Ohm's Law Calculator

What is Ohm's Law?

Ohm's Law is the foundational relationship in electrical engineering, describing how voltage, current, and resistance interact in a circuit. Formulated by Georg Ohm in 1827, it states that current through a conductor is directly proportional to voltage and inversely proportional to resistance.

Combined with the power equation (P = V x I), Ohm's Law enables calculation of any electrical quantity when two others are known. This is essential for circuit design, component selection, wire sizing, and power supply specification.

Ohm's Law applies to resistive (ohmic) circuits. For components like diodes and transistors with non-linear V-I characteristics, the law applies only to small-signal analysis around an operating point.

Formula: V = I x R (Voltage = Current x Resistance) I = V / R R = V / I P = V x I = I^2 x R = V^2 / R

Example Calculation

A 12V power supply drives a 100 ohm resistor. Current = 12V / 100 ohm = 0.12A (120mA). Power dissipated = 12V x 0.12A = 1.44W. The resistor must be rated for at least 1.44W.

When to Use This Calculator

• Calculating the current drawn by a load to size power supplies, fuses, and wire gauges
• Determining the resistance value needed to limit current in a circuit (e.g., current-limiting resistors for LEDs or sensor pull-ups)
• Computing power dissipation in resistive components to select appropriate wattage ratings and thermal management
• Solving for unknown voltage drops across components during circuit analysis and debugging

Common Mistakes to Avoid

• Applying Ohm's Law to non-linear devices (diodes, transistors, LEDs) as if they were resistors — these devices have voltage-dependent resistance and require device-specific models
• Mixing units — using milliamps with volts and ohms without converting gives results off by a factor of 1000; always convert to base units (V, A, Ω) first
• Forgetting to account for power supply internal resistance — the output voltage drops under load, so the actual voltage across the load is less than the open-circuit supply voltage
• Using Ohm's Law at DC to predict behavior in high-frequency circuits — at RF frequencies, parasitic inductance and capacitance dominate, and impedance rather than resistance governs the circuit

How to Interpret Results

• With any two of voltage, current, and resistance known, the other two (and power) are fully determined — the tool solves for whichever quantity you leave blank.
• Power can be found three equivalent ways (V × I, I² × R, V² / R); they must agree, so a mismatch means one input is wrong.
• Doubling the current at fixed resistance quadruples the power (P = I²R) — this is why conductor and resistor heating rises sharply with current.
• Real parts have tolerance and temperature drift; treat the result as nominal and keep design margin, especially against power-dissipation ratings.

Related Standards & References

• Ohm's law (V = I × R) — the foundational DC circuit relation, first published by Georg Ohm in 1827.
• IEC 60027 — Letter symbols to be used in electrical technology, standardizing the V, I, R, and P notation.
• Joule's first law (P = I²R) — relates electrical power dissipation to current and resistance.

Frequently Asked Questions

Does Ohm's Law apply to AC circuits?

Ohm's Law applies to purely resistive AC circuits. For circuits with capacitors or inductors, impedance (Z) replaces resistance: V = I × Z, where Z is a complex number combining resistance and reactance. For simple resistive loads like heaters and incandescent bulbs, the DC form of Ohm's Law works perfectly even with AC.

Why does power have four different formulas?

The four power formulas (P = VI, P = I²R, P = V²/R, P = V²/R) are all derived from Ohm's Law by substitution. Use whichever matches the two values you know: P = VI when you know voltage and current, P = I²R when you know current and resistance, P = V²/R when you know voltage and resistance. All four give identical results for resistive circuits.