Quick answer: Ohm's Law is V = I × R (voltage = current × resistance), and electrical power is P = V × I. Enter any two of the four values — voltage, current, resistance, power — and this calculator finds the other two instantly.
Last reviewed: September 2026
Enter any two values; the other two are calculated automatically.
Key facts
- V = I × R — Ohm's Law: voltage equals current times resistance.
- P = V × I = I²R = V²/R — three equivalent power formulas.
- Any two of V, I, R, P determine the other two uniquely.
- Always convert mA, kΩ, mV to base units (A, Ω, V) before calculating.
This free Ohm's Law calculator solves V = IR and the power formulas for any unknown values. Enter any two of voltage, current, resistance or power and get the other two instantly, with the formulas used shown for every result.
Perfect for students, hobbyists and electricians checking circuits, component ratings and power dissipation. Results update live as you type, with no sign-up required.
How to use
- Enter any two known values among voltage (V), current (A), resistance (Ω) and power (W).
- Leave the two unknowns blank — they are solved automatically.
- Read all four values in the results panel, including the formulas used.
- Check units: convert mA to A and kΩ to Ω before entering.
The Ohm's Law formulas
| Find | Given | Formula |
|---|---|---|
| V | I, R | V = I × R |
| V | P, I | V = P ÷ I |
| V | P, R | V = √(P × R) |
| I | V, R | I = V ÷ R |
| I | P, V | I = P ÷ V |
| I | P, R | I = √(P ÷ R) |
| R | V, I | R = V ÷ I |
| R | V, P | R = V² ÷ P |
| R | I, P | R = P ÷ I² |
| P | V, I | P = V × I |
| P | V, R | P = V² ÷ R |
| P | I, R | P = I² × R |
Worked examples
Find resistance and power from 12 V, 2 A
Given V = 12, I = 2. R = V ÷ I = 12 ÷ 2 = 6 Ω. P = V × I = 12 × 2 = 24 W.
So a 12-volt source driving 2 amps flows through 6 ohms and delivers 24 watts.
Find current and resistance from 100 W, 120 V
Given P = 100, V = 120. I = P ÷ V = 100 ÷ 120 = 0.833 A. R = V² ÷ P = 14,400 ÷ 100 = 144 Ω.
A 100-watt device at 120 volts draws about 0.83 amps with an equivalent resistance of 144 ohms.
Common unit prefixes
| Prefix | Symbol | Multiplier | Example |
|---|---|---|---|
| milli | m | 0.001 | 500 mA = 0.5 A |
| kilo | k | 1,000 | 2.2 kΩ = 2,200 Ω |
| mega | M | 1,000,000 | 1 MΩ = 1,000,000 Ω |
| micro | µ | 0.000001 | 100 µA = 0.0001 A |
Frequently asked questions
What is Ohm's Law in simple words?
Ohm's Law says the current through a conductor equals the voltage across it divided by its resistance (I = V ÷ R). Double the voltage and the current doubles; double the resistance and the current halves.
Which two values do I need to enter?
Any two of voltage (V), current (I), resistance (R) and power (P). The calculator picks the right rearranged formulas and solves the remaining two automatically.
How do I calculate power with Ohm's Law?
Use P = V × I when you know voltage and current, P = V² ÷ R when you know voltage and resistance, or P = I² × R when you know current and resistance. All three are the same relationship rearranged.
Why is my answer wrong by a factor of 1,000?
You probably mixed units. Convert milliamps to amps and kilo-ohms to ohms before calculating: 500 mA = 0.5 A and 2.2 kΩ = 2,200 Ω.
Does Ohm's Law work for AC circuits?
For purely resistive AC circuits, yes — use RMS voltage and current. Circuits with capacitors or inductors need impedance (Z) instead of resistance, which is beyond simple Ohm's Law.
Sources
- OpenStax — University Physics: Ohm's Law and electrical power
- NIST — SI units for electrical quantities
Key takeaways
- V = I × R is the core relationship; P = V × I gives the power.
- Any two of V, I, R, P determine the other two.
- Convert mA, kΩ and mV to base units first.
- The calculator shows which formula it used for transparency.
Explore more tools
Physics tools | All calculators | More tools
Related: Solar Savings Calculator
Comments
Leave a comment