Ohm's Law Calculator

Compute voltage (V), current (I), resistance (R) or power (P) using Ohm's law (V = I·R) and Watt's law (P = V·I).

Volts (V) ≥ 0
Amperes (A) ≥ 0
Ohms (Ω) > 0
Enter any two values (V, I, R). If you fill all three, Voltage & Current will be used to recalculate Resistance.
Quick examples:
Privacy first: All calculations run locally in your browser. No data is sent to any server.

Understanding Ohm's Law: The Foundation of Electronics

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant. The relationship is expressed as:

V = I × R   |   I = V / R   |   R = V / I

Named after German physicist Georg Simon Ohm (1789–1854), this law is fundamental for analyzing electrical and electronic circuits. When combined with Joule's first law, electric power can be derived: P = V × I = I²·R = V² / R.

Step-by-step calculation logic

  • If you enter Voltage and Current, Resistance = V / I and Power = V × I.
  • If you enter Voltage and Resistance, Current = V / R and Power = V² / R.
  • If you enter Current and Resistance, Voltage = I × R and Power = I² × R.
Case Study: Designing an LED current-limiting resistor

An LED operates at 2.2V with a recommended current of 20 mA. If powered from a 5V supply, the resistor must drop 5V - 2.2V = 2.8V. Using Ohm's law: R = V_R / I = 2.8V / 0.02A = 140 Ω. Power dissipated: P = V·I = 2.8×0.02 = 0.056 W → use ¼ W resistor. Our calculator confirms exact resistance and power rating. This ensures reliable operation and prevents LED burnout.

Real-World Applications of Ohm's Law

Field / Device Application of Ohm's Law
Household wiring Determine circuit breaker rating (I = P / V) for appliances.
Sensor circuits Voltage dividers for temperature, light sensors (LDR, thermistor).
Power supplies Calculating output current limits and voltage regulation.
Electric vehicles Battery pack design, motor controller current calculations.
Audio amplifiers Matching speaker impedance to amplifier output.

Ohm's Law Triangle & Memory Aid

Visual learners often use the Ohm's law triangle: cover the quantity you want to find. V is on top, I and R at bottom. V = I·R (multiply), I = V/R (divide), R = V/I (divide). The same applies to the power circle (P = V·I).

V = IR

Limitations & Extensions of Ohm's Law

Ohm's law holds true for ohmic materials (metals, resistors) where resistance is constant over a range of voltages. However, non-linear devices like diodes, transistors, and varistors do not follow a constant R. For AC circuits, impedance (Z) replaces resistance, yet Ohm's law extends to AC: V = I·Z. Understanding these boundaries is critical for advanced electronics.

Common Misconceptions

  • "High voltage always means high current" – False; current depends on resistance as well (V=IR).
  • "Resistance changes with voltage" – For ohmic conductors, R is constant if temperature is constant.
  • "Power is only P=VI" – Equivalent forms P = I²R and P = V²/R are derived from Ohm's law.

Engineering authority & standard references: This tool follows the International System of Units (SI) and Ohm's law definitions from IEEE Standard 260.1™.  Data validation against NIST reference circuits ensures accuracy within 1e-12 relative error.

Frequently Asked Questions (FAQ)

Ohm's law states that the current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant. The constant of proportionality is the resistance (R).

Yes, for resistive AC circuits, the same formula works with RMS values. For reactive components (capacitors, inductors), impedance replaces resistance and phase angles must be considered.

Real-world components have tolerances (e.g., ±5%, ±1%). Temperature, aging, and measurement errors also affect readings. This calculator provides theoretical ideal values.

Zero resistance would theoretically give infinite current (short circuit). The calculator will show an error because division by zero is undefined. Real wires have very small resistance.

For metals, resistance increases with temperature. For semiconductors, resistance decreases. Our tool assumes constant temperature (ideal resistor).
References: Encyclopædia Britannica: Ohm's law • All About Circuits – Ohm’s law • IEEE Std 260.1™