Ohm's Law Power Calculator
Enter any two of the four electrical quantities - voltage (V), current (I), resistance (R), or power (P) - and this calculator instantly solves the remaining two using Ohm's Law and the power equations. Unit prefixes (micro, milli, kilo, mega) are supported for each field. The show-your-work panel traces every formula used.
Formula
Worked example
A 100 W light bulb runs on 240 V mains. Current = P / V = 100 / 240 = 0.417 A. Resistance = V / I = 240 / 0.417 = 576 ohms. Verify: P = I^2 x R = 0.417^2 x 576 = 100 W. Confirmed.
What is Ohm's Law?
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Formulated by Georg Simon Ohm in 1827, it is the cornerstone of circuit analysis. In equation form: V = I x R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. For purely resistive circuits (no capacitors or inductors), these three variables - plus power - are fully determined by any two of the four quantities, which is what this calculator exploits.
The power equations and why they matter
Electrical power is the rate at which energy is transferred in a circuit. The primary power formula is P = V x I (watts equal volts times amperes). Combined with Ohm's Law, two more equivalent forms follow: P = I^2 x R (useful when you know current and resistance, for example when sizing a resistor) and P = V^2 / R (useful when you know voltage and resistance, for example when calculating the power drawn by a known load across a fixed supply). All three give the same answer; this calculator picks the appropriate one based on which two values you supply. Knowing the power dissipated is critical for choosing components with adequate wattage ratings and preventing overheating.
How to use this calculator
Select 'Solve for' to choose which two quantities you want to calculate, then enter the other two in the fields that appear. Each input has a unit-prefix selector (micro, milli, base, kilo, mega) so you can work with signals in the microamp range or power-line quantities in kilovolts without converting by hand. The result panel shows all four values with auto-scaled prefixes, and the 'Show your work' panel traces every formula step. The formula wheel reference table lists all 12 rearrangements at a glance.
Limitations and when to use impedance instead
Ohm's Law as stated here applies to purely resistive DC circuits. In AC circuits, capacitors and inductors introduce reactance, so the total opposition to current flow is impedance (Z, measured in ohms) rather than pure resistance. Impedance has both magnitude and phase, and apparent power (volt-amperes, VA) differs from real power (watts) by the power factor. For AC analysis, replace R with Z and use a complex-power calculator. Similarly, Ohm's Law does not directly apply to non-linear devices such as diodes, transistors or LEDs, where the V-I relationship is governed by semiconductor physics rather than a fixed resistance.
Ohm's Law Formula Wheel
| To find | Use this formula | Equivalent form |
|---|---|---|
| Voltage (V) | V = I x R | V = P / I = sqrt(P x R) |
| Current (I) | I = V / R | I = P / V = sqrt(P / R) |
| Resistance (R) | R = V / I | R = V^2 / P = P / I^2 |
| Power (P) | P = V x I | P = V^2 / R = I^2 x R |
The 12 relationships between voltage (V), current (I), resistance (R), and power (P) for resistive DC circuits.
Frequently asked questions
What is Ohm's Law in simple terms?
Ohm's Law says that doubling the voltage across a resistor doubles the current through it, and doubling the resistance halves the current. In equation form: V = I x R. It holds for any conductor whose resistance stays constant with temperature and voltage, which covers most resistors and wires in ordinary circuits.
How do I calculate power using V, I, and R?
Use whichever of the three power formulas matches the values you know: P = V x I if you know voltage and current; P = I^2 x R if you know current and resistance; P = V^2 / R if you know voltage and resistance. All three give the same result for any valid set of values. The calculator picks the right formula automatically.
What does the resistance of a circuit tell me?
Resistance is the opposition to current flow, measured in ohms. A higher resistance means less current for the same voltage, and less power dissipated. Knowing the resistance helps you choose the correct resistor value, estimate heat generation, and verify that a component will not exceed its power rating.
Can I use Ohm's Law for AC circuits?
Ohm's Law applies to AC circuits if you replace resistance (R) with impedance (Z), which accounts for the additional opposition from capacitors (capacitive reactance) and inductors (inductive reactance). For AC, current and voltage may be out of phase, so you need complex numbers or phasor analysis. This calculator covers DC or resistive AC loads only.
How do I choose the wattage rating of a resistor?
Calculate the actual power the resistor will dissipate using P = V^2 / R or P = I^2 x R, then choose a resistor rated at least twice that value. The standard 2x derating factor accounts for temperature rise, manufacturing tolerances, and intermittent peaks. Common ratings are 0.1 W, 0.25 W, 0.5 W, 1 W, 2 W, 5 W, and 10 W.
What unit prefixes does this calculator support?
Voltage: microvolts (uV), millivolts (mV), volts (V), kilovolts (kV), megavolts (MV). Current: microamps (uA), milliamps (mA), amperes (A), kiloamps (kA). Resistance: ohms, kilohms (kohm), megohms (Mohm). Power: microwatts (uW), milliwatts (mW), watts (W), kilowatts (kW), megawatts (MW). Results are auto-scaled to the most readable prefix.