Ohm’s Law
Calculate voltage, current, resistance, or power using Ohm's Law. Enter any two known values and the calculator instantly finds the missing quantity.
Enter Any Two Values to Find the Third
Ohm's Law Formula
Where:
- \(V\) — voltage across the conductor (volts, V)
- \(I\) — current flowing through the conductor (amperes, A)
- \(R\) — resistance of the conductor (ohms, Ω)
The law states that the current through a conductor between two points is directly proportional to the voltage across them and inversely proportional to the resistance between them. Once two of the three quantities are known, the third follows directly.
The calculator also reports the dissipated power, calculated as \(P = V \times I\), which can also be written as \(P = I^2 R\) or \(P = V^2 / R\).
Ohm's Law – Calculation Example
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Example: a 12 V power supply drives a circuit and you measure a current of 2 A. What is the resistance of the load, and how much power does it dissipate?
\[ R = \frac{V}{I} = \frac{12}{2} = 6 \; \Omega \]
\[ P = V \times I = 12 \times 2 = 24 \; \text{W} \]
Result: resistance is 6 Ω and the circuit dissipates 24 W of power.
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How to Use the Calculator
- Enter the two values you know in the Voltage (V) and Current (A) fields, or in whichever two of the three fields apply to your problem.
- Leave the field for the unknown quantity empty — the placeholder text "leave empty if unknown" reminds you of this.
- Check the Ohm's Law Results panel: the Missing Quantity row shows which value was calculated and how, and the remaining rows list the full set of values: Voltage, Current, Resistance, and Power.
- Enter exactly two values. If all three fields are filled, or only one, the calculator shows an error message instead of a result.
What Is Ohm's Law?
Ohm's Law, formulated by German physicist Georg Simon Ohm in 1827, is one of the most fundamental relationships in electrical engineering and physics. It describes how voltage, current, and resistance interact in an electrical circuit: push more voltage through a given resistance and more current flows; increase the resistance at a fixed voltage and the current drops.
The law is the working basis for circuit analysis. Whether you are sizing a resistor for an LED, checking whether a wire can carry a load, or troubleshooting why a device draws too much current, you will apply \(V = IR\) at some point. The power figure \(P = V \times I\) that the calculator also provides is equally important: it tells you how much energy per second the component converts to heat, which determines whether a resistor, wire, or power supply is rated adequately for the job.
A useful way to interpret the result: if you calculated resistance, the value represents how strongly the load opposes current flow at the given voltage. If you calculated current, the value tells you what the load will draw from the source — a key number for choosing fuses, power supplies, and cable gauge. If you calculated voltage, the value is the electrical "pressure" required to push the given current through the given resistance.
Assumptions, Limitations, and Practical Tips
- DC and resistive loads only. Ohm's Law in this simple form applies to direct current and purely resistive components. For AC circuits with reactive elements, impedance replaces resistance and phase angles come into play — see the Real and Reactive Power calculator for those cases.
- Linear behavior assumed. The law holds for ohmic (linear) components. Diodes, lamps with filaments, and semiconductors have a nonlinear voltage–current relationship, so a single resistance value does not describe them across different operating points.
- Temperature effects. Real resistors change value with temperature. A calculation made at room temperature may not hold when a component heats up under load.
- Division by zero. To calculate current, resistance must be greater than zero; to calculate resistance, current must be greater than zero. The calculator rejects these inputs with an error message.
- No negative values. Voltage, current, and resistance cannot be negative in this context; negative inputs are rejected. In real circuits, current direction matters, but magnitude calculations are what this tool provides.
When a circuit contains several resistors, reduce them to a single equivalent value first — the Parallel Resistance calculator handles the parallel case. If you are designing in electronics, the Zener Diode Resistor calculator applies Ohm's Law to size the series resistor in a voltage regulator, and the Wire Gauge calculator helps you pick a conductor that safely carries the current you just calculated.
Frequently Asked Questions
How do I calculate Ohm's Law by hand without a calculator?
Use the formula that matches your known values: voltage is current times resistance, current is voltage divided by resistance, and resistance is voltage divided by current. A helpful memory aid is the VIR triangle — cover the quantity you want and the arrangement of the other two shows the operation. For example, with a 9 V battery and a 3 Ω resistor, the current is \(I = 9 / 3 = 3\) A. Always double-check that your units are volts, amperes, and ohms before dividing.
What are common unit prefixes like milliamps or kilohms, and how do I convert them?
Calculations only work in base units: volts (V), amperes (A), and ohms (Ω). Convert before computing: 1 mA = 0.001 A, so 20 mA becomes 0.02 A. For resistance, 1 kΩ = 1000 Ω and 1 MΩ = 1,000,000 Ω. A frequent mistake is dividing volts by kilohms and treating the result as amperes — it is actually milliamperes. For example, 12 V across 4 kΩ gives 3 mA, which equals 0.003 A. Small signals in electronics are often quoted in mA, so this conversion comes up constantly.
What happens if resistance is zero, like in a short circuit?
With zero resistance the formula \(I = V / R\) involves division by zero, so the current mathematically approaches infinity. In reality, even wires and power sources have small internal resistance, so the current is very large but finite. This is exactly what a short circuit is: a near-zero resistance path that draws dangerous currents, generating heat \(P = I^2R\) in the source and wiring. This is why fuses and circuit breakers exist. The calculator rejects zero resistance to reflect this physically undefined situation.
Can I use Ohm's Law for AC circuits and household mains?
Only for purely resistive AC loads like heaters and incandescent lamps, where the simple formulas still apply using RMS values. Motors, transformers, and anything with coils or capacitors are reactive loads: their impedance depends on frequency, and voltage and current are out of phase, so real power is less than \(V \times I\). For household circuits, also note that mains voltage is dangerous — rely on rated values and a qualified electrician rather than measurements. Never probe live outlets with a multimeter unless you are trained to do so.
How do I find the power rating a resistor needs?
After the calculator gives you power, choose a resistor rated at least twice that value in watts. For instance, if the circuit dissipates 0.25 W, use a 0.5 W or higher resistor. Running a resistor at its exact rating makes it run extremely hot and shortens its life. The three equivalent formulas \(P = VI\), \(P = I^2R\), and \(P = V^2/R\) let you compute power from whichever pair of values you already know. Undersized resistors can discolor, crack, or even smoke.
Why does my measured resistance not match the calculated value?
Several factors cause this. Real resistors have a tolerance (±5% or ±10% is common), so the actual value differs from nominal. Component resistance changes with temperature — a filament lamp measures a few ohms cold but much more when glowing. Your meter's own leads add a small resistance, and measuring a resistor while it is still in the circuit lets parallel paths distort the reading. None of these mean Ohm's Law failed; they mean the actual resistance differs from what you assumed.
How do I apply Ohm's Law to multiple resistors in a circuit?
You cannot plug a single resistor value into the formulas until you reduce the network to one equivalent resistance. For resistors in series, add the values directly: \(R_{total} = R_1 + R_2 + \dots\). For parallel resistors, the reciprocal rule applies: \(1/R_{total} = 1/R_1 + 1/R_2 + \dots\). Once you have the equivalent value, use it with the source voltage to find total current, then analyze individual branches. The Parallel Resistance calculator on this site handles the parallel case for you.