Ohm's Law Calculator

Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I × R. Combined with the power formula P = V × I, these two equations allow any of the four quantities to be found from any other two. Enter exactly two known values and the calculator solves for the remaining two.

The four equations:

V = I × R | I = V / R | R = V / I | P = V × I = I² × R = V² / R

Ohm's Law is the most fundamental relationship in electrical engineering and electronics. It was formulated by German physicist Georg Simon Ohm in 1827 and remains the starting point for analyzing virtually every resistive circuit — from simple LED resistor calculations to complex power distribution systems. Understanding these relationships is essential for anyone working with electronics, home wiring, or electrical safety.

The Four Variables

Variable	Symbol	Unit	What It Represents
Voltage	V or E	Volts (V)	Electrical potential difference — the "pressure" that drives current
Current	I	Amperes (A)	Rate of charge flow — electrons passing a point per second
Resistance	R	Ohms (Ω)	Opposition to current flow — higher R means less current for the same voltage
Power	P	Watts (W)	Rate of energy transfer — one joule of energy per second

Practical Applications of Ohm's Law

LED resistor calculation: An LED needs 20 mA at 2 V forward voltage on a 5 V supply. The required resistor: R = (5 − 2) ÷ 0.02 = 150 Ω. Power dissipated: P = 0.02² × 150 = 0.06 W, so a standard 1/4 W resistor is fine.
Fuse selection: A 240 V appliance rated at 1,200 W draws I = P ÷ V = 1200 ÷ 240 = 5 A. Choose a 5 A or 6 A fuse.
Wire gauge selection: Knowing the current allows you to calculate the minimum conductor size to limit resistive heating and voltage drop.
Battery life estimation: A 10,000 mAh battery powering a 500 mA device will last approximately 10,000 ÷ 500 = 20 hours.

Ohm's Law in Series and Parallel Circuits

In a series circuit, resistors add directly: R_total = R₁ + R₂ + R₃. The same current flows through each component. In a parallel circuit, the reciprocal rule applies: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃. For two resistors in parallel: R_total = (R₁ × R₂) ÷ (R₁ + R₂).

Limitations of Ohm's Law

Ohm's Law applies to linear (ohmic) resistors at constant temperature. It does not accurately describe components whose resistance varies with voltage or current:

Diodes: Conduct heavily above a threshold voltage and block current in reverse. Resistance is not constant.
Transistors: Resistance changes dramatically with bias conditions — they function as amplifiers and switches.
Light bulbs: Resistance increases significantly as the filament heats up.
AC circuits with reactance: Capacitors and inductors introduce reactance, requiring the use of impedance (Z) rather than pure resistance.

Power and Heat in Resistive Components

Every resistor dissipates power as heat: P = I² × R. A 100 Ω resistor carrying 0.1 A dissipates 0.1² × 100 = 1 W of heat. Always select a component rated at least twice the calculated dissipation for a safety margin in normal use.

Frequently Asked Questions

What is Ohm's Law and what does it calculate?

Ohm's Law states that the voltage across a resistor equals the current through it multiplied by its resistance: V = I × R. Combined with the power formula P = V × I, you can calculate any of the four quantities — voltage, current, resistance, or power — from any other two. It is the foundational relationship used in all resistive circuit analysis.

How do I use this calculator — what values do I enter?

Enter exactly two known values and leave the other two empty. For example, if you know a component has 12 V across it and 2 A flowing through it, enter V = 12 and I = 2; the calculator returns R = 6 Ω and P = 24 W. Any pair of the four variables (V, I, R, P) can be used as the two inputs.

What resistor do I need for an LED?

Use Ohm's Law: R = (Supply Voltage − LED Forward Voltage) ÷ LED Current. A typical red LED has a forward voltage of about 2 V and runs at 20 mA (0.02 A). On a 5 V supply: R = (5 − 2) ÷ 0.02 = 150 Ω. The power dissipated in the resistor is P = I² × R = 0.02² × 150 = 0.06 W, so a standard 1/4 W resistor is adequate.

What is the difference between resistance and impedance?

Resistance (R, measured in ohms) is a pure opposition to current flow that applies equally to DC and AC circuits. Impedance (Z, also in ohms) is the total opposition in an AC circuit, which includes resistance plus reactance from capacitors and inductors. Ohm's Law applies to AC circuits using impedance: V = I × Z.

Why does my light bulb draw more current when cold?

Incandescent light bulb filaments are made of tungsten, which has a resistance that increases with temperature. A cold filament at room temperature has much lower resistance than it does at its operating temperature of around 2,500 °C. At the moment you switch a bulb on, the cold filament draws a surge current 10–15 times higher than normal operating current.

What happens if I exceed the power rating of a resistor?

A resistor that dissipates more power than its rating will overheat. At moderate overloads it will drift in value and degrade over time. At severe overloads it will burn — possibly catching fire or producing smoke. Always select resistors with a power rating of at least twice the calculated dissipation for a safety margin.

Can Ohm's Law be applied to whole circuits, not just individual components?

Yes. Ohm's Law applies to any part of a circuit where there is a single equivalent resistance. For a series circuit, add all resistances; for parallel, use the reciprocal formula. If a 9 V battery drives 0.3 A through a circuit, the total circuit resistance is R = 9 ÷ 0.3 = 30 Ω.

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