–250 sq ft | 5–6 m²/L | Bare paper and joint compound absorb at different rates |
| Light orange-peel / knockdown texture | 300–325 sq ft | 7.5–8 m²/L | Roughly 10–15% more real surface area |
| Heavy knockdown or skip-trowel | 250–275 sq ft | 6–6.5 m²/L | The roller has to push paint into every valley |
| Popcorn / acoustic ceiling | 150–200 sq ft | 3.5–5 m²/L | Enormous surface area, and the texture is highly absorbent |
| Smooth wood, primed | 350–400 sq ft | 8.5–10 m²/L | Behaves like sealed drywall |
| Bare or weathered wood siding | 200–250 sq ft | 5–6 m²/L | End grain and checking pull paint in |
| Smooth or lightly sanded concrete/block filler | 150–200 sq ft | 3.5–5 m²/L | Open pores; a block filler coat is usually needed first |
| Rough exterior stucco or split-face block | 100–150 sq ft | 2.5–3.5 m²/L | The worst case — brush the first coat into the profile |
Two practical consequences. First, the first coat on a porous surface always covers less than later coats, sometimes by 30–40%, because the substrate is still drinking. If you are painting bare drywall without primer, budget the first coat at the low end of the range and the second at the high end. Second, deep-tinted colours in a bright red, orange, or yellow family are formulated on a clear or deep base with very little white pigment, so they hide poorly and often need three coats regardless of coverage — a tinted grey primer is the standard fix.
Do you actually need primer?
Primer is not a cheaper paint, it is a different product with a different job: it seals porosity, it blocks stains, and it creates adhesion on slick surfaces. Modern "paint and primer in one" products are self-priming topcoats — genuinely fine over a sound, previously painted wall in a similar colour, and no substitute for a dedicated primer in the situations below.
| Situation | Primer? | What to use |
|---|---|---|
| Repainting sound walls, similar colour | No | Self-priming topcoat, spot-prime patches only |
| New drywall | Yes | PVA drywall primer — cheap, and it equalises paper vs. joint compound |
| Skim-coated or patched walls | Yes, at least spot | PVA or a high-build primer, or the patches flash through as dull spots |
| Water stains, smoke, nicotine, marker | Yes | Shellac-based stain-blocker — latex primer will let it bleed back |
| Bare wood, tannin-rich (cedar, redwood, knotty pine) | Yes | Shellac or an oil-based stain-blocking primer |
| Glossy oil-based paint or varnish | Yes | Bonding primer after a scuff-sand; do not skip the sanding |
| Very dark to very light colour change | Yes | White primer, then two topcoats — cheaper than three topcoats |
| Bright red, yellow, or orange topcoat | Yes | Primer tinted to a mid-grey; it is the standard trade fix for poor hide |
| Bare masonry, block, or stucco | Yes | Masonry block filler or a masonry-rated primer |
Primer covers at roughly the same rate as paint, so size it the same way: run the calculator with coats set to 1 for the primer order, then again with your real coat count for the finish paint.
Picking a sheen, room by room
Sheen is a durability decision disguised as an aesthetic one. Higher sheen means more resin and less pigment volume, which makes the film harder, less permeable, and easier to scrub — and also far more willing to show every ripple, nail pop, and roller mark in the wall behind it. The calculator suggests a finish when you pick a preset; this is the reasoning behind it.
| Sheen | Typical gloss (60°) | Where it belongs | Trade-off |
|---|---|---|---|
| Flat / matte | 0–5 | Ceilings, low-traffic adult bedrooms, textured walls you want to disappear | Best hide of imperfections; burnishes if you scrub it |
| Matte "scrubbable" | 2–7 | Living and dining rooms in a modern look | Costs more; still not for a hallway with kids and bikes |
| Eggshell | 10–25 | Bedrooms, living rooms, most of the house | The default compromise — washable enough, forgiving enough |
| Satin | 25–35 | Kitchens, kids' rooms, hallways, laundry | Wipes clean; starts to reveal wall flatness under raking light |
| Semi-gloss | 35–70 | Bathrooms, trim, doors, cabinets, utility rooms | Moisture and scrub resistant; shows every prep shortcut |
| Gloss | 70+ | Accent doors, furniture, exterior trim details | Hardest film available; requires near-perfect substrate |
One caveat about bathrooms: semi-gloss is the traditional answer, but a dedicated bathroom or kitchen paint with a mildewcide in an eggshell or satin sheen is now generally a better choice for the walls, with semi-gloss reserved for the trim. The mildew resistance comes from the additive package, not the shine. In a room with an operating exhaust fan, walls in satin will outperform semi-gloss walls in a room without one.
