Sources

paint

Wall paint coverage

Every row below was read off the manufacturer's own product data sheet, not a retailer listing or a search summary. Coverage figures are per coat.

Overlap across all three: 350-400 sq ft/gal. Every product's stated range contains it. All three state their number at roughly the same 4 mils wet, so they are directly comparable rather than measured on different bases.

Behr: dropped, not pending. behr.com serves a Cloudflare block page to both WebFetch and a headless browser, so no Behr figure can be read from a primary source. Rather than leave a permanent gap, SPEC.md section 5 was amended on 2026-08-09 to use Sherwin-Williams as its reference example instead. Do not reintroduce a Behr number from a retailer listing or a search snippet.

Primer coverage

Note the age of this TDS (2011). It is the current document served for the product, but re-check before leaning on it in a second tool.

Coats — manufacturer wording, not our opinion

Both statements below are quoted because the two-coat default is a real cost driver and needs to trace to the manufacturer rather than to us.

Door and window deductions — convention, NOT a manufacturer spec

No paint manufacturer publishes a deduction figure. These are estimating conventions, and the guide must present them as such. The door default does trace to a real door size; the window default does not trace to anything but common practice, because window sizes are not standardized the way doors are.

Triangulation (SPEC section 5.2)

Worked example held constant: 12 ft x 14 ft room, 8 ft ceilings, 1 door, 2 windows, 2 coats. Gross wall area = 2 x (12 + 14) x 8 = 416 sq ft.

Finding, and it belongs in the guide: the three sources land within 7 sq ft of each other on net area (365-372, a 1.9% spread), so the door/window convention barely moves the answer. The coverage constant moves it entirely — this example sits right on the 2-vs-3 gallon boundary, and which side it falls on is decided by 350 vs 400 sq ft/gal alone. That is the honest headline: stop arguing about window deductions and read the number off your can.

Consequence for the tool: default coverage to 350 sq ft/gal (inside all three manufacturer ranges, and the side that avoids a second trip to the store), make it user-editable, and show the result at both 350 and 400 so the boundary is visible instead of hidden behind a single rounded number.

concrete

Derived and printed on the page: 27 cu ft per cu yd means 45 bags of 80 lb, 60 of 60 lb, or 90 of 40 lb per cubic yard. The 1 cu yd bag-vs-truck crossover is a handling judgment, labeled as such in the guide, not a spec.

fence

Reconciliation, and it changed the tool's default. Quikrete's two-bag example is 0.75 cu ft. The geometry disagrees: a 9 in hole 30 in deep is 1.10 cu ft, the 3 in post displaces 0.16, leaving 0.94 cu ft needed. Subtract 6 in of drainage gravel at the base so concrete fills only the top 24 in and the requirement falls to 0.76 cu ft, matching their example within 1%. So the stated figure assumes a gravel base. The calculator exposes gravel depth as a field defaulting to 6 in, and the guide shows this working rather than hiding the discrepancy. Verified live: 9 in / 30 in / 6 in gravel returns 0.76 cu ft per hole, exactly 2.0 bags per post.

deck stain

Default coverage 250 sq ft/gal, mid-band for rough wood, because a weathered deck is the realistic case. Note this tool defaults to one coat, unlike paint, and that comes straight from the manufacturer's own instruction.

drywall

Rates used: 0.009 gal compound, 0.35 ft tape and 1.25 screws per sq ft. The chart is linear across every row from 100 to 1,500 sq ft, so the division is clean. Published typo: the 1,300 sq ft row prints "17.7 Gal." where the series gives 11.7. Every other row follows the rate exactly. We use the rate, not that row, and the guide says so.

grass seed

Rates are derived from those coverages rather than from a rule of thumb: 9.09, 7.50 and 3.03 lb per 1,000 sq ft for a new lawn. The 3:1 finding: all three products' overseeding coverage is almost exactly three times their new-lawn coverage (5,280/1,760, 8,000/2,665, 1,000/330). That consistency is what justifies the single overseeding multiplier, and it is in the guide.

mulch

Bag sizes (1.5 / 2 / 3 cu ft) are as sold, not a spec, and are a user choice because all three share shelf space. 27 cu ft per cu yd is definitional. The 2 cu yd bags-vs-bulk crossover is a handling judgment, labeled as such.

gravel

No manufacturer spec exists for this tool and the guide says so outright. Volume is exact geometry. Tonnage depends on the density of the specific stone at its current moisture, which ranges roughly 1.3-1.7 tons per cu yd across common materials. The calculator defaults to 1.4 and exposes density as an editable field, with the guide telling readers to get their supplier's figure. Do not replace this with a "sourced" constant; there isn't one.

tile

Waste factors (10% straight lay, 15% diagonal or offset, 20% herringbone or complex rooms) are an estimating convention in common use, not a manufacturer spec, and the guide labels them that way. Tile geometry (144 sq in per sq ft) is definitional. Tiles per box is a user input because it varies per product.

wallpaper

Deliberately no hard-coded product constants. Roll width, roll length and pattern repeat vary enough between manufacturers that any default would be wrong more often than right, so all three are inputs defaulted to common American values (about 20.5 in wide, about 33 ft double roll) and labeled "read your roll". The 4 in trim allowance and the half-strip credit per opening are conventions, both explained in the guide.

Verified behavior: the strips-per-roll integer, not area, drives the roll count. On a 7 ft wall with a 33 ft roll, a 9 in repeat still yields 4 strips per roll, but a 25 in repeat drops it to 3 and pushes 24 strips from 6 rolls to 8. That worked example is in the guide and was checked against the live tool.