Transparent method
The formula
Worked example
Example inputs
How to interpret the result
Ohm's law describes a purely resistive, linear relationship between voltage, current and resistance, and it holds precisely for a simple resistor. Components that behave nonlinearly, like diodes or many real-world loads under certain conditions, don't follow the same straight-line relationship, so the law's clean math doesn't transfer directly to every circuit element.
Before you rely on it
What to check
Confirm the component in question actually behaves as a linear resistor before applying Ohm's law directly to it, since nonlinear components need a different analysis.
The common error
Where people go wrong with ohm's law calculator
Applying Ohm's law directly to a nonlinear component, like a diode, expecting the same clean relationship a simple resistor follows. The law's assumptions don't hold for components whose resistance itself changes with voltage or current.
Sensitivity evidence
How current (amps) changes the voltage (volts)
| Current (amps) | Voltage (volts) | Power (watts) |
|---|---|---|
| 1.6 | 160 | 256 |
| 1.8 | 180 | 324 |
| 2worked example | 200 | 400 |
| 2.2 | 220 | 484 |
| 2.4 | 240 | 576 |
Every input, tested
Which input moves the voltage (volts) most
| Input | Tested from | To | Voltage (volts) at each end | Swing |
|---|---|---|---|---|
| Current (amps) | 1.8 | 2.2 | 180 to 220 | 40 (20%) |
| Resistance (ohms) | 90 | 110 | 180 to 220 | 40 (20%) |
Two variables at once
Voltage (volts) by current (amps) and resistance (ohms)
| Current (amps) \ Resistance (ohms) | 80 | 100 | 120 |
|---|---|---|---|
| 1.6 | 128 | 160 | 192 |
| 1.8 | 144 | 180 | 216 |
| 2 | 160 | 200 | 240 |
| 2.2 | 176 | 220 | 264 |
| 2.4 | 192 | 240 | 288 |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Current (amps) | 2 |
| Resistance (ohms) | 100 |
| Voltage (volts) | 200 |
| Power (watts) | 400 |
Where the inputs come from
Enter the current in amps and the resistance in ohms. Measure current with a meter in series with the circuit and resistance with the power off, or take the value from a component's label. Ohm's law states that voltage equals current times resistance, and power equals voltage times current. Use consistent units: milliamps must be converted to amps, and kilohms to ohms. This tool assumes a steady direct current and a purely resistive load.
When the result stops applying
Ohm's law holds for resistors and many wires, but not for diodes, LEDs and lamps whose resistance changes with current or temperature. Alternating current circuits with coils and capacitors add reactance and a power factor. High voltages are dangerous, so calculation is not permission to work on live circuits. Real components have tolerance.
What to compare it with
See combined resistance on the resistor calculator, voltage division on the voltage divider page, LED resistors on the LED resistor calculator and currents from power on the watts to amps calculator. The voltage drop page covers wiring, and the RC time constant calculator adds a capacitor.
People also ask
Frequently asked questions
What is Ohm's law?
The relationship that voltage equals current times resistance, so any one of the three can be found from the other two.
How do I calculate power in a circuit?
Multiply voltage by current in watts. It also equals current squared times resistance, or voltage squared divided by resistance.
Does Ohm's law apply to all components?
No. It describes resistors and ohmic conductors, but diodes, transistors and lamps have resistance that changes with conditions.