How Every Calculator Constant Is Sourced, Cited, and Verified

Every calculator on this site depends on physical constants — how many cubic feet a bag of concrete yields, how wide a deck board actually is, how a grout manufacturer’s coverage chart is read. None of those values are guessed to make a tool look complete. This page is the record of how they are chosen, labeled, and checked.

Source hierarchy

When a number has to live in the data layer, the preference order is:

1. Manufacturer technical documents — product data sheets, installation guides, and coverage charts with a stable URL (Quikrete, USG, Trex, TimberTech, MAPEI, LATICRETE, TEC, ARDEX, and the like).
2. Published standards and trade manuals — for example NIST PS 20 softwood sizes, IRC tables where they apply, or the Chain Link Fence Manufacturers Institute product manual.
3. Labeled trade conventions — only when no technical source clears the bar. The UI and sources block say so; they are never presented as if they came from a data sheet.

Retail product pages, decor blogs, and dead PDFs do not qualify. If a live manufacturer page contradicts an older PDF whose URL now 404s, the live page wins — we have already reversed shipped numbers for that reason rather than keep a convenient but uncheckable figure.

How values are stored and verified

Each constant is stored under src/data/materials/ with a label, numeric value, unit, source citation, source URL, and a verified-on date. Calculator components import those values; they do not hardcode yields or coverage rates in the island script. Every calculator page renders its sources in a visible block so a contractor can open the same document we used.

When a manufacturer revises a sheet, or a reader flags a mismatch, the constant is updated and the verified date moves. A wrong constant is worse than no calculator — that is the standard this site is built on.

What “sourced” has meant on real tools

Concrete bag yields

Bag yields come from Quikrete Concrete Mix Data Sheet No. 1101 (0.30 / 0.45 / 0.60 ft³ for 40 / 60 / 80 lb), read directly from the PDF and cross-checked against Sakrete’s high-strength mix TDS for the same three sizes. Where brands disagree — notably 90 lb yields — that size is left out of the picker rather than averaged into a fake consensus. The common 10% bagged-concrete overage and the “think about ready-mix near a cubic yard” note are trade rules of thumb and are labeled that way on the page; they are not treated as Quikrete specifications.

Deck boards and joists

Board width and gap defaults are tied to currently live manufacturer pages and, for joist spacing, IRC Table R507.7 where it applies. Early draft numbers that depended on a 404’d TimberTech PDF or a Trex install-guide gap that disagreed with Trex’s own worked example were corrected before they could sit as if they were settled. The calculator counts boards and field joists; it does not pretend to size beams for span.

Grout — chart cells before formulas

Grout coverage is product-specific. For each supported preset we store the manufacturer’s chart geometry and use an exact chart-cell lookup when tile size and joint width match a published cell. Outside a calibrated zone (roughly faces at least 3 inches with joints at most 3/8 inch, checked against our own chart dump), the tool warns instead of quietly applying a wider fudge factor. Where the fallback formula still runs, the UI states the observed error direction — usually skewing toward extra material, with a called-out exception for certain MAPEI Keracolor mosaic / wide-joint cells. LATICRETE SPECTRALOCK was a documented gap until August 2026 — its TDS points at a separate estimator — and was closed by locating the actual referenced document, Color Card DS 254.3-0618, with the unit volume corroborated from DS-681-0920. LATICRETE READY-TO-USE Grout is the remaining deliberate omission: it is on the same card, but three tile rows fit the joint-volume model too poorly to fall back on.

Tile — waste as judgment, not TDS

Tile area math is geometry. Box coverage is user-supplied from the carton because no honest handful of brand presets can represent thousands of SKUs. Waste defaults for floor, wall/shower, and diagonal layouts are labeled trade conventions after we could not find TCNA Handbook or manufacturer estimating tables that cleanly define installation cut waste for those modes. Daltile’s familiar ~10% figure is attic stock for repairs and dye lots — a different purpose — so it is not used as a citation for cutting waste. The page says waste is a judgment call the trade does not publish the way bag yields are published.

