Ohm's Law Calculator
Fill in any two of voltage, current, and resistance and the third follows from V = I × R. It is the most-used relationship in electrical work, and the two things worth knowing alongside it are how to get power from the same values and which components do not obey it.
How to use it
- Enter exactly two of voltage, current, and resistance.
- Leave the third blank.
- Read the solved value with its unit.
The relationship and what it means physically
Voltage is electrical potential difference, the pressure pushing charge through a circuit. Current is the rate of charge flow. Resistance opposes that flow. Ohm law states that current is proportional to voltage and inversely proportional to resistance.
The three rearrangements are V = I × R, I = V ÷ R, and R = V ÷ I. A worked case: a 12 volt supply across a 100 ohm resistor gives 0.12 amps, or 120 milliamps.
The inverse relationship between current and resistance is the part that produces surprises. Halving the resistance doubles the current at fixed voltage, which is why a partial short circuit — an insulation fault, a damaged conductor — draws so much current so suddenly. It is also why testing continuity on a de-energised circuit is safer than reasoning about it energised.
Power is what actually causes damage
Ohm law tells you current; the power relationships tell you whether something will overheat. P = V × I, and substituting gives P = I² × R and P = V² ÷ R.
The squared terms matter enormously. Doubling the current through a fixed resistance quadruples the power dissipated, which is why undersized conductors fail so abruptly rather than gradually.
A worked example: 12 volts across 100 ohms gives 0.12 amps and 1.44 watts. A quarter-watt resistor in that position will fail — not eventually, but quickly. Resistors need a power rating with margin, and the usual practice is to specify at least twice the calculated dissipation, more in enclosed or high-ambient locations.
The same arithmetic covers conductors. A 20 amp current through 0.1 ohms of wire resistance dissipates 40 watts as heat in the wire itself, which is roughly a soldering iron distributed along the run. This is the mechanism behind overheated extension leads and loose terminations: a connection that has worked loose adds resistance at a single point, and the I²R heat concentrates there.
Voltage drop over a run
Wire is not a perfect conductor, and over distance its resistance produces a voltage drop that the load never sees.
The resistance depends on gauge, length, and material. Copper at 12 AWG is roughly 1.6 ohms per 1,000 feet; 14 AWG is about 2.5. Because current flows out and back, the effective length is twice the run.
A worked case: 15 amps over a 100 foot run of 14 AWG copper. Round-trip length is 200 feet, resistance is about 0.5 ohms, and the drop is 7.5 volts — over 6 percent of a 120 volt supply. The load sees about 112 volts, and 112 watts are being dissipated in the wall.
The common guidance is to keep drop under 3 percent for branch circuits and 5 percent overall. The fix is a larger conductor, which is why long runs are upsized even when the current alone would not require it. Motors are particularly sensitive: low voltage means higher current draw for the same mechanical work, which increases heating and shortens life.
Components that do not obey Ohm's law
Ohm law describes linear resistances. A good deal of what is in a real circuit is not linear, and applying the law to it gives wrong answers.
- Diodes and LEDs have an approximately fixed forward voltage drop and pass essentially no current below it, then rise steeply. An LED needs a current-limiting resistor sized from the supply voltage minus the forward drop, not from the LED resistance, which is not a meaningful figure.
- Incandescent lamps have a cold resistance several times lower than hot, which is why inrush current at switch-on is much higher than steady-state and why filaments usually fail at the moment of switching.
- Capacitors and inductors oppose changing current rather than current itself, so their behaviour depends on frequency and requires impedance rather than resistance.
- Semiconductors generally are non-linear by design, and their characteristics shift with temperature.
- Batteries have internal resistance that rises as they discharge, so terminal voltage falls under load and the drop grows as the cell ages.
At a glance
| Relationship | V = I × R |
|---|---|
| Power | P = V × I = I²R = V²/R |
| Applies to | Linear resistances, DC or resistive AC |
| Transmitted | Nothing |
Frequently asked questions
How do I get power from these values?
P = V × I, or I²R, or V²/R. The squared terms matter: doubling current quadruples the heat dissipated in a fixed resistance.
What resistor power rating do I need?
At least twice the calculated dissipation, and more in enclosed or hot locations. 12 volts across 100 ohms dissipates 1.44 watts, so a quarter-watt part will fail.
How much voltage drop is acceptable?
Under 3 percent on a branch circuit and 5 percent overall is the usual guidance. Fifteen amps over 100 feet of 14 AWG drops about 7.5 volts, which exceeds it.
Does this work for LEDs?
Not directly. An LED has a forward voltage drop rather than a useful resistance. Size the series resistor from supply voltage minus forward drop, divided by target current.
Read more
Engineering and vehicle math — Squared and cubed relationships explain most of it: heat, scaling, and why your truck runs out of payload before tow rating.