Resistance Calculator
Calculate resistance using voltage, current, or power.
What the resistance calculator does
The resistance calculator finds the resistance of a circuit element from any two of three quantities: voltage across it, current through it, and power it dissipates. Enter the two values you know, leave the third field empty, choose DC or AC, and press Calculate. The result is shown in ohms (Ω) with automatic scaling to kΩ or MΩ when the value is large.
The three formulas behind the tool
All three come from Ohm’s law, V = I × R, combined with the power relation P = V × I.
| Known values | Formula | Example |
|---|---|---|
| Voltage and current | R = V / I | 12 V and 0.5 A → 24 Ω |
| Voltage and power | R = V² / P | 230 V and 2000 W → 26.45 Ω |
| Current and power | R = P / I² | 2 A and 10 W → 2.5 Ω |
The calculator picks the formula automatically based on which fields are filled. If you fill all three, it uses voltage and current and ignores power.
How to use it
- Measure or read the voltage across the component in volts.
- Measure the current through it in amperes. A multimeter in series, or a clamp meter for larger currents, both work.
- If you only know the power rating, enter it in watts instead of one of the other values.
- Select DC for batteries, power supplies and electronic circuits. Select AC for mains-powered loads and enter RMS values.
- Press Calculate and read the resistance.
Worked examples
A heating element. A kettle is rated 2200 W at 230 V. R = 230² / 2200 = 52900 / 2200 = 24.0 Ω. That is the resistance while hot; a cold element measures noticeably lower because the resistance of nichrome rises with temperature.
An LED series resistor. A 9 V supply, an LED that drops 2.1 V, and a target current of 15 mA. The resistor sees 9 − 2.1 = 6.9 V, so R = 6.9 / 0.015 = 460 Ω. The nearest standard value is 470 Ω. The LED resistor calculator does this in one step.
A car headlight bulb. 55 W at 12 V draws I = 55 / 12 = 4.58 A, so R = 12 / 4.58 = 2.62 Ω. Bulb filaments are another case where the hot resistance is several times the cold reading.
DC versus AC mode
In DC mode the calculation is exact for any resistor. In AC mode the tool assumes a purely resistive load and RMS voltage and current, which is correct for heaters, incandescent lamps and resistors. For loads with inductance or capacitance, such as motors and transformers, the ratio of RMS voltage to RMS current is the impedance magnitude, not the resistance. Use the inductive reactance or capacitive reactance calculator to separate the reactive part.
Units and prefixes
| Prefix | Symbol | Value | Typical use |
|---|---|---|---|
| milliohm | mΩ | 0.001 Ω | Shunts, wiring, contacts |
| ohm | Ω | 1 Ω | Heaters, speakers, power resistors |
| kilohm | kΩ | 1000 Ω | Signal circuits, pull-ups, dividers |
| megohm | MΩ | 1,000,000 Ω | Insulation, high-impedance inputs |
Typical resistance values
- 1 m of 2.5 mm² copper cable: about 7 mΩ
- 60 W incandescent bulb at 230 V (hot): about 880 Ω
- 8 Ω loudspeaker: 6 to 7 Ω measured with DC
- Dry human skin, hand to hand: 100 kΩ or more; wet skin can fall below 1 kΩ
- Good insulation between conductors: above 1 MΩ
Common mistakes
- Mixing peak and RMS values in AC. A 230 V RMS supply peaks at 325 V. Use RMS for both voltage and current.
- Entering milliamps as amps. 20 mA is 0.02 A. The error changes the result by a factor of a thousand.
- Using the cold resistance of a heater or bulb. Rated power applies at operating temperature.
- Treating a motor as a resistor. The running current of a motor depends on load and power factor, so V / I gives impedance, not winding resistance.
Related tools
The Ohm’s law calculator solves for any of V, I, R or P. The power dissipation calculator tells you how large a resistor you need so it does not overheat. For dividing a voltage between two resistors, use the voltage divider calculator. Resistors in series add directly; for parallel combinations use the resistors in parallel calculator.
External references
Frequently asked questions
Can I use this to find the resistance of a resistor without measuring current?
Yes, if you know the power it dissipates and the voltage across it. Otherwise measure it directly with a multimeter in the ohms range, with the circuit powered off.
Why does my measured resistance differ from the calculated value?
Temperature is the usual reason. Metal conductors rise in resistance as they heat up, so a value calculated from rated voltage and power describes the hot component, while a meter reads it cold. Meter lead resistance also adds a few tenths of an ohm at low values.
Does the AC result include power factor?
No. The AC mode assumes a resistive load with a power factor of 1. For inductive or capacitive loads the result is the apparent impedance.
What happens if I enter zero current?
Division by zero is undefined, so the calculator asks for a non-zero current. Physically, zero current at a non-zero voltage means an open circuit with effectively infinite resistance.
Is this tool suitable for mains work?
The arithmetic is, but measuring live mains circuits is dangerous. Use rated values from the nameplate, or have measurements taken by a qualified electrician.