About
Methodology and Sources
The conventions every calculator follows, and the sources behind the numbers it starts with.
Conventions
Your inputs come first. A calculator never substitutes its own figure for one you have entered. Defaults exist so the page shows a meaningful example; each is labelled and sourced in the assumption table.
Every component is a separate line. Allowances, losses, fees, taxes and rounding appear as their own rows in the breakdown, never folded into a single figure. If a line surprises you, you can see exactly what caused it.
Units are converted, not approximated. Conversions use exact factors, and the result of a calculation is the same whichever unit system you enter it in.
Rounding happens at the end. Intermediate values are kept at full precision; only the displayed figures are rounded, and quantities that are bought in whole units are rounded up.
Averages are averages. Where a default is a national or typical average, it is a starting point, not a claim about your situation.
How a paint quantity is built
Every quantity calculator follows the same five steps, and shows each one in its breakdown:
- Area. Measured surfaces, or counts multiplied by standard sizes (a door is 20 sq ft, a window 15 sq ft), less the openings.
- Surface. A texture or siding factor where the surface is rougher than the test panel the coverage figure assumes.
- Coverage and coats. Area divided by coverage per coat, multiplied by the number of coats.
- Allowance. A small percentage for the roller, the brush and a touch-up supply: 5% on walls and ceilings, 10% on trim and doors, 15% on windows and cabinets.
- Cans. Rounded up to quarts, gallons and 5-gallon pails, or to 1, 2.5, 5 and 10 litre cans in metric, never down.
Sources for default figures
| Figure | Source | Used by |
|---|---|---|
| Coverage of wall, ceiling and trim paint | Technical data sheets of Sherwin-Williams, Benjamin Moore, Behr and PPG | Interior quantity calculators, two-tone, striped wall, sample pots |
| Primer coverage by surface | Kilz, Zinsser and Sherwin-Williams primer data sheets | Primer, accent wall, coats, cabinet paint |
| Masonry, elastomeric and block filler coverage | Behr, Sherwin-Williams Loxon and PPG Speedhide data sheets | Stucco and masonry, block filler |
| Deck and fence stain coverage | Behr, Cabot, Olympic and Thompson’s data sheets | Deck stain, fence stain, masonry sealer |
| Floor coatings and clear finishes | Rust-Oleum, Behr, Minwax, Varathane and Bona data sheets | Concrete floor, epoxy kit, porch floor, wood stain, hardwood floor finish |
| Roof coating rates | Elastomeric, silicone and aluminium roof coating data sheets | Roof coating |
| Volume solids and film thickness | The 1,604 sq ft per gallon per mil relation; product data sheets | Spread rate and film thickness, thinning |
| Drying, recoat and cure times | Manufacturers’ data sheets by product class, at 77 °F and 50% humidity | Drying and recoat, painting time, crew schedule |
| Spray tip flow | Airless tip charts at 2,000 psi (water) | Spray tip, sprayer usage |
| Gloss levels | Master Painters Institute (MPI) gloss and sheen levels | Sheen selector |
| Paint densities | Manufacturers’ safety data sheets | Paint weight |
| Production rates | Site figures consistent with RSMeans daily outputs and Painting Contractors Association guidance | Painting time, production rate, project planner, crew schedule, painting bid, DIY versus pro |
| Painting and wallpapering prices | Published 2025 cost surveys (Angi/HomeAdvisor, HomeGuide, Fixr), rounded | Painter cost, cost per square foot, wallpaper cost, paint versus wallpaper |
| Painters’ wages | US Bureau of Labor Statistics, Occupational Employment and Wage Statistics, May 2023 | Painting bid, painter cost |
| Wallpaper roll sizes | Standard US and European roll sizes printed on roll labels | Wallpaper rolls, paste, cost, peel-and-stick |
| Unit conversions | NIST Handbook 44 (US gallon 3.785411784 L, exact) | Every calculator in metric mode |
The figures themselves are collected in the reference tables: coverage rates, can sizes and conversions, production rates, drying and recoat times, sheens and standard sizes. Site estimates, such as surface factors, room templates and condition factors, are labelled as such wherever they are used.
What the calculators do not model
- Your exact product. Coverage, drying times and recommended film thickness vary between products, even within one brand. The data sheet for your paint is the authority; every coverage field accepts your figure.
- The state of your surfaces. Chalking, peeling, stains, mildew and repairs change the prep, the primer and sometimes the coats. The calculators ask about the surface where it matters, but cannot see it.
- Colour hiding in detail. The coats rules use five depth steps. Some colours, especially saturated reds and yellows, can need more than the rule suggests.
- Weather. Exterior calculators give quantities and times, not forecasts. Drying times assume steady conditions.
- Local prices. Cost defaults are national ranges. Local labour markets, access, and the painter’s own costs move real quotes well outside them.
- Safety and regulation. The calculators do not assess lead paint, asbestos in old textures, working at height or ventilation. Pages mention these where they arise; they are not a substitute for the relevant rules.
Testing
Every calculator carries a set of test cases with hand-worked answers in both unit systems. The tests run automatically before each update to the site, and a calculator that fails any of them is not published. When an error is reported and confirmed, a test case for it is added so that it cannot come back.