Ohm’s Law Calculator
Enter any two of voltage, current, resistance, and power, and this calculator solves the other two using Ohm’s Law (V = I × R) and the power law (P = V × I). Unit selects handle millivolts, milliamps, kilohms, and the rest, so you can mix prefixes freely.
Enter exactly two values and clear the rest. The calculator fills in the other two.
How Ohm’s Law works
Ohm’s Law ties together the three basic quantities of a simple electrical circuit. Voltage is the push that drives charge, current is the flow of charge, and resistance is the opposition to that flow. They relate by V = I × R: raise the voltage and current rises; raise the resistance and current falls. Power — the rate energy is delivered — adds a fourth quantity through P = V × I. Because these two equations connect all four, knowing any two lets you compute the rest, which is exactly what this calculator does.
How it’s calculated
The tool converts each entry to base units (volts, amps, ohms, watts), then figures out which two you supplied. It derives the missing pair from the appropriate identities: V = I × R, I = V ÷ R, R = V ÷ I, and P = V × I (equivalently P = I²R or V²/R). If power is one of your knowns, it back-solves for the matching V and I. Results are shown in base units.
Assumes an ideal ohmic resistor with constant resistance. Real components vary with temperature, and reactive parts (capacitors, inductors) require impedance, not plain resistance.
The four formulas
| Find | From V & I | From V & R | From I & R |
|---|---|---|---|
| Voltage V | — | — | I × R |
| Current I | — | V ÷ R | — |
| Resistance R | V ÷ I | — | — |
| Power P | V × I | V² ÷ R | I² × R |
Any two known quantities determine the other two.
Worked example
A component has 12 V across it and a resistance of 4 Ω. The current is I = V ÷ R = 12 ÷ 4 = 3 A, and the power dissipated is P = V × I = 12 × 3 = 36 W. If instead you knew the current was 3 A through 4 Ω, you would get the same 12 V and 36 W.
Common mistakes
- Entering three or four values — supply exactly two and clear the others.
- Forgetting prefixes: 10 kΩ is 10,000 Ω, and 5 mA is 0.005 A.
- Mixing DC formulas into reactive AC circuits, where impedance replaces resistance.
- Confusing power (watts) with energy (watt-hours) — power is the rate, not the total.
Where it is used
- Sizing resistors and current-limiting components in electronics.
- Checking whether a part will exceed its power rating.
- LED and hobby-circuit design.
- Teaching and studying basic circuit theory.
Frequently asked questions
What is Ohm’s Law?
Ohm’s Law states that voltage equals current times resistance: V = I × R. Rearranged, current I = V ÷ R and resistance R = V ÷ I. It holds for resistive (ohmic) components where resistance stays constant across a range of voltages.
How do you calculate power in a circuit?
Power is voltage times current: P = V × I. Combining with Ohm’s Law gives two more forms: P = I² × R and P = V² ÷ R. Any of these works depending on which two quantities you know.
Which two values do I need to enter?
Any two of the four — voltage, current, resistance, or power. With two known values the calculator can always derive the remaining two using V = I × R and P = V × I. Leave the two you do not know blank.
What units does it accept?
Voltage in millivolts, volts, or kilovolts; current in milliamps or amps; resistance in ohms, kilohms, or megohms; and power in milliwatts, watts, or kilowatts. The tool converts everything to base SI units internally, so you can mix prefixes.
Does Ohm’s Law work for AC circuits?
For purely resistive AC circuits, yes, using RMS values. When capacitors or inductors are present, resistance is replaced by impedance and phase matters, so the simple V = I × R form no longer captures the full behavior.