When to bump the waste factor
Bump to 15% if the surface is textured (stucco, popcorn ceiling) or porous (new drywall, unprimed wood). Bump to 20% if you're spraying instead of rolling — an airless sprayer puts a meaningful fraction of the material into the air and onto the drop cloths, and back-rolling behind the gun consumes more still. Keep 10% for previously painted interior walls, and drop to 5% only if you are painting one flat wall with a roller and no cut-in.
Worked example: a 16 × 20-ft living room with 9-ft ceilings
The wall perimeter is 2 × (16 + 20) = 72 ft, so gross wall area is 72 × 9 = 648 sq ft. Subtract two doors at 20 sq ft each and three windows at 15 sq ft each — 85 sq ft of openings — and the paintable wall is 563 sq ft. Add the 16 × 20 = 320 sq ft ceiling and the total paintable area is 883 sq ft. A 10% waste factor makes that 971 sq ft. At 350 sq ft per gallon, one coat is 2.78 gallons, and two coats is 5.55 — rounded up, 6 gallons.
In practice you would buy this as two products, not one: a 5-gallon pail of wall colour plus a gallon of ceiling flat, because 320 sq ft of ceiling at two coats is about 2 gallons on its own and ceiling paint is a different (dead-flat, high-hide, slow-drying) formulation. Now change one input — assume the walls carry a knockdown texture, so coverage drops to 275 sq ft/gal and the waste factor goes to 15%. The same room becomes 883 × 1.15 = 1,015 sq ft, one coat is 3.69 gallons, and two coats is 7.4 → 8 gallons. Texture alone added a third to the order.
Worked example: a 12 × 15-ft kitchen, walls only
Kitchens usually get walls only, because the ceiling is broken up by cabinets, soffits, and lighting. Perimeter is 2 × (12 + 15) = 54 ft; at an 8-ft ceiling that is 432 sq ft gross. Two doors and two windows remove 40 + 30 = 70 sq ft, leaving 362 sq ft. With the ceiling unchecked there is nothing to add, so with 10% waste that is 398 sq ft, one coat is 1.14 gallons, two coats is 2.28, and you buy 3 gallons of satin.
The third gallon is not waste. Wall cabinets typically leave 18 inches of backsplash and a few narrow returns that eat paint out of proportion to their area because every one is a cut-in, and you want a sealed quart left over for the inevitable grease-cleaner rub-through above the range.
Sequencing: the order pros work in
Ceiling first, then walls, then trim. The reason is entirely practical — ceiling roller spatter lands on the walls, and wall cut-in is easier to correct than trim. Within each surface, cut in the edges of a wall and roll it while the cut-in is still wet, wall by wall. If you cut in the whole room and then come back to roll, the cut-in band dries at a different film thickness than the rolled field and you get a visible "picture frame" or hat-banding effect, especially in flat and matte sheens under raking light.
Other things pros do differently: they box their paint — combining all the gallons of one colour into a single 5-gallon pail and stirring — so that no can-to-can tint variation shows up as a shade change halfway across a wall. They maintain a wet edge and always roll back into the section they just finished. They roll floor-to-ceiling in overlapping W or vertical passes rather than in patches. And they do the prep: filling, sanding, caulking, and vacuuming take longer than the painting and are the entire difference between a job that looks good and one that looks painted.
Common failure modes
- Flashing — dull patches where a repair, patch, or unprimed spot absorbed more than the surrounding wall. Fix by priming the whole wall, not by adding a third coat.
- Lap marks — visible overlap lines from rolling into a section that had already started to skin. Work faster, in smaller sections, and out of direct sun or a running heater.
- Roller stipple mismatch — using a ½-inch nap on the field and a brush at the edges produces different textures. Match the nap to the surface (¼–⅜ inch on smooth, ½ inch on light texture, ¾ inch on heavy texture and masonry) and cut in with a roller where you can.