Drywall sheets

Sheet face coverage (32 / 40 / 48 sq ft for 4×8 / 4×10 / 4×12) comes from USG’s Sheetrock installation guide and matches the sizes National Gypsum’s own calculator offers. Opening deductions follow USG’s “large openings yes, small boxes no” guidance without inventing a square-foot cutoff USG never published — you enter the large-opening total. The 10% waste default matches an installation-waste scenario in a USG environmental declaration used for modeling; the UI states that it is not purchasing guidance or a code rule, and notes that National Gypsum’s calculator excludes waste entirely.

Fence posts, pickets, and concrete

Wood, vinyl, and chain link use different takeoff math on purpose. Picket width starts from NIST PS 20-25’s dressed-dry 1×6 face and stays editable. Chain-link spacing and footing baselines cite CLFMI’s product manual section 3.2.A. Post embedment uses the deeper endpoint of Quikrete’s current Setting Posts in Concrete guide (⅓–½ of above-ground length), with gravel called out separately; an older Quikrete sheet’s “⅓ of overall post length” wording is cited literally and not rewritten into a false equivalence. Local frost or product depth overrides the baseline when greater. Concrete volume reuses the same hole-minus-post geometry and Quikrete bag yields as the concrete calculator. Wood spacing, picket waste, and concrete overage remain labeled conventions where no national rule applies.

Shingles — the square that isn’t 100

Every roofer says three bundles make a square, and CertainTeed’s own data sheet defines the square as 100 sq ft. But the coverage line on the spec sheets does not agree with the shorthand: GAF Timberline HDZ, Owens Corning TruDefinition Duration, and CertainTeed Landmark each publish 98.4 sq ft per three bundles, while IKO Cambridge publishes a true 100. That is a 1.6% shortfall on three of the four brands — about one extra bundle on a 20-square roof, which is exactly the kind of gap that turns into a second trip. So the calculator uses the brand’s published figure rather than the round number, and the page shows which document each came from. Starter strip and hip-and-ridge cap are counted from their own sell sheets (GAF WeatherBlocker and Seal-A-Ridge), because they are sold by linear foot, not by square, and estimates that fold them into the field count come out short.

Wire, conduit, and what the NEC actually requires

The electrical tools read straight from NFPA 70. Conduit interior areas come from Chapter 9 Table 4, conductor areas from Table 5, and the fill limits — 53% for one conductor, 31% for two, 40% for more than two — from Table 1, with the 31% two-conductor case being the strict one because it exists to prevent jamming rather than to cap heat. Ampacity comes from Table 310.16 (2023 edition) for copper and aluminum, 14 AWG through 4/0, at all three temperature columns. The distinction the tool refuses to blur: the 90°C column is almost never the number you are allowed to use, because termination ratings usually pull you back to the 60 or 75°C value, so the calculator says which column it applied and why. Voltage drop is handled the same way — the 3% branch-circuit and 5% combined figures come from 210.19(A) Informational Note No. 4, and an informational note is a recommendation, not an enforceable requirement. The result is labeled as such. None of this replaces an inspector or a licensed electrician, and the pages say so.

Waste and overage

Almost every tool carries an adjustable waste or overage input. Defaults are not one global percentage pasted across trades:

• Some start from a sourced modeling assumption that is then labeled carefully (drywall’s USG EPD scenario).
• Some start at 0% because the buy unit already rounds up (vinyl panels) or the published takeoff omits a percentage (chain-link mesh length).
• Many — bagged concrete, tile layouts, wood pickets — are trade conventions shown as such, because that is all the technical record supports.

Pros order over net quantity because material is sold in whole units, sites are imperfect, and a short order costs more than a spare board. The calculator defaults are starting points you can change; they are not a promise that your layout needs exactly that percentage.

What we refuse to invent

If a constant lacks a defensible source, the build stops or the field becomes a user input — we do not fabricate brand presets, frost maps, or “industry standard” percentages to look authoritative. Cost and labor estimators are out of scope: they need local prices and productivity rates this site does not pretend to know. Health, tax, and legal calculators are also out of scope.

Corrections

A wrong constant is worse than no calculator. If you find one, use the contact page — corrections ship with an updated verified date on the sources block for that tool.