Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law (V=IR). Enter any two values to instantly compute the remaining electrical quantities.
Type into any 2 fields. The calculator will automatically lock those as your inputs and solve the other 2 fields.
Applying a standard 50% safety margin, the resistor should be rated for at least 9 W.
High power dissipation! Ensure adequate ventilation or use a chassis-mount power resistor.
R = V / I
P = V * I
About Ohm's Law Calculator
Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = IR. Combined with the power formula P = VI, you can derive any of the four electrical quantities given any two known values. This calculator supports all six combinations of two known values and displays results with appropriate SI prefixes (milli, kilo, mega, etc.) for readability. Essential for electronics hobbyists, electrical engineers, and students.
Built and maintained by Meet Shah · Last updated
What this tool is used for
- Sizing a series resistor for an LED at a given supply voltage.
- Checking whether a resistor's power rating is adequate for the current through it.
- Working out the current a load will draw before choosing a supply.
- Confirming a measured value against what the circuit should produce.
- Comparing two resistor choices on dissipated power rather than resistance alone.
Frequently Asked Questions
- What is Ohm's law?
- V = I × R — voltage equals current times resistance. Rearranged it gives I = V/R and R = V/I, and combined with P = V × I it yields the twelve formulas of the power wheel, all derived from those two relationships.
- Does it apply to every component?
- No — it holds for ohmic materials where resistance is constant. Diodes, LEDs and transistors are non-linear, which is precisely why an LED needs a series resistor: its current rises almost vertically past the forward voltage with nothing to limit it.
- How is power calculated?
- P = V × I, or equivalently I²R and V²/R. The squared forms are the useful ones for sizing a resistor — doubling the current quadruples the heat, which is how a component rated correctly for voltage still burns out.
- What changes in AC circuits?
- Resistance becomes impedance, which includes reactance from capacitors and inductors and varies with frequency. Ohm's law still holds in the form V = I × Z, but the quantities are complex and current can lag or lead the voltage.
- Why does resistance change with temperature?
- Because thermal vibration impedes charge carriers. Metals increase resistance as they heat; semiconductors decrease it, which is what makes thermal runaway possible — more current means more heat means less resistance means more current.
Common errors and gotchas
- Mixing units — milliamps with volts and kilohms — which produces an answer a thousand times out.
- Ignoring power dissipation, so the arithmetic is right and the resistor burns.
- Applying it to a non-linear component. An LED or a diode does not obey a fixed resistance.
- Using the supply voltage rather than the voltage across the component in question.
- Forgetting that a real supply has internal resistance, so the loaded voltage is lower than the rated one.