Ohm’s Law Calculator

Ohm's Law Calculator

Fill in any two fields and click Calculate. The other two will be computed.

Formula Reference Card

V = I × R
Voltage
I = V / R
Current
P = V × I
Power
R = V / I
Resistance
P = I² × R
P = V² / R

Ohm's Law Calculator — Voltage, Current, Resistance & Power

Ohm's Law is one of the foundational relationships in electronics and electrical engineering. This free online Ohm's Law calculator lets you solve for any two unknowns — Voltage (V), Current (I), Resistance (R), and Power (P) — by entering any two known values. It's an essential tool for hobbyists, students, and professionals working with circuits.

What Is Ohm's Law?

Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality: V = I × R. Stated simply, doubling the voltage doubles the current (at constant resistance), and doubling the resistance halves the current. This relationship was discovered by German physicist Georg Simon Ohm and published in 1827.

The Four Key Formulas

The four electrical quantities — Voltage (V), Current (I), Resistance (R), and Power (P) — are interrelated through six core equations:

  • V = I × R — Voltage equals current times resistance.
  • I = V / R — Current equals voltage divided by resistance.
  • R = V / I — Resistance equals voltage divided by current.
  • P = V × I — Power equals voltage times current.
  • P = I² × R — Power equals current squared times resistance.
  • P = V² / R — Power equals voltage squared divided by resistance.

Practical Applications

Ohm's Law is used every day in electronics design. When you choose a resistor to protect an LED, you are applying Ohm's Law. When an electrician sizes wiring for a circuit, the same principle governs how much current a given voltage will drive through a conductor. Battery-powered devices rely on these relationships to determine battery life. Audio amplifiers, motor controllers, and power supplies are all designed using Ohm's Law at their core.

Tips for Using This Calculator

Enter any two of the four values (V, I, R, P) and the calculator will derive the remaining two using the appropriate formulas. If you enter three values, it uses the first pair it finds. Always ensure values are in base SI units: Volts, Amps, Ohms, and Watts. For milliamps, divide by 1000 before entering (e.g., 20 mA = 0.02 A).

Frequently Asked Questions

What units should I enter in the calculator?
Enter voltage in Volts (V), current in Amperes (A), resistance in Ohms (Ω), and power in Watts (W). For milliamps convert: 50 mA = 0.05 A. For kilohms convert: 10 kΩ = 10000 Ω.
Can I calculate all four values from just one?
No — you need at least two known values to solve for the others. With only one value there are infinitely many valid combinations of the remaining three quantities.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies directly to DC (direct current) resistive circuits. For AC circuits, a more general form uses impedance (Z) instead of resistance, but the principle V = IZ is identical in structure.
What is the difference between resistance and impedance?
Resistance (R) is the opposition to current flow in a purely resistive element and is constant regardless of frequency. Impedance (Z) includes the effects of capacitance and inductance and varies with frequency in AC circuits.
Why is power important in circuit design?
Every component has a maximum power rating. If the actual power dissipated in a resistor exceeds its rating it will overheat and fail. Calculating power ensures you choose components with adequate ratings for safe and reliable operation.