Resistance Calculator

Enter every measurement Resistance Calculator asks for, then press Calculate. The result stays blank while you are still typing.

Press Calculate after the entries are in.

What Resistance is for

Resistance Calculator is the page to open when you need resistance from measurements you already have. It belongs with the other electrical calculators because that is the work this arithmetic supports. Type the entries first. The result stays hidden until you press Calculate, so a half-finished form never flashes a number you might mistake for the answer.

How to fill in this calculator

Voltage. Enter the value this part of the job needs. The unit list on this field includes millivolts, volts, kilovolts.
Current. Enter the value this part of the job needs. The unit list on this field includes milliamps, amps, kiloamps.
Power. Enter the value this part of the job needs. The unit list on this field includes milliwatts, watts, kilowatts.
Resistance. Enter the value this part of the job needs. The unit list on this field includes milliohms, ohms, kiloohms.

The formula in plain language

Ohm's law says voltage equals current times resistance. Power in watts equals voltage times current. On single-phase AC, divide power by voltage and by the power factor. On three-phase line-to-line voltage, also divide by the square root of 3. Voltage drop on a wire is the round-trip factor times current times the wire's resistance. Copper resistance is about 10.371 ohm-circular-mils per foot.

A worked example

A 12 volt circuit with a 2 ohm load draws 6 amps and uses 72 watts. A 1,500 watt space heater on a 120 volt circuit draws 12.5 amps.

Voltage, current, resistance, and power describe one circuit. This resistance result belongs with watts from voltage and current and voltage drop along the wire, using the same voltage and the same load.

Mistakes that move the answer

Mixing peak voltage with RMS voltage doubles the confusion. Household outlets are quoted as RMS.
A power factor of 1 is fine for a heater. A motor is lower, often near 0.8, so it draws more current than the watt label suggests.

How to check the result

After Calculate shows the resistance result, redo the last operation with a pencil using the rounded figures on the screen. Matching that check means the inputs were read correctly. If you edit a label or a sentence in this HTML file, reload the page. The form and this article both live in the file, so the edit is what visitors will read.