Ohm's Law Calculator
Solve for voltage, current, resistance, or power in any DC circuit using Ohm's Law. Ideal for electronics students, hobbyists, and engineers troubleshooting circuits.
About this calculator
Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. The core formula is V = I × R, meaning voltage equals current multiplied by resistance. Rearranging gives I = V / R and R = V / I. Electrical power adds a fourth variable: P = V × I, where power in watts equals voltage times current. These four equations let you find any unknown quantity as long as you know two others. For example, if you know a resistor's resistance and the voltage across it, you can calculate exactly how much current flows through it — critical for selecting safe component ratings and avoiding overheating.
How to use
Suppose you have a 12 V battery connected to a 47 Ω resistor and want to find the current. 1. Select 'Current from V and R' as the calculation type. 2. Enter Voltage = 12 V. 3. Enter Resistance = 47 Ω. 4. The calculator computes I = V / R = 12 / 47 ≈ 0.255 A (255 mA). 5. To find power, switch to the Power mode: P = V × I = 12 × 0.255 ≈ 3.06 W. Use this to confirm the resistor's wattage rating is sufficient.
Frequently asked questions
What is Ohm's Law and why is it important in electronics?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, given a constant resistance: V = I × R. It is the foundational principle for analyzing and designing electrical circuits. Without it, engineers could not determine safe operating currents, select appropriate resistors, or diagnose faults. It applies to most resistive components under steady-state DC conditions.
How do I calculate power using Ohm's Law?
Power is calculated using the formula P = V × I, where P is in watts, V is voltage in volts, and I is current in amperes. If you only know two of the three base variables (V, I, R), you can derive power using equivalent forms: P = I² × R or P = V² / R. For example, a 12 V circuit drawing 2 A dissipates P = 12 × 2 = 24 W. This matters when choosing components rated for adequate power handling.
When does Ohm's Law not apply to a circuit component?
Ohm's Law applies strictly to ohmic (linear) materials where resistance remains constant regardless of voltage or current. It does not accurately describe non-linear components such as diodes, transistors, LED junctions, or thermistors, whose resistance changes with temperature or applied voltage. It also breaks down under very high frequencies where reactive (inductive or capacitive) impedance dominates. In those cases, more advanced formulas involving impedance (Z) are required.