Paint Calculator Guide: How Many Gallons by Room Size
The painter walks in, looks at the room, and says "two gallons." How do they know? It is not magic - it is one number (350 sq ft per gallon) and one rule (always two coats). Here is how to estimate paint for any room size, plus the quick chart most painters keep in their head.
How many gallons of paint for a room?
The shortcut: one gallon covers about 175 sq ft with two coats on smooth drywall. That is 350 sq ft of single-coat coverage, applied twice. Most interior latex paint specs quote 350-400 sq ft per gallon per coat - same answer, different framing.
By room size, expect:
- Small bedroom (10x10 ft, 8 ft ceilings): 1.5 gallons walls only, 2 gallons with ceiling
- Standard bedroom (12x10): 2 gallons walls, 2.5 with ceiling
- Large bedroom or guest room (14x12): 2.5 gallons walls, 3 with ceiling
- Living room (18x14, 9 ft ceilings): 3-4 gallons
- Open kitchen/living combo: 4-5 gallons
- Whole 3-bedroom house interior (walls only): 8-12 gallons
- Whole house including ceilings and trim: 12-18 gallons
These ranges assume two coats and standard 350 sq ft per gallon coverage. Bump the high end by 20-30% for textured walls, light-over-dark color changes, or fresh drywall that has not been primed.
For an exact number, use the paint calculator with your specific room dimensions and door/window count.
Why do I always need two coats?
One coat works only when you are refreshing the exact same color over a clean, primed, recently painted surface. Every other situation needs two coats:
- Color change: the old color shows through one thin coat
- Light over dark: needs 2-3 coats plus a tinted primer
- Dark over light: 2 coats; very saturated colors may need 3
- New drywall: primer + 2 finish coats - never paint raw drywall
- Repainting in similar color: 2 coats give a uniform sheen and hide imperfections
If you are wondering what happens when you only do one coat: brush strokes show, the color reads slightly different across the wall, and the paint film is thin enough that it can be damaged by cleaning. Two coats is the standard for a reason.
How do doors and windows change the calculation?
Subtract them from the wall area. A standard interior door is 3 ft x 7 ft = 21 sq ft. An average residential window is 3 ft x 5 ft = 15 sq ft. In a 12x10 bedroom with one door and two windows, that is 21 + 30 = 51 sq ft removed from the wall area - meaningful enough to drop you from 2.5 gallons to 2 gallons.
Bathrooms and kitchens have proportionally more openings (vanities, mirrors, cabinets, appliances) and small wall areas, so they often only need 1-2 quarts rather than full gallons. Buy quarts for accent walls and small bathrooms - you save 30-40% vs gallons.
Should I include the ceiling in my calculation?
If you are painting it, yes - and remember most ceilings are flat white in a different paint type (ceiling paint is thicker and flatter than wall paint to hide flaws and avoid drips). A 12x10 ceiling is 120 sq ft, which adds about 0.7 gallons in two coats of ceiling paint. Most rooms need 1 gallon of ceiling paint regardless of size because the minimum purchase is 1 gallon.
Trim is usually painted with a different finish too (semi-gloss or satin instead of flat or eggshell). One quart of trim paint typically covers the door, window casings and baseboards of an entire bedroom.
How much primer do I need?
Use primer separately when painting raw drywall, going light over dark, or covering stains. Modern "paint and primer in one" products work for refreshing existing paint in similar colors - but they do not replace dedicated primer for new construction.
Primer coverage is similar to paint - about 200-300 sq ft per gallon (slightly less than finish paint because of higher solids). For a whole 3-bedroom house with new drywall, plan on 4-6 gallons of primer plus the 12-18 gallons of finish paint.
What if the walls are textured?
Textured walls (popcorn ceilings, orange peel, knockdown) absorb 20-40% more paint because the textured surface has more area than its measurement suggests. For these, bump your calculation up by 25%. A room that calls for 2 gallons in the calculator may need 2.5 gallons in practice on textured walls. Buy the extra paint up front - matching the exact color from the same dye lot later is impossible.
For other interior projects like wallpaper, the wallpaper calculator handles pattern repeat and waste in a similar way - useful when an accent wall makes more sense than another can of paint.