Free, browser-only calculators for contractors, electricians, and homeowners. Every tool runs entirely in your browser – nothing is sent to a server, no signup, no ads inside the calculator. Each page links to the relevant code reference (NEC table, IRC section) and explains the formula so you can sanity-check the result.

Electrical

  • NEC Wire Size Calculator – Pick the right conductor for any circuit by amperage, distance, voltage, and material. References NEC 310.16 ampacity tables and includes voltage drop and derating.
  • Voltage Drop Calculator – Verify that the conductor you picked also keeps voltage drop inside the recommended NEC 210.19(A) targets (3% branch / 5% total). Copper or aluminum, single or three phase.
  • Electrical Load Calculator (NEC 220) – Residential standard-method service-load calculation. Returns total VA, amps at 240 V, and the recommended service size.
  • Conduit Fill Calculator – Add conductors row-by-row by insulation type (THHN, XHHW, RHH) and size, pick the conduit type and trade size, and the calculator returns total fill against the NEC Chapter 9 Table 1 allowable percent (53/31/40).
  • Box Fill Calculator (NEC 314.16) – Counts conductors, devices, internal clamps, fixture supports, and EGC allowance and reports required cubic inches against standard outlet box volumes from NEC Table 314.16(A).
  • Motor FLA & Branch Circuit Calculator – Looks up motor FLA from NEC Tables 430.247-250 by HP / voltage / phase, applies the 125% conductor factor (430.22), and returns the recommended OCPD per Table 430.52 (250% inverse-time breaker, 175% time-delay fuse, 300% non-time-delay fuse).
  • Grounding Conductor Sizer – Returns the GEC per NEC Table 250.66 (sized from the largest service-entrance ungrounded conductor) or the EGC per NEC Table 250.122 (sized from the branch-circuit OCPD). Copper or aluminum.

New to NEC code lookups? Start with the NEC Code Lookup Hub for plain-English explanations of which calculator answers which code section. Want to read the actual NEC text alongside the calculators? See How to Read the NEC Codebook Online (Free, Legally).

Construction

  • Concrete Volume Calculator – Cubic yards and 80 lb bag count for slabs, footings, and round columns, with a configurable waste allowance.
  • Construction Markup Calculator – Convert margin to markup and back, with optional overhead allocation, so your bid math actually pencils out.
  • Construction Unit Converter – Inches/feet/decimal-feet, square feet to square yards, cubic yards to cubic feet, gallons to cubic feet, and the unit conversions that come up most often on takeoffs.
  • Drywall, Mud, and Tape Estimator – Sheets, joint compound, tape, and screws for a hang-and-finish job. Configurable sheet size, ceiling include/exclude, and waste allowance.
  • Roofing Material Estimator – Squares, bundles of asphalt shingles, underlayment, ice and water shield, drip edge, and nails for a residential re-roof, with pitch-based waste.
  • Stair Stringer Calculator – Picks the riser count for a given total rise and checks the result against IRC R311.7 (max riser, min tread, max variance), then returns run length so you can confirm the stair fits the floor space.

Energy

  • Solar Panel Payback Calculator – Estimate years to break-even from system size, install cost, your electric rate, and sun hours. Includes the 30% federal residential clean-energy tax credit.

How these calculators are built

Every calculator is a single self-contained HTML file with no external JavaScript, no fonts, and no tracking. The math runs locally so you can use them on a job-site phone with no signal, and so they keep working even if this site is offline (just save the page). The source files live in the project repository if you want to inspect or fork them.

Have a calculator you wish existed? Open an issue on the repo or send a note via the contact page and we’ll add it to the queue.

Calculators are for planning and estimating only. They are not a substitute for a stamped engineering design or a permitted electrical drawing. When in doubt, hire the appropriate licensed professional.