Electrical calculators that show their working
Ohm's law, watts to amps, voltage drop and wire sizing from the NEC tables, electricity cost and bench electronics. Every result shows the formula and table it came from, with the code caveats that matter.
Power & conversions
Convert between watts, amps, volts, kW and kVA for DC, single-phase and three-phase circuits.
- Ohm's law calculatorEnter any two of voltage, current, resistance or power and get the other two instantly, with the Ohm's law formula wheel, worked examples and a live circuit explainer.
- Watts to amps calculatorConvert watts to amps for DC, single-phase and three-phase circuits with power factor.
- Amps to watts calculatorConvert amps to watts for DC, single-phase and three-phase circuits.
- Volts to watts calculatorConvert volts to watts using current (or resistance).
- Volts to amps calculatorConvert volts to amps from watts or from ohms.
- kW to amps calculatorConvert kilowatts to amps for single-phase and three-phase systems at 120, 208, 240, 480 V and more, with power factor and a quick-reference chart.
- Amps to kW calculatorConvert amps to kilowatts for DC, single-phase and three-phase circuits with power factor.
- kVA to amps calculatorConvert kVA to amps for single-phase and three-phase transformers and generators.
- Amps to kVA calculatorConvert amps to kVA or VA for single-phase and three-phase circuits.
- Joules to watts calculatorConvert joules to watts over a time period, or watts to joules.
- HP to amps calculatorFind motor full-load amps from horsepower using NEC Tables 430.248 and 430.250, or compute from efficiency and power factor.
- 3 phase power calculatorCalculate three-phase kW, kVA, kVAR and current from voltage, amps and power factor, or solve for current from a kW load.
- Power factor calculatorCalculate power factor from kW and kVA (or volts, amps and watts), see kVAR, and size the capacitor bank needed to correct to a target power factor.
- Transformer calculatorCalculate transformer full-load primary and secondary current from kVA, pick a standard kVA size for a load, and work out turns ratio.
Wiring & code
Size conductors and breakers from the NEC tables, check voltage drop and conduit fill.
- Voltage drop calculatorCalculate voltage drop for copper and aluminum wire, single-phase, three-phase and DC, using NEC Chapter 9 Table 8 resistance.
- Wire size calculatorFind the minimum wire gauge for a load using NEC Table 310.16 ampacity, terminal temperature limits, ambient and bundling correction, 240.4(D) and a voltage-drop target.
- Wire size and ampacity chartAWG and kcmil wire ampacity chart from NEC Table 310.16 for copper and aluminum at 60, 75 and 90 °C, with resistance, mm² equivalents and the 240.4(D) limits.
- Breaker size calculatorSize a circuit breaker from watts or amps using the NEC 125% continuous-load rule and 240.6(A) standard ratings, with the matching minimum copper wire.
- Conduit fill calculatorCheck EMT and PVC Schedule 40 conduit fill for THHN/THWN-2 conductors using NEC Chapter 9 Tables 1, 4 and 5.
- Wire resistance calculatorCalculate the resistance of copper or aluminum wire by AWG/kcmil, length and temperature, using NEC Chapter 9 Table 8 values and its temperature-correction equation.
Energy & cost
Turn watts into kilowatt-hours and dollars, appliance by appliance.
- Watts to kWh calculatorConvert watts to kilowatt-hours from hours of use per day, and see energy per day, month and year with the cost at your electricity rate.
- Electricity cost calculatorWork out what an appliance costs to run per hour, day, month and year from its watts, hours of use and your electricity rate.
- Electric bill calculatorEstimate your monthly electric bill appliance by appliance.
Electronics bench
Bench tools for resistors, LEDs, dividers, capacitors and batteries.
- mAh to Wh converterConvert battery capacity from mAh or Ah to watt-hours using nominal voltage.
- Voltage divider calculatorSolve a resistive voltage divider for Vout, R1 or R2, see the divider current and power, and get the nearest standard E24 resistor values.
- Parallel resistor calculatorCalculate total resistance of resistors in parallel or series, up to 10 resistors, and find which resistor to add in parallel to reach a target value.
- Resistor color code calculatorDecode 4, 5 and 6-band resistor color codes to ohms and tolerance, or enter a value to get its bands.
- LED resistor calculatorCalculate the current-limiting resistor for one LED or a string of LEDs from supply voltage, forward voltage and current.
- Capacitor calculatorCapacitors in series and parallel, stored energy and charge, RC time constant, reactance at a frequency, and 3-digit capacitor code decoding, in one tool.
- Battery life calculatorEstimate battery runtime from capacity and load current or watts, with usable depth of discharge, inverter efficiency and sleep/active duty cycles.
Built around the code tables
The wiring tools use values transcribed from the 2023 National Electrical Code: ampacity from Table 310.16, temperature and bundling factors from 310.15, conductor resistance from Chapter 9 Table 8, conduit areas from Chapter 9 Tables 4 and 5, motor currents from 430.248 and 430.250. Each tool checks its formulas against published worked examples. The sources page lists them all.
Calculations run in your browser as you type. No sign-up, nothing sent to a server.
What a calculator can't do
Codes are adopted locally, often with amendments, and your inspector has the final word. A calculator doesn't see the wire's route, the equipment's listing or the condition of an old panel. Use these tools to plan and to check your understanding; have a licensed electrician design and install the work, and get the permit.
Got a question the tools don't answer? Ask it in plain English and get an explanation with the relevant formula or code section.