Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law. Enter any two known values to find the unknown parameters.
Amperes (A)
Ohms (Ω)
Ohm's Law is the fundamental relationship between voltage, current, and resistance in electrical circuits. German physicist Georg Ohm published it in 1827. It states that current through a conductor is directly proportional to voltage and inversely proportional to resistance.
The Core Formula
V = I × R
Voltage = Current × Resistance
Voltage (V)
Electrical pressure that pushes electrons through a circuit. Measured in volts. Think of it like water pressure in a pipe.
Current (I)
Flow rate of electrons through the circuit. Measured in amperes (amps). Like the volume of water flowing through a pipe.
Resistance (R)
Opposition to current flow. Measured in ohms (Ω). Similar to pipe diameter - narrower pipes restrict flow more.
| To Find | Formula | When You Know |
|---|---|---|
| Voltage (V) | V = I × R | Current and Resistance |
| Voltage (V) | V = P / I | Power and Current |
| Current (I) | I = V / R | Voltage and Resistance |
| Current (I) | I = P / V | Power and Voltage |
| Resistance (R) | R = V / I | Voltage and Current |
| Resistance (R) | R = V² / P | Voltage and Power |
| Power (P) | P = V × I | Voltage and Current |
| Power (P) | P = I² × R | Current and Resistance |
| Power (P) | P = V² / R | Voltage and Resistance |
| Device | Voltage | Current | Resistance |
|---|---|---|---|
| AA Battery | 1.5V | Varies by load | Internal: ~0.15Ω |
| Car Battery | 12V | Up to 500A (starting) | Internal: ~0.02Ω |
| USB Port | 5V | 0.5A - 3A | Device dependent |
| US Wall Outlet | 120V | 15A - 20A (circuit) | Device dependent |
| LED (typical) | 2-3V | 20mA | ~150Ω (with resistor) |
| 60W Light Bulb | 120V | 0.5A | 240Ω (hot) |
Example 1: Finding Current
You have a 12V battery connected to a 100Ω resistor. How much current flows?
Example 2: Choosing a Resistor for an LED
You want to power a 2V LED with a 9V battery at 20mA. What resistor do you need?
Voltage across resistor = 9V - 2V = 7V
R = V / I = 7V / 0.02A = 350Ω
Use nearest standard value: 330Ω or 390Ω
Example 3: Power Dissipation
A 10Ω resistor has 2A flowing through it. How much power does it dissipate as heat?
P = I² × R = (2A)² × 10Ω = 4 × 10 = 40W
You would need a 50W+ resistor to avoid burning it up!
Does Ohm's Law apply to all circuits?
Ohm's Law applies to ohmic materials - resistors, wires, and simple conductors. It does not directly apply to non-ohmic components like diodes, transistors, or LEDs, which have non-linear voltage-current relationships. For AC circuits with capacitors and inductors, you need to consider impedance instead of simple resistance.
Why is current denoted by I?
The symbol I comes from the French phrase "intensité du courant" (current intensity), used by André-Marie Ampère in the early 1800s. The unit of current (ampere) is named after him. Voltage (V) and resistance (R) use more intuitive abbreviations.
What happens if resistance is zero?
If resistance approaches zero, current approaches infinity according to I = V/R. This is a short circuit - extremely dangerous as it can cause fires or explosions. Superconductors have zero resistance but require special conditions (extreme cold) and have current limits.
How do I calculate resistance for multiple resistors?
For series resistors: R_total = R1 + R2 + R3 + ... For parallel resistors: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... Two parallel resistors simplify to: R_total = (R1 × R2) / (R1 + R2). Series increases resistance; parallel decreases it.
Can I use Ohm's Law for AC circuits?
For purely resistive AC circuits, yes. But most AC circuits have inductance and capacitance, creating reactance. You then use impedance (Z) instead of resistance: V = I × Z. Impedance combines resistance and reactance and depends on frequency.
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