Ohm's Law Calculator

Lock any two variables and the simulator solves the rest in real time. Covers Voltage, Current, Resistance, and Power with a live circuit diagram.

Electrical Workbench
🔒 Lock two variables to solve. Click a lock icon to toggle it.
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V Voltage SOLVED
V
I Current SOLVED
A
R Resistance SOLVED
Ω
P Power SOLVED
W
⚡ Short Circuit detected. Resistance = 0 causes unbounded current flow.
Live Circuit Diagram
V = 12 V R = 6 Ω I = 2 A P = 24 W
V = I x R P = V x I P = V² / R P = I² x R
Key Terms Explained
Ohm's Law
The foundational rule of circuit analysis: Voltage equals Current multiplied by Resistance (V = I x R). Formulated by Georg Ohm in 1827, it describes how these three quantities are always proportionally linked in a linear conductor.
Impedance (Z)
The AC equivalent of resistance. Impedance includes both resistance (from resistors) and reactance (from inductors and capacitors), measured in Ohms. Unlike resistance, impedance changes with the frequency of the AC signal.
Voltage (V)
Also called Potential Difference or EMF. The electrical pressure that pushes current through a circuit, measured in Volts (V). Think of it like water pressure: more voltage means more force driving electrons through the wire.
Current (I)
The rate of electron flow through a conductor, measured in Amperes (Amps, A). The symbol I comes from the French word "intensite." One Amp equals one Coulomb of charge passing a point per second.
Resistance (R)
The opposition a material offers to current flow, measured in Ohms (the omega symbol). Resistors convert electrical energy into heat. Lower resistance means more current flows for a given voltage.
Power (P)
The rate at which electrical energy is transferred or consumed, measured in Watts (W). P = V x I. A 100W light bulb consumes 100 Joules of energy every second. Also expressible as P = I squared times R or P = V squared divided by R.
Short Circuit
A fault condition where resistance drops to near zero, causing current to spike toward infinity. The enormous current flow generates intense heat in wires and components, creating a fire and explosion hazard. Circuit breakers and fuses are designed to interrupt this condition.
Ohm (unit)
The SI unit of electrical resistance, symbolized by the Greek letter omega. One Ohm is defined as the resistance between two points when a constant potential difference of one Volt produces a current of one Ampere.

The Complete Guide to Ohm's Law and Electrical Power

Whether you are sizing a resistor for a LED circuit, troubleshooting an amplifier, or designing a power supply, Ohm's Law is the first calculation you will reach for. This guide explains the four core variables, the three equations that connect them, and the practical rules that keep circuits safe.

How to Use This Calculator

The tool has four variable cards: Voltage (V), Current (I), Resistance (R), and Power (P). Each card has a lock icon in the upper right corner. Click a lock to designate that variable as a known input. Lock any two variables, then adjust their sliders or type a value directly. The remaining two variables update instantly to satisfy both Ohm's Law (V = I x R) and the power formula (P = V x I).

Use the unit toggles at the top to switch between Amps and milliamps, Ohms and kilohms, and Watts and milliwatts. The slider range and displayed unit adjust automatically, so you can work comfortably with microelectronics (milliamps, kilohms) or mains-level circuits (amps, low ohms) without rescaling everything by hand.

If Resistance is set to zero, the tool shows a Short Circuit warning and disables the infinite result. This reflects the real behavior: a dead short allows current limited only by the source's internal resistance, which is typically so low that the current is destructive.

The Four Formulas at a Glance

Ohm's Law generates a family of four interchangeable formulas. Given any two of the four variables, you can solve for the other two without a calculator if you know these:

V = I x R - Voltage equals Current times Resistance. The base form of Ohm's Law.

P = V x I - Power equals Voltage times Current. Often called the power formula or the "West Virginia" formula (W = V x A).

P = V squared / R - Power from Voltage and Resistance only. Useful when you know the supply voltage and the load resistance.

P = I squared x R - Power from Current and Resistance only. This form explains why transmission line losses fall so sharply at high voltages (lower current for the same power).

Resistance vs. Impedance: When Does It Matter?

For DC circuits and purely resistive AC loads (heaters, incandescent bulbs, resistors), resistance and impedance are numerically the same and the terms are interchangeable. Impedance becomes a distinct concept in AC circuits containing capacitors or inductors. These components store and release energy, creating reactance that opposes current in a frequency-dependent way. The total impedance Z = square root of (R squared + X squared), where X is the net reactance. At audio frequencies, a speaker's impedance might be rated at 8 Ohms but swings between 4 and 60 Ohms depending on frequency. At DC (0 Hz), a capacitor looks like infinite impedance (an open circuit) and an inductor looks like zero impedance (a short).

Safety: Why Zero Resistance Is Dangerous

Ohm's Law predicts that if R approaches zero, current I = V / R approaches infinity. Real circuits are limited by the source's internal resistance, but that limit is usually very small. A standard 12V car battery has an internal resistance of around 0.01 Ohms, which means a dead short could draw over 1,000 Amps before the battery voltage collapses. At those currents, P = I squared times R means tens of kilowatts are dissipated as heat in the cables within seconds. Fuses and breakers are sized to interrupt the circuit before that heat can start a fire or cause components to explode.

Frequently Asked Questions

Resistance (measured in Ohms) is the opposition to current flow in a DC circuit or a purely resistive AC circuit. It is constant regardless of frequency. Impedance is the broader term used in AC circuits. It includes both resistance (R) and reactance (X), which comes from capacitors and inductors that store and release energy. Impedance changes with frequency. For DC circuits and resistors only, Resistance and Impedance are numerically equal. This tool focuses on the DC Ohm's Law relationship where the two terms are interchangeable.
Doubling the voltage quadruples the power consumption, assuming resistance stays constant. Because P = V squared divided by R, power scales with the square of voltage. For example, a 10-ohm resistor at 10V consumes 10W. At 20V (double), it consumes 400 / 10 = 40W - four times as much. This squared relationship is why high-voltage transmission lines are so efficient: transmitting power at 10x the voltage means current drops to 1/10th, and power lost as heat in the wires (P = I squared times R) drops to 1/100th.
Zero resistance causes a short circuit. With R = 0, Ohm's Law gives I = V / R, which approaches infinity. In practice, the current spikes to the maximum the power source can deliver. This enormous current releases huge amounts of heat (P = I squared times R) in wires, connectors, and the source itself - hot enough to melt insulation, start fires, or cause batteries and capacitors to explode. Fuses and circuit breakers exist specifically to interrupt this runaway current before it causes physical damage.
Power (Watts) = Voltage (Volts) multiplied by Current (Amps). This is the power formula P = V x I. One Watt means one Joule of energy is transferred or consumed every second. A 60W light bulb on a 120V circuit draws 0.5 Amps. A 1200W microwave on 120V draws 10 Amps. For resistive loads, you can also express power as P = I squared times R or P = V squared divided by R, which let you calculate power knowing only two of the three Ohm's Law variables.
Click the lock icon next to any variable to lock it as a known input. When two variables are locked, the calculator solves for the other two automatically as you adjust the slider or type a value. For example, lock Voltage and Resistance, then drag the Voltage slider - Current and Power update in real time. You need at least two locked variables to get a unique solution, since Ohm's Law has four variables and two independent equations.