Free calculators for contractors & technicians
No sign-up. Every result cites the code section or rule of thumb behind it, so you can check the math yourself.
Calculators
Voltage Drop Calculator
Single-phase & three-phase voltage drop by wire size, distance, and load. Flags NEC 3%/5% thresholds.
ElectricalWire & Breaker Ampacity
NEC Table 310.16-based ampacity lookup by wire gauge, insulation, and temperature rating, with derating notes.
ElectricalConduit Fill Calculator
NEC Chapter 9 conduit fill — how many conductors fit in EMT, and what trade size you actually need.
ElectricalElectrical Box Fill
NEC 314.16 box fill — conductors, devices, and grounds vs. box volume, so you pick the right box the first time.
ElectricalEMT Conduit Fill Chart
How many THHN wires of one size fit each EMT trade size, 1/2" to 4", checked against NEC Annex C.
ElectricalBox Fill Chart
Max 14–6 AWG conductors for every standard metal box in NEC Table 314.16(A).
ElectricalMotor Full-Load Current
NEC Table 430.248/430.250 FLC lookup by horsepower and voltage, with the 125% conductor sizing rule applied.
ElectricalTransformer Sizing
Convert between transformer kVA and full-load amperage, either direction, single- or three-phase.
ElectricalDwelling Load Calculator
NEC 220.82 optional method — total service load and minimum service size from square footage, fixed appliances, and HVAC.
ElectricalEV Charger Circuit Sizing
NEC 625 branch-circuit sizing for EV charging equipment — breaker and wire size from the charger's continuous rating.
HVACBTU Load Estimator
Rough heating/cooling BTU load by room size, climate zone, insulation, and sun exposure — a sizing starting point.
HVACSuperheat & Subcooling
Superheat and subcooling from your gauge readings — R-410A, R-22, R-32, and R-454B saturation PT data built in.
PlumbingDrain Pipe Sizing
Fixture-unit based drain, waste & vent pipe sizing for common residential fixture combinations.
PlumbingSeptic Tank & Drainfield Sizing
Starting-point tank size by bedroom count, compared against real state minimums, plus rough drainfield sizing by soil percolation rate.
CarpentryStair Stringer Calculator
Code-compliant riser height, tread count, and stringer length for a straight-run stair, from total floor-to-floor rise.
RoofingRoof Pitch & Shingle Calculator
Roof area, rafter length, and shingle/underlayment material estimate from plan dimensions and pitch.
WeldingMIG/Stick Amperage & Wire Speed
MIG and Stick starting-point settings — amperage, voltage, and wire feed speed by steel thickness or electrode size/type.
MachiningCNC Feeds & Speeds
RPM and feed rate for end-mill milling by material, tool diameter, flutes, and chip load — a dial-in starting point.
Guides
- How to calculate box fill under NEC 314.16
What counts toward box fill, the per-conductor volumes of Table 314.16(B), the 2020 rule for more than four grounds, and worked examples for device boxes and junction boxes.
- Conduit fill rules: how NEC Chapter 9 sizes a raceway
The 53 / 31 / 40 percent limits of Chapter 9 Table 1, how to use Tables 4 and 5, the notes on grounds, nipples, cables and rounding, and worked EMT examples.
- Voltage drop formula, with worked examples
The K-factor voltage drop formula for single- and three-phase circuits, how to solve it for wire size, the NEC 3% and 5% recommendations, and four worked examples.
- Sizing a motor branch circuit under NEC Article 430
Table FLC versus nameplate current, the 125% conductor rule, Table 430.52 protective devices, overload percentages, Table 430.22(E) duty cycles and the 430.24 rule for several motors.
- Spindle speed and feed rate formulas, explained
RPM from surface speed (3.82 × SFM ÷ D and its metric form), feed from chip load, radial chip thinning, metal removal rate, and worked milling, turning and drilling examples.
- Superheat and subcooling: what they are and how to calculate them
The two measurements, where to take them, the arithmetic with a PT chart, which one to charge by for fixed orifice and TXV systems, and worked examples in °F/psig and °C/bar.
Why these numbers, and why trust them?
Every calculator on this site is built directly from published reference sources — NEC (National Electrical Code), IPC (International Plumbing Code), IRC (International Residential Code), and industry-standard trade reference tables — not guessed. Each calculator page shows the underlying table or formula in plain language, with the source cited, so you can verify it yourself or check it against the current edition adopted in your jurisdiction. See the disclaimer for the full picture on how to use these results responsibly.