- Blocking — freshly painted doors or windows sticking to their jambs. Latex reaches full cure in 14–30 days, not the 4 hours on the label; leave doors ajar for a few days.
- Peeling in a bathroom — almost always a moisture problem, not a paint problem. Fix the fan before you repaint.
When to call a pro
Three situations reliably justify hiring out: ceilings above about 12 feet or over a stairwell, where the staging is the real job and a fall is the real risk; any surface in a home built before 1978 that you would need to sand, where lead-safe work practices and containment apply; and lacquer or conversion-varnish cabinet finishing, which needs spray equipment and ventilation you do not own. Extensive plaster repair is a fourth — patching plaster to an invisible finish is a separate trade from painting.
Common room sizes
Already know your room is a standard size? These pages give the answer, the arithmetic, and the one-coat and 9-ft-ceiling variants:
- How much paint for a 12×12 room — 3 gallons walls-only, 4 with the ceiling.
- How much paint for a 10×10 room — 2 gallons walls-only, 3 with the ceiling.
- How much paint for an average bedroom — gallon counts for every size from 10×10 to 14×16.
Frequently asked questions
Do I need primer?
If you're going lighter over darker, painting new drywall, or covering stains — yes. Primer is roughly the same coverage as paint, so run the calculator once with coats set to 1 to size the primer order. Over a sound, previously painted wall in a similar colour, a self-priming topcoat is genuinely enough; over bare wood, bare masonry, glossy oil paint, or any stain, a dedicated primer is not optional.
Why round up to whole containers?
Paint stores sell by the gallon (or quart for trim, by the liter in metric). Running out mid-project means a second trip, and a freshly-mixed container of the same colour can shift slightly in tint — which matters on a wall. Buying one extra is the cheap safety margin, and a sealed leftover quart is the only realistic way to touch up in two years.
What about trim?
This calculator focuses on wall and ceiling coverage. For baseboards, door casings, window casings, and door faces, use the trim paint calculator — trim paint is a different product (usually semi-gloss or a waterborne enamel) and is sized in quarts rather than gallons, because linear trim uses far less material than its prominence suggests.
Does one gallon really cover 400 square feet?
Only on the surface the manufacturer tested: a smooth, sealed, primed panel at the specified wet film thickness. On a light orange-peel texture expect 300–325 sq ft; on heavy knockdown 250–275; on a popcorn ceiling 150–200; on rough stucco as little as 100. The number on the can is a ceiling, not an average, which is why the calculator lets you override it in Advanced options.
Should I buy 5-gallon pails or individual gallons?
Buy a pail once you need four or more gallons of the same colour — it is usually cheaper per gallon, and more importantly it removes can-to-can tint variation entirely. Below four gallons, individual cans are more flexible and much easier to handle; a full 5-gallon pail weighs around 55 lb. If you do buy separate cans of one colour, box them: pour them all into one bucket and stir before you start.
How long should I wait between coats?
Follow the label, but the general rule for interior latex is 4 hours minimum at 70°F and 50% relative humidity, longer when it is cold or humid. Recoating too early lifts the first coat and leaves streaks. Dry-to-touch is not recoat-ready, and neither is recoat-ready the same as cured — full cure takes two to four weeks, which is when the film reaches its final hardness and washability.
Can I paint over wallpaper?
It is a last resort, not a shortcut. It only has a chance if the paper is fully adhered with no lifting seams, is not vinyl-faced, and gets an oil-based or shellac primer first — water-based primer will re-wet the old paste and cause bubbling. Even done correctly the seams telegraph through. Removing the paper is more work now and far less work than fixing a failed paint job later.
How much paint does the ceiling take on its own?
Ceiling area is simply length × width, so a 12 × 12 room is 144 sq ft — under half a gallon per coat on a smooth ceiling. Buy a gallon anyway, in a dedicated dead-flat ceiling paint: it is formulated to dry slowly so you can keep a wet edge overhead, and to hide better in one or two coats. A textured or popcorn ceiling is a different problem, running 150–200 sq ft per gallon, so budget roughly double.
Planning the full project?
These calculators usually get run in the same afternoon.