Ohm’s Law is the single most useful equation in electronics. It states that voltage equals current multiplied by resistance, and from that one relationship any of the three quantities can be found from the other two. A circuit designer uses it to choose resistor values, to check that a power supply can deliver the required current, and to predict what happens when a component changes. This calculator solves the law for whichever value you leave blank, and also reports the power dissipated, which matters because a resistor that handles the voltage and current may still overheat if it cannot dissipate the resulting power. Leave exactly one field empty, and the missing value appears with its unit. Because the three quantities are linked so tightly, the calculator also serves as a consistency check: if you already know all three values from a datasheet, entering any two should reproduce the third, and a mismatch points to a measurement or specification error worth investigating.
Quick Answer: Enter any two of voltage, current and resistance, leave the third blank, and click Solve to get the missing value plus power.
Free Online Tool
Ohm’s Law Calculator
Enter any two values, leave the third blank, and solve.
How to Use the Ohm’s Law Calculator
- Decide which quantity you need: voltage in volts, current in amps, or resistance in ohms.
- Enter the two quantities you know, using consistent units of volts, amps and ohms.
- Leave the unknown quantity’s field completely empty.
- Click Solve to compute the missing value and the power dissipated in watts.
- Check the power figure against your components’ ratings before building the circuit.
Key Features
- Solves for voltage, current or resistance from the other two values.
- Power calculation included with every result, since power ratings are a common oversight.
- Unit labelled outputs in volts, amps, ohms and watts, so results are unambiguous.
- Input validation that asks for exactly two known values instead of guessing.
- Instant local calculation for bench work and circuit design checks.
Best Practices and Tips
- Use amps, not milliamps, in the current field, converting by dividing milliamps by one thousand first.
- Always check power after solving. A resistor value can be electrically correct yet thermally impossible for a small part.
- When a result looks surprising, check for a unit slip first, since a factor of one thousand is the most common error.
- For circuits with several resistors, solve each resistor separately using the voltage across that resistor and the current through it.
Common Mistakes to Avoid
- Entering all three values, which leaves the calculator nothing to solve and usually means one value is inconsistent.
- Mixing milliamps with amps, which throws the result off by a factor of one thousand.
- Forgetting power entirely and choosing a resistor that burns out under load despite having the right resistance.
Frequently Asked Questions
What is Ohm’s Law?
Ohm’s Law states that the voltage across a conductor equals the current through it multiplied by its resistance, written as V equals I times R. It applies to resistive circuits where resistance stays constant, and it is the foundation for nearly all basic circuit analysis.
Why is power reported too?
Power, in watts, equals voltage times current, and it tells you how much heat a component must dissipate. A resistor rated for a quarter watt will overheat and fail if the circuit dissipates one watt through it, even when its resistance value is exactly right.
Can I use milliamps?
Yes, but convert first: one amp equals one thousand milliamps. Entering a milliamp figure as if it were amps makes the computed voltage or resistance one thousand times too large, so divide by one thousand before entering the current.
Does Ohm’s Law work for every component?
It works exactly for resistors and approximately for many conductors. Components such as diodes and transistors are non linear, meaning their effective resistance changes with voltage, so the law gives only an approximate snapshot for them at one operating point.