How we calculate
Every result on this site can be traced to a formula and a published source. This page explains how.
Sources
Structural and safety limits come from the model codes most US jurisdictions adopt: the International Residential Code for stairs, decks, framing and roofing, and the National Electrical Code for wiring. Coverage rates, bag yields and product sizes come from manufacturer data sheets, and planning figures from government agencies (DOE, EPA, BLS, BEA), standards bodies and university extension services. Each calculator lists its sources with links under the result.
The math on every page
Each result shows the formula with your own numbers substituted, so you can check it with a pencil. The page below each calculator explains the formula in plain language with a worked example, and the examples are part of our automated tests, so the text and the calculator cannot drift apart.
Units, rounding and waste
Switch between feet and inches and metric at any time. Internally every calculator works in metric units and converts only for display, which avoids rounding errors from mixing feet and inches. Things you buy, such as bags, sheets, boxes and boards, are rounded up to the next whole unit. Material calculators add a waste allowance you can change, 10% unless the trade uses another figure, and the page says which.
Cost estimates
Cost calculators combine published national cost ranges with state factors: labor from Bureau of Labor Statistics wage data and materials from Bureau of Economic Analysis regional price parities. They give a low, typical and high range with the date of the underlying data. They are planning figures, not quotes; get at least three written bids for work you hire out.
Review
Our editorial team checks each calculator against its sources before publishing and again when codes or guidance change. The date of the last review is shown on every calculator page.
Limits
Results are estimates for planning and buying, not engineering or legal advice. Your local building department decides which code edition and amendments apply, and structural, electrical and gas work often needs a permit and an inspection. Call 811 before you dig, and use a licensed professional where your jurisdiction requires one.
Corrections
If you find an error, email jonas@sport-calculator.com. We fix confirmed errors and update the review date.