Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values to find the other two.
Everyday Uses
Hobby electronics
Size the resistor for an LED circuit so it glows instead of burning out.
Car audio and 12 V projects
Check current draw and wire gauge for amplifiers, lights, and accessories.
Home circuit sense
Understand why a 2,000 W heater on a 10 A circuit trips the breaker.
Physics class
The V = IR triangle solved in any direction, with power included.
Frequently Asked Questions
What is Ohm's Law and what are its formulas?
Ohm's Law states that Voltage (V) = Current (I) × Resistance (R), or rearranged: I = V ÷ R and R = V ÷ I. It describes the linear relationship between voltage, current, and resistance in a conductor at constant temperature. Named after German physicist Georg Ohm (1827), it is the foundational law of electrical circuit analysis. Units: Voltage in Volts (V), Current in Amperes (A), Resistance in Ohms (Ω).
How is electrical power calculated from Ohm's Law?
Power (P) in watts = V × I. Combined with Ohm's Law, this yields three equivalent power formulas: P = V × I (power from voltage and current), P = I² × R (power from current and resistance), P = V² ÷ R (power from voltage and resistance). Example: a 12 V circuit drawing 2 A dissipates P = 12 × 2 = 24 watts. These formulas are used for sizing resistors (to avoid overheating), designing amplifiers, and calculating circuit efficiency.
What are the real-world applications of Ohm's Law?
Ohm's Law is used in virtually every electrical and electronics application: calculating the correct resistor value to limit current through an LED (R = (V_supply − V_LED) ÷ I_LED); sizing fuses and circuit breakers; designing voltage dividers; calculating heat dissipation in resistors; analysing audio amplifier output stages; and troubleshooting short circuits or high-resistance faults in wiring. It applies to DC circuits and, with modifications (using impedance instead of resistance), to AC circuits.
Does Ohm's Law apply to all materials and components?
No — Ohm's Law strictly applies only to ohmic (linear) conductors, where resistance stays constant regardless of voltage or current. Most metals and resistors are ohmic. Non-ohmic components include: diodes (one-way current flow), LEDs, transistors, light bulbs (resistance increases with temperature), thermistors, and superconductors. For these, current is not proportional to voltage. The law also breaks down at very high electric fields or temperatures.
What is the difference between resistance and impedance?
Resistance (R, in ohms) is opposition to DC current flow and is frequency-independent. Impedance (Z, also in ohms) is the AC equivalent — it includes resistance plus reactance (opposition from capacitors and inductors, which varies with frequency). Z = √(R² + X²), where X is reactance. In a DC circuit Z = R. In audio, RF, and power electronics, impedance matching is critical to maximise power transfer — a guitar amplifier's speaker impedance must match its output stage.