How this Ohm's law calculator works
Ohm's law describes the relationship between voltage, current and resistance in an electrical circuit: V = I × R. Voltage (measured in volts) is the push that drives charge, current (in amperes) is the rate of flow, and resistance (in ohms, Ω) opposes that flow. Rearranging the same equation gives I = V ÷ R and R = V ÷ I.
Electrical power — the rate at which energy is used, measured in watts (W) — is P = V × I. Substituting Ohm's law into that expression produces the two handy forms P = I²R and P = V²/R. Together these let the calculator start from any two of the four quantities and derive the remaining two.
The four base formulas (the Ohm's law wheel)
| To find | Formula | Units |
|---|---|---|
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) | volts |
| Current (I) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) | amperes |
| Resistance (R) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² | ohms (Ω) |
| Power (P) | P = V × I | P = I² × R | P = V² ÷ R | watts (W) |
Reference note: these relationships assume a simple linear (ohmic) DC resistance. Real components, AC circuits and non-ohmic devices can behave differently, but the formulas above cover the everyday cases this calculator is built for.
Frequently asked questions
- What is Ohm's law?
- Ohm's law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. It is written V = I × R, where V is volts, I is amperes and R is ohms.
- How do I calculate current?
- Rearrange Ohm's law to I = V ÷ R: divide the voltage by the resistance. For example, 12 V across 4 Ω gives 12 ÷ 4 = 3 A.
- What is the power formula?
- Electrical power is P = V × I. Substituting Ohm's law gives the equivalent forms P = I²R and P = V²/R, so you can find power from any two of V, I and R.
- How are V, I, R related?
- They are linked by V = I × R. Knowing any two lets you find the third: I = V ÷ R and R = V ÷ I. Power connects through P = V × I.
- What units are used?
- Volts (V) for voltage, amperes (A) for current, ohms (Ω) for resistance and watts (W) for power. Scaled units such as mA, kΩ and mW appear when values are very small or large.
- Can I find power from V and R?
- Yes — use P = V² ÷ R. For example, 12 V across 4 Ω gives 12² ÷ 4 = 144 ÷ 4 = 36 W. The calculator does this automatically.