Ohms Law Calculator
Solve for voltage, current, resistance, or power. Enter any two known values to calculate the other two instantly.
Your Results
- Select what you want to solve for — voltage (V), current (I), resistance (R), or power (P).
- Enter any two known values — the calculator solves for all four using V=IR and P=VI relationships.
- Leave the unknown fields blank — you only need two inputs — the calculator derives the rest automatically.
- Check the power result — watts tell you the heat and energy dissipated in the circuit or component.
- Use for component selection — resistance values help select resistors; power values determine the wattage rating needed.
What Is Ohm's Law?
Ohm's Law states that the voltage (V) across a conductor equals the current (I) flowing through it multiplied by its resistance (R): V = I × R. This single relationship is the foundation of all electrical circuit analysis. From it, current can be expressed as I = V ÷ R, and resistance as R = V ÷ I.
A fourth quantity — power (P) — relates to the others through P = V × I. Combining this with Ohm's Law gives additional forms: P = I² × R and P = V² ÷ R. Together these four formulas allow you to solve for any unknown in a circuit as long as two values are known. The calculator above handles all combinations automatically.
Ohm's Law Formula Wheel
All 12 forms of the Ohms Law relationships:
| Solve for | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current (I) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (R) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (P) | P = V × I | P = I² × R | P = V² ÷ R |
Any two known values are enough to solve for all four quantities. The calculator automatically selects the correct formula based on which values you provide.
Common Circuit Examples
| Circuit | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| LED indicator | 3.3V | 20mA | 165Ω | 66mW |
| 60W incandescent bulb | 120V | 500mA | 240Ω | 60W |
| 1500W space heater | 120V | 12.5A | 9.6Ω | 1,500W |
| EV charger (Level 2) | 240V | 32A | 7.5Ω | 7,680W |
| Car headlight | 12V | 4.2A | 2.9Ω | 50W |
| Laptop charger | 19.5V | 3.3A | 5.9Ω | 65W |
Ohm's Law applies to resistive (DC) circuits and AC circuits with purely resistive loads. For AC circuits with inductive or capacitive loads, impedance (Z) replaces resistance (R) in the formula.
Core Formulas
The four essential electrical relationships:
Current = I = V ÷ R (amps = volts ÷ ohms)
Resistance = R = V ÷ I (ohms = volts ÷ amps)
Power = P = V × I (watts = volts × amps)
Power alt. = P = I² × R or P = V² ÷ R
Ohm's Law applies to DC circuits and resistive AC loads. In AC circuits with motors, capacitors, or inductors, use impedance (Z) instead of resistance — the formula structure remains the same: V = I × Z.
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Frequently Asked Questions
What is Ohm's Law in simple terms?
Ohms Law says that voltage equals current times resistance: V = I × R. Think of it like water in a pipe: voltage is the water pressure, current is the flow rate, and resistance is how narrow the pipe is. More pressure (voltage) pushes more water (current) through a narrow pipe (resistance). Double the resistance with the same voltage and the current halves.
How do I use this calculator?
Select what you want to solve for (voltage, current, resistance, or power) from the dropdown. Then enter any two of the remaining values — leave the others blank. The calculator automatically applies the correct formula and returns all four values. You only ever need two inputs to solve the full circuit.
Does Ohm's Law work for AC circuits?
Ohm's Law in its basic form (V = I × R) applies to DC circuits and purely resistive AC loads like heaters and incandescent lights. For AC circuits that include motors, transformers, capacitors, or coils, resistance is replaced by impedance (Z), which accounts for the phase shift between voltage and current. The formula becomes V = I × Z, but the same mathematical structure applies.
What is the difference between resistance and impedance?
Resistance (R, measured in ohms Ω) opposes current flow in DC and AC resistive circuits and converts electrical energy to heat. Impedance (Z, also in ohms) is the AC equivalent — it combines resistance with reactance, which is the opposition to current from inductors (coils) and capacitors. For purely resistive loads, Z = R. For reactive loads, Z = √(R² + X²) where X is the reactance.
What two values do I need to solve the rest?
You can solve the circuit if you know any two of these four values: voltage (V), current (I), resistance (R), and power (P). The calculator uses Ohms Law (V = I × R) plus the power relationships to compute the remaining values.
How do I calculate power in watts?
Power is calculated as P = V × I. Using Ohms Law, you can also write power as P = I² × R or P = V² ÷ R, which is useful when you know resistance and one other value.