Electrical Calculators
Ohm's Law, voltage drop, conduit fill and other basic DIY electrical math. 10 free calculators.
Household electrical work leans on a handful of formulas - Ohm's Law, voltage drop over a wire run, how many conductors legally fit in a conduit - that are simple once you have them, but easy to get wrong (or skip) without a reference.
These calculators handle that basic electrical math directly: current, resistance and power from Ohm's Law, voltage drop and minimum wire size for a run, and conduit fill percentage against the standard NEC limits.
Last reviewed September 2026
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All 10 Electrical calculators
Ohm's Law Calculator
Use this Ohm's Law calculator to get resistance in ohms and power in watts from any voltage and current reading. Includes the formulas and a worked example.
Voltage Drop Calculator
Use this voltage drop calculator to find the drop on a wire run in volts and percent, plus the smallest wire gauge that stays within the 3% guideline.
Parallel Resistance Calculator
Parallel resistance calculator for up to four resistors. Enter each value in ohms to get the combined total, plus the formula and how it differs from series.
Watts, Amps & kVA Calculator
Use this watts to amps calculator to convert watts, kVA and amps for single-phase or three-phase circuits. Adjust power factor for motors and electronics.
Conduit Fill Calculator
Use this conduit fill calculator to check wire fill against NEC Chapter 9 limits. Get the fill percentage and the most wires of one size the conduit allows.
EV Charger Circuit Calculator
Use this EV charger circuit calculator to find the minimum breaker and wire gauge for a Level 2 charger, sized to the NEC 125% rule for copper or aluminum.
Ceiling Fan Size Calculator
Use this ceiling fan size calculator to get a recommended blade diameter from your room's square footage, with notes on downrods and large rooms.
Smoke & CO Detector Placement Calculator
Use this smoke and CO detector placement calculator to estimate minimum alarm counts from bedrooms, stories and fuel-burning appliances, per IRC rules.
Electrical Box Fill Calculator
Use this box fill calculator to check NEC volume for your wires. Get the required cu in., the most conductors your box fits, and a pass or fail check.
Recessed Lighting Layout Calculator
Use this recessed lighting layout calculator to get a fixture count and grid spacing from room length, width and ceiling height. Ambient light only.