Calculator 05 / Paint
Paint Calculator
Paint coverage is per coat, so the quantity is paintable area × coats ÷ the spread rate on the can, and then waste on top. Estimate gallons of paint needed for a room's walls, based on coverage rate and number of coats.
How this calculator works
Coverage rate is the figure on the can, and it is the number most worth questioning — it is for one coat, on the surface the label describes, and many labels say so outright with wording like "up to" and "on smooth surfaces." Anything rougher or more porous than that takes more paint than the label suggests. Primer is not counted here at all: it is a separate product with its own coverage rate, and it does not count as one of your coats.
Worked example: a 14 × 12 ft bedroom
Walls only, 8 ft ceiling, 40 sq ft of door and window to deduct, two coats at 350 sq ft per gallon, 10% waste:
- Perimeter = 2 × (14 + 12) = 52 ft
- Wall area = 52 × 8 − 40 = 376 sq ft
- Exact gallons = (376 × 2) ÷ 350 = 2.15 gal
- With 10% waste = 2.36 gal
- Rounded up = 3 gallons
Add the ceiling and the same room needs four. That single checkbox moves the order by a whole gallon, which is worth checking before you leave the shop rather than after.
Coverage: start from your label
The 350 default is a planning figure, not a property of paint. Replace it with the rate printed on the can you are buying, or on the product's technical data sheet; if the label gives a range, use the low end for anything but a smooth, already painted wall. No general table can stand in for that number, because coverage varies by product as much as by surface. The table below shows which surfaces pull real coverage below the label, and why.
| Surface | Why coverage drops | What to enter |
|---|---|---|
| Smooth, primed or previously painted | Closest to the conditions a label describes | The label rate |
| Textured (orange peel, knockdown, popcorn) | More surface than its flat outline, and your tape measures only the outline | The low end of the label range, or the data sheet's rate for textured surfaces |
| Bare, unprimed drywall | The paper face and the joint compound absorb unevenly | Prime first. USG lists its First Coat drywall primer at about 180–200 sq ft per gallon |
| Masonry, block, stucco | Rough and porous | The rate on a product made for masonry |
| Primer | A separate product with its own rate, not one of your coats | Run the calculator again with coats set to 1 and the primer's rate |
Choosing a sheen
Sheen does not change how much paint you need, but it changes how the room reads and how it wears. The rule of thumb is that shine and washability rise together, and so does how mercilessly the finish shows every flaw underneath it.
| Sheen | Where it belongs | Trade-off |
|---|---|---|
| Flat / matte | Ceilings, low-traffic rooms | Hides defects; hard to clean |
| Eggshell | Living rooms, bedrooms | The usual compromise |
| Satin | Hallways, children's rooms | Wipeable; starts showing roller marks |
| Semi-gloss | Kitchens, bathrooms, trim | Durable; shows surface flaws |
| Gloss | Doors, cabinets, high wear | Toughest; needs the best prep |
Where paint orders go wrong
- Counting primer as a coat. It is a different product with a different coverage rate, and it is ordered separately. Two finish coats over primer means two, not one.
- Trusting the coverage on the can. That number is often a best case on a smooth primed wall. Texture and porosity both eat into it.
- Buying gallons separately. Tint varies between batches. Buy the job at once and box it — pour all the cans into one bucket and mix — which makes a color difference between walls less likely.
- Deducting every opening. Cutting in around a window takes paint and time even though the glass takes none. Deduct the large openings and let the waste factor absorb the rest.
- Rounding down to save a gallon. Running out mid-wall can leave a lap mark that is hard to hide, and the replacement gallon may not match.
Paint coverage guide
How much a gallon really covers depends on the surface, the coats and whether you prime first. Our paint coverage guide works through a 12 × 12 room step by step and gives gallons for common room sizes.
Where these numbers come from
Spread rate varies more by product than by any general rule, so the manufacturer's own label and technical data sheet outrank this calculator every time. The one primer figure on this page comes from that primer's manufacturer; the rest is safety and air quality. Homes built before 1978 may contain lead-based paint: before scraping or sanding, read the EPA rule below and confirm the requirements with the EPA or a lead-safe certified renovator.
- USG Sheetrock Brand First Coat Primer — Manufacturer's product page: approximate coverage of 180–200 sq ft per gallon, and what a drywall primer is for.
- EPA Renovation, Repair and Painting Rule — Who must be lead-safe certified when disturbing paint in housing built before 1978, and when the rule applies to homeowners.
- EPA on volatile organic compounds — Paints are a listed source of VOCs. Ventilate while painting and follow the label's precautions.
Do I need less paint for a second coat?
Usually not — a second coat covers the same area, so this calculator multiplies total area by the number of coats rather than reducing coverage for later coats.
What if I need less than a full gallon?
If your total comes in under a gallon, most stores sell quarts (¼ gallon) — round up to the nearest quart instead of buying a full gallon.
Can I save, share, or print a calculation?
Yes — use the Save, Copy link, and Print buttons under your results. Saved calculations can be grouped into a named project on the My Projects page for a combined job estimate with cost totals. Everything is stored only in your browser, never uploaded — see the Privacy Policy for details.
Estimates only, based on the measurements you enter. Coverage rates vary by paint brand, surface texture, and application method — check the product label or technical data sheet for the rate, and confirm quantities with your paint supplier or the manufacturer before you buy.