What Is Ohm's Law?
Ohm's Law is the foundational relationship in electrical engineering, stating that the voltage across a conductor is directly proportional to the current flowing through it, provided temperature remains constant. Published by Georg Simon Ohm in 1827, this simple relationship underlies the analysis of virtually every electrical circuit.
Beyond voltage, current, and resistance, the related power law adds a fourth variable: power. These four quantities describe the complete electrical behavior of any resistive element in a circuit.
What This Calculator Does
This calculator solves for any two unknown electrical quantities when two are known. You never need to remember which formula to use: enter what you know and the calculator finds the rest.
- Inputs: Any two of: Voltage (V), Current (I), Resistance (R), or Power (P)
- Outputs: The remaining two unknown quantities
How the Calculation Works
V = I × R
I = V / R
R = V / I
P = V × I = I² × R = V² / R
- V (Voltage): The electrical potential difference measured in volts (V). Think of it as the pressure pushing current through the circuit
- I (Current): The rate of charge flow measured in amperes (A). This is the amount of electrons passing a point per second
- R (Resistance): The opposition to current flow measured in ohms (Ω). Higher resistance means less current for the same voltage
- P (Power): The rate of energy conversion measured in watts (W). Power dissipated as heat in a resistor equals I² × R
How to Use the Calculator
- Identify which two values you know from your circuit
- Enter those two values in the appropriate fields
- Leave the other two fields empty
- Click Calculate to solve for the unknowns
- Results highlighted in blue are the calculated values
Example Calculations
Example 1: Finding Current
A 12 V battery is connected to a 120 Ω resistor. Using I = V / R: current = 12 / 120 = 0.1 A (100 mA). Power dissipated = V × I = 12 × 0.1 = 1.2 W.
Example 2: LED Current Limiting
An LED needs 20 mA (0.02 A) of current from a 5 V supply. The LED drops 2 V, leaving 3 V for the resistor. Using R = V / I: R = 3 / 0.02 = 150 Ω. Power in the resistor = I² × R = 0.02² × 150 = 0.06 W. A quarter-watt resistor is more than adequate.
Example 3: Power Dissipation Check
A 1 kΩ resistor in a 24 V circuit draws I = 24 / 1000 = 24 mA and dissipates P = 24 × 0.024 = 0.576 W. A standard quarter-watt (0.25 W) resistor would overheat. A half-watt or 1 W resistor is required.
Real-World Scenarios
Electronics Prototyping
Every time you add an LED, motor driver, or sensor to a microcontroller circuit, Ohm's Law determines whether the component will receive the correct current and whether the supply can handle the load.
Electrical Fault Diagnosis
Technicians use Ohm's Law to diagnose faults. Measuring voltage and current in a circuit segment and then calculating the expected resistance lets you identify short circuits, open circuits, and degraded components.
Heating Element Design
Electric heaters, toasters, and hair dryers are essentially precision resistors. The power formula P = V² / R allows engineers to specify the exact resistance needed to produce a target heating power from the supply voltage.
Why This Calculation Matters
Ohm's Law is not just for electronics engineers. Electricians use it to size wiring and breakers. Hobbyists use it to safely design LED and battery-powered projects. Understanding the relationship between voltage, current, resistance, and power prevents equipment damage, fires, and personal injury.
Common Mistakes to Avoid
- Confusing units: Current must be in amperes, not milliamps. Convert 20 mA to 0.02 A before entering it in the formula
- Applying Ohm's Law to non-linear components: Ohm's Law applies to resistors and purely resistive loads. Diodes, transistors, and capacitors do not follow this simple relationship
- Ignoring power ratings: Calculating the correct resistance value is not enough. You must also ensure the resistor's power rating exceeds the calculated wattage
- Assuming fixed resistance: Real resistors change value with temperature. For precision work, use resistors with low temperature coefficients