How our calculators work

Calculation approach

Every calculator is a pure, documented formula: the same inputs always produce the same outputs, with no hidden adjustments. Each calculator page explains its own math in a "How this calculator works" section, usually with a worked example you can reproduce by hand, and each engine lives in the project's source code alongside the unit tests that verify it; the project repository also documents every formula, rounding rule, and default assumption in a single reference file (docs/CALCULATOR_FORMULAS.md). Nothing about the math is proprietary or obscured — if you disagree with a formula, you can see exactly what it does and tell us why via the contact page.

Rounding rules

• Whole purchasable units always round up. Boards, bags, sheets, boxes, and pavers are computed with a ceiling function: 22.04 sheets means you buy 23. The only exception to strict ceiling is floating-point noise — a computed 22.000000000001 is treated as exactly 22, not pushed to 23, so you never buy an extra unit because of computer arithmetic.
• Ready-mix concrete is never rounded down. An order that comes up short mid-pour is far more expensive than a small surplus, so concrete volumes only round upward.
• Costs round to 2 decimals. Every cost line is rounded to the cent, with a correction for the classic floating-point artifacts (0.1 + 0.2 ≠ 0.30000…4).
• Measurements display at sensible precision. Displayed measurements scale their precision to their magnitude: values of 1,000 or more show whole numbers, hundreds show one decimal, single and double digits show two decimals, and values under 1 show three — enough precision to be useful, not enough to look falsely exact.

Waste factors

There is no single correct waste number. A straight-laid rectangular floor might waste 5–10%; the same floor laid diagonally, or a room with many closets and jogs, can waste 15% or more. Each calculator ships a default waste factor appropriate to its project type and pattern, shows that default with the results, and lets you change it. Treat the default as a starting point and adjust it for your layout, your skill level, and how easy returns are at your supplier.

Unit conversions

All conversions use exact standard factors from one shared, tested library:

• 12 inches per foot
• 27 cubic feet per cubic yard
• 2,000 pounds per ton (US short ton)
• 2.54 centimeters per inch (exact); 3.280839895 feet per meter
• 3.785411784 liters per US gallon; 4 quarts per gallon

Area and volume conversions (square meters to square feet, cubic meters to cubic yards) are derived from these same base factors, so results are consistent across every calculator.

Cost estimates

All prices come from you.The site ships no built-in price data by design: material prices vary by region, supplier, and week, and a hard-coded "national average" goes stale fast and creates false confidence. Cost fields accept your local store or supplier prices; if you don't enter a price for an item, the calculator shows the quantity and simply omits the cost line rather than inventing one.

The codebase includes a framework for optional, broad "reference range" pricing, but it is intentionally empty: any range added later must carry a named source and a date, must be labeled approximate, and is removed rather than left to go stale.

Source review

We cite a source when a claim rests on a published document or an authoritative organization — safety guidance (call 811 before digging, CPSC generator safety, EPA lead and asbestos guidance), code-related reminders, and trade-association practice. Quantities that reflect common industry practice rather than a citable standard — typical bag yields, screw counts per joint, coverage rates — are labeled as assumptions on the page instead of being dressed up with decorative citations. The full list is on the sources page.

Limitations

• Results are planning estimates for budgeting and shopping, not build documents.
• Nothing here is engineering, structural, or electrical advice. Spans, footings, loads, and electrical connections need span tables, local code, and licensed professionals.
• Product specifications vary by manufacturer — board widths, bag yields, box coverage, and paint spread rates should be checked against the actual product you buy.

See the full disclaimer for details.

Testing

Every calculation engine has automated unit tests, including hand-verified worked examples — scenarios computed manually first, then locked in as tests so a future code change that alters results fails loudly. Shared conversion and rounding helpers are tested the same way. Before any deploy, the project must pass:

• npm test — unit tests for engines, unit conversions, helpers, and URL state
• npm run test:e2e — end-to-end browser tests against a production build
• npm run typecheck, npm run lint, and npm run build