How We Build and Check Our Calculators

Every calculator on this site is built to be checked. This page explains where our formulas and assumptions come from, how we test them and how we keep them up to date.

1. Formulas you can verify

Each calculator uses standard geometry and unit conversions, shown in a formula section on its page with a worked example. You can check any result by hand. Where a tool works in both US and metric units, it converts at exact factors (for example, 1 ft = 0.3048 m) and rounds only the final answer.

2. Assumptions from published sources

Many estimates depend on a typical value rather than pure math, such as the yield of an 80 lb bag of concrete, the weight of a cubic yard of crushed stone or the coverage of a gallon of paint. We take these from published sources: building codes such as the International Residential Code and the National Electrical Code, industry standards from bodies like ACI, ANSI, ASTM, NCMA, TCNA and the Gypsum Association, government sources such as NREL, ENERGY STAR and the US Department of Labor, and manufacturers' product data. Each calculator lists the sources it relies on in a Sources & standards section. Where a value varies by product or region, we state the figure we use and, where practical, let you change it.

3. Testing before publishing

• Every formula is tested against hand-worked examples in both US and metric units before a calculator is published.
• The worked examples and charts on each page are checked against the live calculator, so the page and the tool agree.
• Code-based checks, such as stair riser limits and wire ampacity, are checked against the current code tables.

4. Rounding and waste

Materials are sold in whole units, so the calculators round up bags, sheets, boxes, bundles and gallons. Volumes and weights are shown to two decimal places. Waste allowances are editable inputs with sensible defaults, because the right figure depends on the layout, the material and your skill. Where waste is included in a result, the result says so.

5. Reviews and corrections

We review every page at least once a year, and whenever a referenced code or standard is updated. Each page shows its last-updated date. If you find an error or an outdated assumption, please tell us. Corrections are checked and published quickly, and we note significant changes on the page.

6. What our results are and aren't

Results are estimates for planning, budgeting and ordering. They are not engineering, structural, electrical or legal advice. Building codes are adopted and amended locally, so for permitted or structural work, follow your local building department and a qualified professional. See our disclaimer for details.