Paint Calculator
The paint question is really three questions: how much wall is actually there, how many times it gets covered, and how the result maps onto what paint stores sell. This calculator answers all three. It starts with the wall area from your room dimensions, subtracts the standard openings the trade uses (20 sq ft per door, 15 per window, 40 for a sliding or French pair), multiplies by the number of coats — two is the standard, and skipping it is why single-coat jobs look blotchy — and divides by Sherwin-Williams' published 350 sq ft per gallon. The result comes back twice: rounded up to whole gallons for simple buying, and as a gallon-plus-quarts mix that buys only what the job needs. Options cover ceilings (one coat), primer for bare drywall or stains, and the honest caveat that dark-over-light or heavily textured walls can drop real coverage to 250-300 sq ft per gallon.
The short answer
One gallon of paint covers about 350 sq ft per coat — a 12×10 room with an 8-foot ceiling and one door and two windows has 302 sq ft of wall, which at the standard two coats needs 604 sq ft, or about 1.7 gallons. Buy 2 gallons, or 1 gallon plus 3 quarts and skip the leftover.
2 is standard.
Coverage uses Sherwin-Williams’ published 350 sq ft per gallon per coat — dark-over-light or textured walls can drop that to 250-300, so the gallon figure is a floor, not a ceiling.
Paint to buy
1 gal + 3 qt
604 sq ft to paint at two-coat math — 1.73 gallons exact.
- Whole gallons instead
- 2 gal
- Net wall area
- 302 sq ft
Same coverage with margin for touch-ups; the gallon-and-quart mix above buys only what the job needs.
352 sq ft of wall minus standard openings (20 sq ft per door, 15 per window).
How to use Paint Calculator
- 1
Measure the room and wall height
Length, width and wall height in feet. Standard ceilings are 8 ft; 9-10 ft in older homes changes the answer materially.
- 2
Count the openings
Doors (20 sq ft each), windows (15 each) and sliding or French pairs (40 each). Default to 1 door and 2 windows for a typical bedroom-sized room.
- 3
Set coats and buy
Two coats is the trade standard — most color changes will look patchy without them. Buy the gallon-plus-quart mix, or whole gallons when you want touch-up margin.
Why use this tool
- Two-coat math by default — the single most skipped step in paint estimates
- Standard door, window and sliding-door deductions built in
- Sherwin-Williams' published 350 sq ft/gallon coverage, not a guess
- Gallon + quart buying mix alongside the simple whole-gallon figure
- Ceiling and primer options with their own coverage math
- Private — room dimensions never leave the browser
Frequently asked questions
- How much paint do I need for a 12x10 room?
- About 1.7 gallons at two coats: 352 sq ft of wall minus a door (20) and two windows (30) leaves 302 sq ft, doubled for two coats is 604, and at 350 sq ft per gallon that is 1.73 gallons. Buy 2 gallons, or 1 gallon plus 3 quarts.
- How many square feet does a gallon of paint cover?
- About 350-400 sq ft per coat on smooth interior walls — Sherwin-Williams publishes 350 as the planning figure. Dark paint over light, or heavily textured surfaces, can drop real coverage to 250-300, and two coats therefore eat a gallon for every 175 sq ft of wall.
- How much paint do doors and windows save?
- The trade-standard deductions are 20 sq ft per standard door, 15 per window and 40 for a sliding or French pair. On the 12×10 room those openings remove 50 sq ft — about a third of a gallon across two coats. Count them on paneled doors and trim you plan to paint separately; the deduction only applies if the opening stays unpainted.
- Do I really need two coats?
- For most color changes, yes: one coat leaves the old color ghosting through, and the second coat is what produces the even color the swatch promised. Two coats are already the default here. A same-color refresh or a paint-and-primer product can sometimes do one — but plan two and treat one as a bonus.
- Should I buy gallons or gallons plus quarts?
- Whole gallons when you want margin for touch-ups and future scuffs — paint stores advise slightly over rather than under. The gallon-plus-quart mix buys only what the job needs; a quart covers about 87 sq ft, which is usually the exact remainder of a room-sized job.
- When does the room need primer?
- On bare drywall or plaster, over stains or smoke damage, and under big color swings (light paint over dark, or saturated reds and blues). Primer gets its own coverage math — one coat over the net wall area — and most primers cover 250-350 sq ft per gallon depending on the product.
- Does this work for ceilings and accent walls?
- For a ceiling, tick the ceiling option — it adds one coat over the full length × width area, since ceilings almost always get a single coat. For an accent wall, size the room as usual and treat the one saturated wall as its own job: it still needs the same two-coat math on its own area.
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