Paint Coverage Calculator

A paint label may promise hundreds of square feet per gallon, yet a real room has doors, windows, texture, sheen choices, and more than one coat. This Paint Coverage Calculator turns those details into a can count for the walls of one room. It is best when the main question is, “How much wall paint should I have on hand?” rather than a full decorating budget. Enter the room length, width, height, opening count, surface type, paint type, and number of coats; the result reports net wall area, paint needed in gallons, the coverage rate used, and the rounded purchase count.

Use it before a weekend repaint, a landlord touch-up, or a color-change test so you can buy enough paint without filling the closet with extra cans. The calculator also helps compare choices: textured walls use a lower coverage rate than smooth walls, eggshell and semi-gloss are adjusted upward from flat paint, and the built-in 10 percent allowance covers roller loading, small measuring errors, and final touch-ups.

Paint coverage versus a full home paint estimate

This page is deliberately focused on coverage area and cans. It assumes the painted surface is the room’s walls, subtracts standard-sized openings, applies surface and sheen adjustments, and rounds up to whole cans. If you need to enter exact square footage for doors, windows, and other unpainted areas, or you want to include ceiling and trim, use the home paint calculator. For a more general construction-style estimate, compare the broader paint calculator, and use the square footage calculator when you need to measure irregular rooms first.

Assumptions and calculation

The form has two unit modes. In feet mode, it uses the entered length, width, and height directly. In meters mode, it first works in square meters and then converts the net area to square feet by multiplying by 10.764, because the coverage rates are stored as square feet per gallon. Wall area is the perimeter times height:

$\text{wall area} = 2 \times \left(\text{length} \times \text{height} + \text{width} \times \text{height}\right)$

Openings are then removed. The calculator subtracts 20 square feet for each door and 15 square feet for each window in feet mode. In meters mode it subtracts 1.86 square meters per door and 1.39 square meters per window before conversion. The net area is never allowed to fall below zero.

Surface coverage rates are 400 square feet per gallon for smooth walls, 350 for textured walls, and 300 for new drywall. Paint type then changes the amount: flat uses a factor of 1, eggshell uses 0.95, and semi-gloss uses 0.9. Because the code divides by that factor, eggshell and semi-gloss increase the gallon estimate.

$\text{paint gallons} = \frac{\text{net square feet}}{\text{coverage rate}} \times \frac{\text{coats}}{\text{paint factor}} \times 1.10$

Finally, the page displays paint needed rounded to one decimal place and rounds the purchase recommendation up to the next whole can.

Example

Suppose the room is 12 feet long, 10 feet wide, and 8 feet high, with one door and two windows. The wall area is:

$2 \times \left(12 \times 8 + 10 \times 8\right) = 352\ \text{square feet}$

The calculator subtracts 20 square feet for the door and 30 square feet for the two windows, leaving 302 square feet of net wall area. Choose smooth walls, eggshell paint, and two coats. Smooth coverage is 400 square feet per gallon, so the base single-coat amount is 302 ÷ 400 = 0.755 gallons. Eggshell divides that by 0.95, two coats double it, and the 10 percent allowance brings the underlying total to about 1.748 gallons. The result displays 1.7 gallon(s) because the interface rounds to one decimal place, and the main recommendation says to buy 2 cans.

Coverage benchmarks to sanity-check the result

Many interior wall paints advertise coverage around 350 to 400 square feet per gallon on smooth, previously painted surfaces. Rough plaster, heavy knockdown texture, fresh drywall, repairs, or thirsty masonry can reduce that noticeably. The calculator’s 400, 350, and 300 square-foot rates intentionally map to smooth, textured, and new drywall situations so the estimate changes before you reach the store. Manufacturer labels still matter: if your exact paint says 250 square feet per gallon for a porous surface, treat the calculator as a planning guide and buy according to the label.

Coat count is equally important. A near-identical repaint may cover in one coat, but dramatic color changes, patched areas, smoke staining, and glossy old paint usually require two coats after suitable prep. The calculator does not add primer gallons, so a new-drywall project needs a separate primer estimate.

Money-saving tips

Measure before buying, then round up only once. Buying a single extra can is often cheaper than a second trip, but buying far more than the rounded recommendation ties up cash and creates disposal problems. Keep walls within the same sheen and color family when possible; deep reds, bright whites over dark colors, and very high-sheen finishes tend to need more coats. Patch, sand, clean, and prime stains before painting so expensive finish paint is not wasted as a sealer. If you are painting several rooms, combine the net areas and buy larger containers only when the color, sheen, and product are identical.

Pitfalls that make coverage look wrong

The biggest mistake is confusing floor area with wall area. A 120-square-foot room can have more than 300 square feet of wall surface once height is included. Another mistake is counting a closet or alcove as part of the room but not adding its extra wall surface. Standard openings are also approximations; patio doors, bay windows, and wall-to-wall built-ins can change net area dramatically. Finally, do not expect the calculated gallons to equal the exact remaining paint in a can. Brushes, rollers, trays, and textured surfaces retain paint, which is why the calculator keeps the 10 percent allowance.

Sources

• Sherwin-Williams, Pure White SW 7005 product page — manufacturer coverage and product-data context for interior paint.
• Sherwin-Williams, Peeling and cracking problem solver — preparation issues that affect paint performance.
• EPA, Volatile Organic Compounds’ Impact on Indoor Air Quality — background for ventilation and indoor coating choices.