Methodology
Where the formulas come from, what they assume, and how they are tested.
Sources by category
| Category | Calculators | Primary references |
|---|---|---|
| Bulk Materials | 14 | Published aggregate densities; supplier product specifications |
| Landscaping | 6 | Trade coverage rates; USDA soil volume guidance |
| Concrete & Masonry | 9 | ACI 318, ACI 117; ready-mix supplier yield tables |
| Roofing | 10 | IRC chapter 9; NRCA Roofing Manual; manufacturer coverage rates |
| Framing | 8 | IRC chapters 5–8; AWC span tables; nominal vs dressed lumber sizes |
| Interior | 11 | Manufacturer coverage rates (paint, board, adhesive, grout) |
| Exterior | 5 | Manufacturer coverage rates; IRC chapter 7 |
| Electrical | 9 | NFPA 70 (National Electrical Code), tables 8, 9, 310.16 and chapter 9 |
| HVAC & Plumbing | 9 | ACCA Manual J principles; IPC/UPC fixture unit tables |
| Automotive | 6 | Standard engine geometry and drivetrain ratio formulas |
Assumptions that apply everywhere
- US customary units. Densities, board measures and standard sizes follow US trade practice. A UK or metric user should check unit conventions before relying on a result.
- Compacted, in-place quantities. Bulk material figures assume the material as installed, not as loose-loaded on a truck. Loose volume is typically 15–25% greater.
- Nominal versus actual dimensions. Lumber calculators use actual dressed sizes (a 2×4 is 1.5 × 3.5 in) unless the page says otherwise. Board foot calculations use nominal size, because that is how lumber is sold.
- Waste allowances are trade defaults, not code. Where a page adds a waste margin it says so and shows the exact requirement alongside. Complex layouts, diagonal patterns and small jobs justify more.
Where the density figures come from
Bulk material weights vary with moisture, gradation and source quarry — sometimes by more than 10%. We use mid-range published values for typical dry material and state them on each page. For a delivery priced by the ton, ask your supplier for the density of the specific product; a 10% error on density is a 10% error on your invoice.
Electrical calculators
Voltage drop, conductor sizing, conduit fill and breaker selection follow National Electrical Code tables. They assume standard installation conditions: 75 °C terminations, no more than three current-carrying conductors in a raceway, and 30 °C ambient. Derating for temperature, bundling, continuous loads and local amendments is not applied automatically. Code-covered work must be designed and inspected by a qualified person.
How results are tested
Each calculator carries at least two test cases worked out by hand from the reference formula. The site does not build unless every case passes within tolerance. A separate check runs the JavaScript actually embedded in each published page and verifies it returns the same answers — catching the case where the source is right but the page is wrong.
Test cases are never generated by running the calculator. Doing so would record whatever the code currently does, including its bugs, and call it correct.
Corrections
If you believe a formula or a constant is wrong, tell us and cite your source. We would rather be corrected than confidently wrong.