Ohm's Law Calculator

Ohm's Law relates voltage, current, and resistance for an ohmic element under suitable conditions. Its familiar form is V = IR. If two of those three quantities are known, the third can be calculated. This page also combines the relationship with basic electrical-power equations.

The main form is V = I × R. Rearranging gives I = V/R and R = V/I. The calculator uses positive magnitudes and internally converts unit prefixes so the equations are solved in volts, amperes, ohms, and watts.

If voltage and resistance are known, use I = V/R. If voltage and power are known, use I = P/V. If resistance and power are known, current can be found from I = √(P/R) for the basic positive-magnitude resistive case.

If current and resistance are known, use V = IR. If current and power are known, use V = P/I. If resistance and power are known, use V = √(PR), using positive magnitudes for the basic resistive calculation.

If voltage and current are known, use R = V/I. If voltage and power are known, use R = V²/P. If current and power are known, use R = P/I². The result assumes a basic resistive relationship at the operating point.

Multiply voltage by current: P = VI. For example, 120 V at 5 A gives 600 W. In AC systems with reactance or phase shift, real power may also depend on power factor, so a simple VI calculation may represent apparent rather than real power.

Use R = V²/P. For example, a 120 V resistive load dissipating 600 W has a resistance of 24 Ω. This assumes a basic resistive load and does not model temperature-dependent or nonlinear resistance changes.

For a basic resistive circuit, use I = √(P/R). Once current is known, voltage follows from V = IR. The calculator assumes positive magnitudes and does not infer current direction, AC phase angle, or nonlinear device behavior.

Voltage is measured in volts, current in amperes, and resistance in ohms. Electrical power is measured in watts. Prefixes such as milli-, micro-, kilo-, and mega- scale these units without changing the underlying equations.

V represents voltage, or electric potential difference. The SI unit is the volt. In the basic relationship V = IR, voltage equals current multiplied by resistance. This calculator supports microvolts, millivolts, volts, and kilovolts.

I represents electric current. Its SI unit is the ampere. Current can be calculated from voltage and resistance with I = V/R, or from power and voltage with I = P/V for the basic resistive case.

R represents electrical resistance, measured in ohms. Resistance describes opposition to current in a resistive element. In AC circuits with significant reactance, impedance Z may be the more appropriate quantity instead of resistance alone.

P represents electrical power, measured in watts. For a basic resistive circuit, P can be calculated as VI, I²R, or V²/R. Power is a rate of energy transfer, not the same as energy consumption over time.

Ohm's Law itself relates V, I, and R, while power adds P = VI. Combining the equations gives P = I²R and P = V²/R. This is why entering any two of V, I, R, and P can solve the complete basic resistive set.

The basic V = IR form applies directly to resistive behavior. AC circuits containing inductors or capacitors can require complex impedance and phase relationships. In those cases, use impedance Z and the appropriate AC power relationships rather than simple resistance alone.

Resistance is the real opposition associated with resistive elements. Impedance is the broader AC quantity that can include both resistance and reactance. Impedance can vary with frequency and is often represented as a complex quantity in AC circuit analysis.

An LED is nonlinear and does not normally behave like a constant resistor across its operating range. Ohm's Law can still be applied to external resistors and local operating-point relationships, but a device model or manufacturer I-V data is needed for the LED itself.

Diodes have nonlinear current-voltage characteristics, so a single constant resistance does not usually describe them accurately. Basic Ohm's Law remains useful for resistors around the diode circuit, while diode behavior should be modeled with an appropriate I-V relationship.

At least two independent positive values are required from voltage, current, resistance, and power. If three or four values are entered, the calculator checks whether they are mutually consistent before accepting the calculation.

Yes. Current is I = P/V, and resistance is R = V²/P. For example, 120 V and 600 W give 5 A and 24 Ω in a basic resistive circuit.

Yes. Use R = P/I². Voltage then follows from V = P/I. This combination is fully supported by the calculator and does not require you to select a separate “solve for resistance” mode.

Watts measure power, which is a rate of energy transfer. Watt-hours measure energy accumulated over time. A 100 W device operating for 5 hours uses 500 Wh of energy, assuming its power stays at 100 W.

Only when the applied voltage is held constant in the basic V = IR relationship. In a real circuit, changing resistance can also change source voltage, operating point, temperature, control behavior, or other system conditions, so the broader circuit context matters.

It is commonly used to determine resistor current, voltage drop, resistance value, or power dissipation when enough quantities are known. Final resistor selection should also consider tolerance, temperature coefficient, voltage rating, power derating, and environmental conditions.