Ohm's Law Calculator
Calculate V, I, R, or P
Enter any two values to calculate the other two.
Result
| Voltage (V) | — |
|---|---|
| Current (I) | — |
| Resistance (R) | — |
| Power (P) | — |
How to Use This Ohm's Law Calculator
Every electronics bench has a sticky note with the Ohm's law wheel on it—because you always know two values and need the other two. Whether you are sizing a LED resistor or checking why a fuse blew, entering voltage, current, resistance, and power should not require four separate formulas on scratch paper.
- Enter exactly two values. Fill in any two of Voltage (V), Current (I), Resistance (R), or Power (P). Leave the other two blank.
- Press Calculate. Results populate all four fields in the result table—including the two you did not enter.
- Press Clear to reset. Use Clear when starting a new problem so old inputs do not carry over.
For wire run losses, see the Voltage Drop Calculator. For color-band identification, use the Resistor Calculator. For appliance running costs from wattage, try the Electricity Calculator.
Ohm's Law Calculator Formulas and Practical Applications
Ohm's law and the power law work as a system—know any two, get the rest for a simple resistive load.
V = I × R
P = V × I = I² × R = V² / R
Given voltage and resistance: I = V / R, P = V² / R. Given current and power: V = P / I, R = P / I². Example: 12 V across 470 Ω draws I = 12/470 ≈ 0.026 A (26 mA) and dissipates P = 144/470 ≈ 0.31 W—use at least a ½ W resistor for safety margin.
Variable Reference
- Voltage (V): Volts across the load
- Current (I): Amperes through the load
- Resistance (R): Ohms
- Power (P): Watts dissipated or delivered
Frequently Asked Questions
Why do I need exactly two inputs?
Two independent values fully determine the circuit state for a simple resistive load. One value is underdetermined; three or more over-constrains the system and may be inconsistent.
Does this work for AC circuits?
It applies to resistive AC loads at RMS voltage and current. Reactive components (capacitors, inductors, motors) need impedance calculations beyond basic Ohm's law.
How do I pick resistor wattage?
Calculate power with this tool, then choose a resistor rated at roughly double the calculated wattage for derating and ambient temperature margin.
Which fields can I leave blank?
Enter exactly two of Voltage (V), Current (I), Resistance (R), or Power (P). Leave the other two empty before pressing Calculate.
What does the Clear button do?
Clear resets all four input fields and the result table so you can start a new problem without stale values.