Formulas
Each calculator is a small, self-contained program with a documented formula, written separately from the page design. Quantities follow standard geometry and manufacturer coverage data, such as bag yields, coverage per gallon and bundles per square. Every page lists its formula under "How this calculator works", and each result can be expanded to show the math.
Testing
Every calculator ships with automated tests: hand-checked reference cases, edge cases (zero, minimums, very large values, unit switching), and whole-unit rounding checks. Tests run on every change before anything is published.
Price data
- Prices are US national ranges for the current year, compiled from published cost guides, manufacturer data, and government sources (EIA, DOE, IRS, ICC).
- Each value is stored with its source and review date. The full list is public in our Cost Index.
- Where no single published source exists, we use an editorial estimate and label it as one.
- A "local cost level" setting shifts labor and installed prices for low- and high-cost areas. These multipliers are deliberately coarse; local quotes are always better.
Review schedule
Price datasets are reviewed at least quarterly. Policy-driven values such as tax credits, energy rates and building-code limits are checked whenever the underlying rules change. Each page shows its last-reviewed date and data version.
Corrections
If you find an error, please tell us. We verify reports, fix confirmed errors, add a regression test so the error can't come back, and update the page's review date.
Independence
Advertisers and lead partners have no input into our calculations or content. Sponsored links are labeled. Estimates are for planning only and are not professional, engineering, legal or tax advice. Always confirm with licensed professionals and your local building department.