Ohm's Law Calculator

Ohm's Law is the foundation of all electrical circuit analysis. This calculator lets you find voltage, current, resistance, or power when you know any two of the four values — instantly, with no formula lookup required.

What Is Ohm's Law?

Ohm's Law states that the voltage across a conductor is directly proportional to the current through it, given the resistance is constant. It's expressed as V = I × R and applies to almost all resistive electrical circuits.

How to Use This Calculator

1. Enter any two known values from: voltage (V), current (I), resistance (R), or power (P).
2. Select the appropriate units for each value.
3. Click Calculate to solve for the remaining unknowns.

Ohm's Law Formulas

• Voltage: V = I × R
• Current: I = V ÷ R
• Resistance: R = V ÷ I
• Power: P = V × I = I² × R = V² ÷ R

Example Calculations

Find current: A 12V battery connected to a 100Ω resistor.

• I = V ÷ R = 12 ÷ 100 = 0.12 A (120 mA)

Find power: A 120V circuit drawing 5A.

• P = V × I = 120 × 5 = 600 Watts

Find resistance: 9V applied, 30mA current.

• R = V ÷ I = 9 ÷ 0.030 = 300 Ω

Why Use This Calculator?

Ohm's Law is used in circuit design, troubleshooting, motor sizing, and power calculations. Whether you're a student, hobbyist, or professional electrician, this calculator resolves any Ohm's Law problem with any two known values.

Common Mistakes to Avoid

• Confusing milliamps and amps — always convert mA to A (divide by 1,000) before using the formula.
• Applying Ohm's Law to non-linear components — diodes, transistors, and LEDs don't follow Ohm's Law at all voltages. The formula applies mainly to resistors and simple resistive loads.
• Ignoring units — make sure voltage is in volts, current in amps, and resistance in ohms before calculating.

Frequently Asked Questions

What does V = IR mean?

V is voltage (volts), I is current (amps), and R is resistance (ohms). The formula says: voltage equals current times resistance. If you know any two, you can find the third.

What is the power formula in Ohm's Law?

P = V × I. You can also express it as P = I² × R or P = V² ÷ R — all three give the same result for a given circuit.

Can Ohm's Law be used for AC circuits?

Yes, but AC circuits also involve impedance (Z) — a combination of resistance and reactance. For simple resistive AC loads, Ohm's Law applies directly. For circuits with capacitors or inductors, use impedance instead of resistance.

What is a good current for an LED?

Most standard LEDs operate best at 10–20 mA. Use Ohm's Law to calculate the correct series resistor for your supply voltage.

How much resistance does 120V at 15A represent?

R = V ÷ I = 120 ÷ 15 = 8 Ω. This is the equivalent resistance of a 1,800-watt load on a 120V circuit.

Conclusion

Ohm's Law is simple, powerful, and used in virtually every electrical application. This calculator makes it accessible for anyone — from students solving textbook problems to technicians diagnosing circuits in the field.

Related: Resistor Calculator | Voltage Drop Calculator | Electricity Calculator