Ohms Law Calculator

Enter any two electrical values to solve voltage, current, resistance, and power.

100% Free No Sign-Up Calculated in Your Browser

Voltage (V)

Current (A)

Resistance (ohms)

Result

Enter values to calculate.

Summary

Waiting for input

Detail

Waiting for input

Enter any two of voltage, current, and resistance to solve for the missing value and the resulting power, using Ohm's Law.

How It Works

How Ohms Law Calculator Works

Ohm's Law relates the three values with the formula V = I × R (voltage equals current times resistance), which can be rearranged to solve for whichever value you leave blank: current = voltage ÷ resistance, or resistance = voltage ÷ current.

Once all three values are known — either because you entered them or the calculator solved for the missing one — power is calculated using Joule's Law, P = V × I, giving you the circuit's power consumption in watts alongside the electrical values.

Worked Example

See It In Action

With a known voltage of 12V and resistance of 4 ohms, current works out to 3A (12 ÷ 4), and power is 36W (12 × 3).

Real-World Use Cases

Who Uses Ohms Law Calculator and Why

Sizing a current-limiting resistor for an LED given a supply voltage, the LED's forward voltage, and a target current.
Working out how much current a known resistor will draw from a given supply voltage.
Checking a component's power dissipation before choosing a resistor's wattage rating so it doesn't overheat.
Verifying a circuit design's numbers by hand before building it, especially for a first electronics project.

Common Mistakes

Mistakes to Avoid

Filling in all three of voltage, current, and resistance instead of leaving one blank — the calculator won't cross-check that your three numbers actually satisfy V = I × R, so a mismatched trio produces a power figure that isn't meaningful.
Mixing up which unit is expected (volts vs. millivolts, ohms vs. kilohms) and getting a result that's off by a factor of 1,000.
Forgetting to also check the resulting power (wattage) figure — a resistor sized correctly for resistance can still fail if it's not rated to dissipate the power the circuit actually puts through it.

Pro Tips

Tips for Best Results

Leave exactly one field blank so the calculator solves for it — filling in all three risks entering numbers that don't actually satisfy Ohm's Law together.
Once you have all three values, check the power result against your component's wattage rating — a resistor rated for less power than it's dissipating will overheat and can fail.

Troubleshooting

Fixing Common Problems

My result doesn't match what I expected for a known LED circuit. — Double check you used the LED's forward voltage drop, not the full supply voltage, in your V = I × R setup — the resistor only needs to drop the difference between supply voltage and the LED's forward voltage.

I got a negative number. — In basic DC analysis this usually just means a reversed reference direction in how you set up the problem, not an impossible result — recheck your original inputs rather than assuming an error.

Glossary

Terms Explained

Ohm's Law: The fundamental relationship V = I × R linking voltage, current, and resistance in a circuit.

Joule's Law: The formula P = V × I used to calculate electrical power once voltage and current are known.

FAQ

Frequently Asked Questions

What happens if I fill in all three values instead of just two?

The calculator uses your entered values as-is without cross-checking that they satisfy Ohm's Law together, so if all three numbers don't actually match the formula, the power result may not be meaningful — leave one field blank to have the calculator solve it for you.

Which two values are most useful to start from?

It depends on what you know about your circuit — voltage and resistance is common when sizing a resistor for a known supply voltage, while current and resistance is useful when you know a component's rated draw and its resistance.

What does a negative value mean?

In basic DC circuit analysis, a negative result usually indicates a reversed reference direction rather than a physically impossible outcome — double-check your original inputs if a result surprises you.

What is this commonly used for?

Ohm's Law calculations are a everyday task in basic electronics, such as sizing a current-limiting resistor for an LED given a supply voltage and the LED's forward voltage and desired current.