Ohm's Law and the power equation
Ohm's Law says the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. Combined with the electrical power equation, four quantities are linked — voltage (V, volts), current (I, amps), resistance (R, ohms) and power (P, watts):
Because each quantity can be written in terms of two others, knowing any two values lets you derive the rest. This calculator takes exactly two inputs and solves the remaining two.
Worked example
A 12 V supply drives a 24 Ω resistor. What current flows, and how much power is dissipated?
The Ohm's Law wheel at a glance
The "Ohm's Law wheel" arranges the twelve rearrangements so you can pick the formula that matches your two known values:
| To find | From V & I | From V & R | From I & R | From P & ... |
|---|---|---|---|---|
| Voltage (V) | — | — | I×R | P÷I or √(P×R) |
| Current (I) | — | V÷R | — | P÷V or √(P÷R) |
| Resistance (R) | V÷I | — | — | V²÷P or P÷I² |
| Power (P) | V×I | V²÷R | I²×R | — |
Prefixes, AC, and staying safe
Real circuits span huge ranges, so engineers use prefixes: 1 kΩ = 1,000 Ω, 1 MΩ = 1,000,000 Ω, 1 mA = 0.001 A, 1 µA = 0.000001 A. Convert everything to base units (volts, amps, ohms, watts) before calculating, then convert the answer back.
Two caveats worth knowing:
- Ohm's Law is for resistive, linear loads. Diodes, transistors and other semiconductors are non-linear, so a single resistance value doesn't describe them. For AC circuits with capacitors or inductors, you replace resistance with impedance (Z), which depends on frequency.
- Power means heat. A resistor rated for ¼ W will overheat if you ask it to dissipate 6 W. Always check the power result against your component's rating.