What Ohm's Law Calculator helps you do

Solve the relationships between voltage, current, resistance and power by entering any two values. Ohm's law and the power equations are combined so the remaining quantities are calculated for you, which is handy for circuit design and troubleshooting. Enter the pair you know and read the other two.

Circuit values are solved locally in your browser and are never uploaded.

How it works

These converters apply a fixed factor or formula to the value you enter, inside the page. They do not fetch live rates, measurement standards or device data, so the result depends only on the number and the units you choose. Values are formatted to a handful of decimals, with scientific notation for very small or very large results. Where a formula needs a second input, such as voltage, hours or a power factor, the page asks for it rather than assuming a value. The factor itself comes from the unit definition, so it is the same today as it will be next year; only the input changes.

How to use Ohm's Law Calculator

Enter any two of voltage, current, resistance or power.
The tool computes the remaining two values.
Change one value to see how the rest respond.
Check the four results against the pair you trust most.

When to use it

Find the resistance needed to set a target current in a simple circuit.
Check the power a resistor will dissipate before choosing its rating.
Debug a circuit by confirming which values are consistent with Ohm's law.
Work out the current through an LED series resistor.
Estimate the voltage drop across a shunt.
Check a bench supply setting against a load's expected draw.

When not to use it

Do not use it for reactive AC impedance; it models resistance, not reactance.
Do not mix values from different components or branches.
Do not treat the result as a thermal or safety rating.

Example

12 V across 6 ohms gives 2 A and 24 W. 0.5 A through 100 ohms gives 50 V and 25 W. 230 V at 10 A gives 23 ohms and 2300 W. 5 V at 0.02 A gives 250 ohms and 0.1 W. Any valid pair resolves to the same four quantities.

What you can do with Ohm's Law Calculator

Enter any two of voltage, current, resistance or power.
Compute the remaining two values.
Update the results as you change an input.
Handle fractional values and small currents.
Keep all four quantities visible together.
Run the calculation locally with no upload.

Example: check the unit before trusting the number

Confirm the source unit and the target unit before reading the result. A correct number in the wrong unit is the most common conversion mistake, especially between US and imperial measures.

Sanity-check the magnitude with a rough mental estimate. If kilometres should be larger than miles and the result is smaller, the direction is reversed.

Round the result for the destination. A calculator can show six decimals, but an invoice, label or form usually needs two or three, and the rounding rule may be specified.

Keep the original value and unit in your notes. If the destination rejects the result, the source is the only way to recompute without guessing what was entered.

Check whether the unit has regional variants. Gallons, pints, tons and acres all have more than one definition, and picking the wrong one can shift the result by several percent without any arithmetic error.

If the answer will drive a purchase, a dose or a safety decision, verify it against a second source or the manufacturer's own table. A calculator removes arithmetic mistakes, not definition mistakes.

What to know before you start

Models ideal resistive DC relationships; reactive AC impedance is not handled.
The two values you enter must refer to the same component or branch.
It does not model temperature coefficients or non-linear devices.
The result is a calculation, not a measurement.

Common mistakes

Entering values from two different components.
Using AC RMS values as if they were DC without accounting for impedance.
Ignoring the resistor's power rating after calculating dissipation.
Entering more than two values that contradict each other.

Troubleshooting

If a result is blank, fewer than two inputs were provided.

If the values disagree, one of the entered numbers may belong to another branch.

If power looks huge, check whether the voltage and current were entered in the right units.

If all four fields are filled, the tool solves from voltage and current first.

Ohm's Law Calculator