Room Painting Calculator
Estimating how much paint you need before heading to the hardware store prevents both frustrating mid-project shortages and the waste of returning half-used gallons. The calculation starts with the paintable wall area: the total perimeter of the room multiplied by the ceiling height, minus the area occupied by doors and windows. Standard interior doors subtract roughly 21 square feet each, and a typical window subtracts about 15 square feet. Most interior walls need two coats for full coverage, so the net wall area is multiplied by the number of coats before dividing by the paint's coverage rate. Standard interior paint covers approximately 350 square feet per gallon on a smooth surface. Ceiling paint is simpler: just the floor area multiplied by the number of coats. Trim paint is sold by the quart and covers baseboards, door frames, and window casings. This calculator handles all three surfaces in one pass. Enter your room dimensions, check which surfaces you are painting, enter the number of doors and windows, and pick your coat count.
Wall paint: -- gal | Ceiling paint: -- gal | Trim: -- qt
Paint coverage guide
| Surface | Coverage per gallon | Typical finish |
|---|---|---|
| Interior walls | 350 sq ft | Eggshell or satin |
| Ceiling | 350 sq ft | Flat or matte |
| Trim (per quart) | ~100 linear ft | Semi-gloss or gloss |
| Primer | 200 to 300 sq ft | Flat (apply before finish coats) |
Tips for accurate paint estimation
Paint coverage rates assume smooth, properly primed surfaces. Textured walls (orange peel, knockdown, or heavy texture) absorb more paint, reducing effective coverage by 10% to 20%. Round up to the nearest gallon to avoid running short mid-coat. Buying one extra quart as a touch-up reserve is good practice for most rooms.
Room painting: frequently asked questions
How much paint do I need for a 12x12 room?
A 12 x 12 room with a 9-foot ceiling has a wall area of about 378 square feet (perimeter of 48 feet times 9-foot height). Deducting a standard door (21 sq ft) and two windows (30 sq ft) leaves about 327 square feet of paintable wall area. At 350 sq ft coverage per gallon, one gallon covers one coat. For two coats, you need 2 gallons of wall paint, plus about 1 gallon for the ceiling (144 sq ft divided by 350).
How many coats of paint do I need?
Most painting projects use two coats for even, opaque coverage. A single coat may be acceptable when repainting with the same or a similar colour, or when using a premium paint-and-primer product. If you are making a dramatic colour change (dark to light or vice versa), three coats may be needed. New drywall almost always requires a primer coat plus two finish coats for best results.
How do I calculate paint for ceilings?
Ceiling paint coverage is simply the length of the room multiplied by the width. A 12 x 12 room has a 144 sq ft ceiling. At 350 sq ft per gallon per coat, one gallon covers the ceiling in a single coat, and you will need about 0.82 gallons for two coats (round up to one gallon). Flat or matte finish paints are typically used on ceilings to hide imperfections and reduce glare.
Does primer count as a coat?
A primer is not a substitute for finish paint coats. Primer seals the surface and provides adhesion for the finish coat, but it does not provide the colour coverage or washability of finish paint. If you prime new drywall or a dramatic colour change, you still need the full number of finish coats on top. This calculator is designed for finish paint only; add primer separately if your project requires it.
What is the coverage rate for standard interior paint?
Most manufacturers rate standard interior paint at 350 to 400 square feet per gallon on a smooth surface. This calculator uses 350 sq ft per gallon, which is a conservative estimate that accounts for some texture and roller waste. Premium paints and speciality finishes may have different coverage; always check the product label for the manufacturer's stated coverage rate.
References
- Painting and Decorating Contractors of America (PDCA): industry coverage rate standards
- US Environmental Protection Agency (EPA): low-VOC and zero-VOC paint guidance
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Coverage rates are estimates; always check the product label. See our methodology.