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Calculate voltage, current, resistance, or power using Ohm's Law. Enter any two values to find the others.
V = I × R
V = P ÷ I
V = √(P × R)
I = V ÷ R
I = P ÷ V
I = √(P ÷ R)
R = V ÷ I
R = V² ÷ P
R = P ÷ I²
P = V × I
P = I² × R
P = V² ÷ R
V = I × R
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. This fundamental relationship is essential for all electrical calculations.
Ohm's Law is one of the most fundamental principles in electrical engineering, discovered by German physicist Georg Simon Ohm in 1827. It defines the relationship between three key electrical quantities: voltage (V), current (I), and resistance (R). The law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them, expressed as V = I × R. This simple yet powerful equation forms the foundation of virtually all circuit analysis and electrical design. Whether you are sizing a resistor for an LED, calculating the current draw of a motor, determining the voltage drop across a wire, or troubleshooting a faulty circuit, Ohm's Law provides the essential mathematical framework. Combined with the power equation (P = V × I), it enables engineers and technicians to solve for any electrical parameter when two values are known.
Determine which two electrical quantities you already know. You need any two of voltage (V), current (I), resistance (R), or power (P) to calculate the remaining values.
Choose the appropriate formula variation. For voltage: V = I × R. For current: I = V ÷ R. For resistance: R = V ÷ I. For power: P = V × I.
Make sure all values use base units: volts (V), amps (A), and ohms (Ω). Convert milliamps to amps (divide by 1,000) or kilohms to ohms (multiply by 1,000) before calculating.
Plug in your values and compute the result. Verify by substituting the result back into another form of the equation. For example, if you found current, check that V = I × R gives the correct voltage.
Ohm's Law is essential for selecting component values, sizing wires, and ensuring circuits operate within safe electrical limits.
Technicians use Ohm's Law to diagnose faults by measuring voltage and current to identify open circuits, short circuits, and failed components.
Understanding the relationship between voltage, current, and resistance is critical for preventing electrical hazards like overheating and fires.
| Voltage (V) | Current (A) | Resistance (Ω) | Power (W) |
|---|---|---|---|
| 5 V | 0.5 A | 10 Ω | 2.5 W |
| 12 V | 1.0 A | 12 Ω | 12.0 W |
| 24 V | 2.0 A | 12 Ω | 48.0 W |
| 120 V | 1.0 A | 120 Ω | 120.0 W |
| 120 V | 10.0 A | 12 Ω | 1,200 W |
| 230 V | 5.0 A | 46 Ω | 1,150 W |
| 240 V | 20.0 A | 12 Ω | 4,800 W |
| 480 V | 10.0 A | 48 Ω | 4,800 W |
If resistance is zero and voltage is applied, the formula predicts infinite current, which represents a short circuit. In practice, every conductor has some resistance, but very low resistance paths can cause dangerously high currents and are a major safety concern.
Power can be calculated by combining Ohm's Law with the power formula. The three variations are: P = V × I, P = I² × R, and P = V² ÷ R. Each form is useful depending on which two values you know.
Ohm's Law is used to size wires and fuses, design LED circuits with current-limiting resistors, troubleshoot electrical faults, calculate heating element specifications, and determine battery discharge rates in electronics projects.
Ohm's Law helps determine safe operating limits for circuits. It allows engineers to calculate maximum current flow, select appropriate wire gauges, size circuit breakers correctly, and prevent overheating or fire hazards in electrical installations.
Ohm's Law applies perfectly to ohmic materials like most metals at constant temperature. Non-ohmic materials such as diodes, transistors, and thermistors do not follow a linear voltage-current relationship and require more advanced analysis.