Ohm's Law Calculator
The Ohm's Law Calculator computes resistance and power from the voltage and current you enter. It is a practical tool for electronics students, hobbyists, and technicians who need to verify circuit values quickly.
Ohm's Law Calculator
V
A
What the result means
Resistance is the ratio of voltage to current: R = V ÷ I. A higher voltage with the same current means more resistance, while a higher current with the same voltage means less resistance. Power is the product of voltage and current, representing the rate at which electrical energy is converted to heat or work.
How to use this calculator
- 1Enter the voltage across the component in volts (V).
- 2Enter the current flowing through the component in amperes (A).
- 3Press Calculate to see the resistance in ohms (Ω) and the power in watts (W).
- 4Review the formula shown with the result to confirm the relationship.
- 5Adjust either input and recalculate to explore how resistance and power change.
The formula
The calculation uses a standard, verifiable formula. Here it is in its simplest form.
R = V / I
P = V × I
Where:
R = Resistance (Ω)
V = Voltage (V)
I = Current (A)
P = Power (W)
What each variable means
| Symbol | Name | Description |
|---|---|---|
| V | Voltage | The electrical potential difference across the component, measured in volts. |
| I | Current | The flow of electric charge through the component, measured in amperes. |
| R | Resistance | The opposition to current flow, measured in ohms. |
| P | Power | The rate of energy conversion, measured in watts. |
Step-by-step example
Example: 12 V across a 2 A circuit
Voltage:12 VCurrent:2 A
- 1Resistance = 12 ÷ 2 = 6 Ω
- 2Power = 12 × 2 = 24 W
- 3The component resists 6 Ω and dissipates 24 W.
Result
R = 6 Ω, P = 24 W
What changes the result
- Resistance depends on the material, length, and cross-sectional area of the conductor.
- Temperature changes resistance in most materials — metals increase resistance when hot.
- Power dissipation determines how much heat a component must handle.
Edge cases to be aware of
Unusual situations handled correctly
- If current is zero, resistance is undefined (division by zero) — the calculator returns 0.
- Very small currents produce very large resistance values, which may be unrealistic for real components.
- AC circuits with reactance (capacitors or inductors) do not follow simple Ohm's Law for impedance.
Common mistakes
Avoid these errors
- Using milliamps (mA) without converting to amps — 500 mA is 0.5 A, not 500 A.
- Confusing voltage drop across a component with the supply voltage.
- Applying Ohm's Law to AC circuits with reactive components as if they were purely resistive.
Assumptions
- The circuit is purely resistive (DC or resistive AC).
- Voltage and current are measured at the same point in the circuit.
- The component obeys Ohm's Law linearly over the operating range.
Limitations
- Does not account for reactance, inductance, or capacitance.
- Assumes ideal conditions without temperature effects or wire resistance.
- Real components may have non-linear voltage-current relationships.
Frequently asked questions
What is Ohm's Law?+
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the points and inversely proportional to the resistance. It is expressed as V = I × R.
How do I calculate resistance if I know voltage and current?+
Divide the voltage by the current: R = V ÷ I. For example, 12 V ÷ 2 A = 6 Ω.
Why does power matter in a circuit?+
Power tells you how much energy the component converts per second. If a resistor cannot dissipate the power as heat, it will overheat and fail. Always check the power rating of components.
Does Ohm's Law work for AC circuits?+
For purely resistive AC circuits, yes. For circuits with capacitors or inductors, you need impedance (Z) instead of resistance, and the relationship becomes V = I × Z.