Formulas you can check
Every calculator page shows its formula, a worked example, and a "How we calculated this" breakdown of your own numbers. Volumes use standard geometry (for example, cubic yards = length × width × depth ÷ 27 with consistent units). Product-specific values — bag yields, coverage per gallon, bricks per square foot — come from published product data or common industry standards and are editable where they vary.
Units
Inputs default to U.S. customary units (feet and inches, square feet, cubic yards, pounds, tons, gallons). Switch to metric on any calculator with dimensions; values are converted with exact factors (1 ft = 0.3048 m, 1 in = 2.54 cm, 1 yd³ = 0.764555 m³, 1 gal = 3.78541 L, 1 short ton = 0.907185 t).
Waste and rounding
Waste allowances are shown as editable percentages with typical ranges. Countable items (bags, sheets, boards, bundles) are rounded up because you cannot buy part of one. Volumes and weights are shown to two or three decimals; order to your supplier's increments.
Costs
Cost calculators start from typical national ranges taken from published cost data, and you can replace them with your own values. Default values that are only examples are labeled as such. Results are planning ranges, not quotes — local labor rates, access, permits and material choices change prices substantially, especially in New York City.
Testing
Each calculator is covered by automated tests that check known answers (for example, a 10 × 10 ft slab at 4 in = 1.23 yd³; a 2 × 4 × 8 ft board = 5.33 board feet; a 6/12 roof pitch = 26.57°) and that every calculator returns valid results with its default inputs.
Limits
Calculators do not size structural members, footings or beams. Span tables in your adopted building code — or an engineer — determine those. Always confirm quantities with your supplier and requirements with your building department.