Every calculator must show a useful result, the assumptions behind it, a worked example and the situations that still require professional verification.
Formula selection
We use established geometry, unit conversions and trade formulas appropriate to the stated calculator. Each tool page explains its formula in plain language and includes a worked example so visitors can independently check the approach.
Inputs and units
Current tools use US customary units such as feet, inches, square feet, cubic yards, AWG and volts. Inputs are labelled with their unit, invalid, blank and nonfinite values show field-level errors and suppress the estimate, and results are rounded only for display.
Result testing
Automated checks cover default results, invalid inputs, zero rules, whole-item boundaries, units and the simplified electrical model across all nine tools. We also verify that each public page loads, exposes its own title and description, links to its related calculators and contains the correct safety notice where needed.
Electrical model
Voltage drop uses a fixed 75°C resistance approximation: K = 12.9 for copper and 21.2 for aluminum in Ω·cmil/ft. One-way length is multiplied by 2 for DC/single phase or √3 for balanced three phase. AC assumes unity power factor, neglects reactance and does not model actual conductor temperature. Three-phase inputs are line current and line-to-line voltage. The common-size list spans 14 AWG through 4/0, including 3 AWG, but omits other AWG sizes. Zero current or distance does not produce a wire recommendation.
Sources and assumptions
Tool pages link to relevant method references when a manufacturer, standards body or industry resource can clarify the calculation. References support the method; they do not mean that the source endorses Calc for Trades. Project conditions and local requirements can still change the correct answer.
Electrical and structural limits
Voltage-drop and wire-size results are planning guidance and do not replace NEC ampacity, derating, terminal, protection or local-code checks. Stair, stringer and roof results are geometry estimates—not structural designs or cut plans. Safety-sensitive decisions must be verified by an appropriately qualified professional.
Corrections and updates
When a formula, explanation or limitation changes materially, the review date is updated. If you find a possible error, send the calculator name, inputs, displayed result and expected result through the Contact Us page.