Ohm’s Law Calculator

Voltage, current, resistance and power are linked by a simple rule called Ohm’s law. If you know any two of these values in a circuit, you can work out the other two. This calculator does that maths for you, quickly and without mistakes.

It is useful for students, hobbyists and anyone learning electronics. Use it to check a school project, choose a resistor, or see how much current a device will draw.

This tool is part of our physics calculators collection, where you can find more tools for science and engineering problems.

What Is the Ohm’s Law Calculator?

The Ohm’s Law Calculator is a free online tool that finds voltage, current, resistance or power in a simple electrical circuit. You give it any two values, and it calculates the missing two.

Ohm’s law is named after Georg Ohm, who described this link in the 1820s. It is one of the first ideas taught in electronics. For harder maths, try our scientific calculator for advanced calculations.

How It Works and the Formulas

The main formula is V = I × R: voltage equals current multiplied by resistance. The second key formula is P = V × I: power equals voltage multiplied by current. All the other forms in this article come from these two.

Each letter stands for one quantity, with its own unit:

• Voltage (V) — measured in volts; the unit is the volt. Voltage is the push that drives charge around a circuit, like pressure in a water pipe.
• Current (I) — measured in amperes, often shortened to amps; the unit is the ampere. Current is the rate at which charge flows.
• Resistance (R) — measured in ohms; the unit is the ohm, written Ω. Resistance slows the flow of current.
• Power (P) — measured in watts; the unit is the watt. Power is the rate at which electrical energy turns into heat, light or movement.

Many students use the Ohm’s law triangle: V at the top, I and R at the bottom. Cover the value you want: cover V for I × R, I for V ÷ R, and R for V ÷ I.

How to Use the Ohm’s Law Calculator

1. Decide which two values you already know, such as the voltage of a battery and the resistance of a component.
2. Type those two values into the matching boxes, using volts, amps, ohms and watts as labelled.
3. Leave the other two boxes empty. The calculator needs exactly two values.
4. Press Calculate. The tool will show all four results: voltage, current, resistance and power.
5. Check that the results make sense for your circuit. Press Reset to start again.

Worked Example

Suppose a circuit is powered by a battery of V = 12 volts, and the resistance is R = 4 ohms. What are the current and the power?

First, find the current using I = V ÷ R: I = 12 ÷ 4 = 3 amps. So I = 3 amps. Next, find the power using P = V × I: P = 12 × 3 = 36 watts. So P = 36 watts. The full result is V = 12 volts, I = 3 amps, R = 4 ohms and P = 36 watts.

A second, shorter example: a 60 W bulb runs on 120 V. Its current is I = P ÷ V = 60 ÷ 120 = 0.5 A. Its resistance is R = V ÷ I = 120 ÷ 0.5 = 240 Ω. So a 60 watt bulb on 120 volts draws 0.5 A and has 240 Ω resistance.

Ohm’s Law Formulas at a Glance

This table gives all 12 rearrangements, grouped by what you are solving for.

Solving for	Form 1	Form 2	Form 3
Voltage (V)	V = I × R	V = P ÷ I	V = √(P × R)
Current (I)	I = V ÷ R	I = P ÷ V	I = √(P ÷ R)
Resistance (R)	R = V ÷ I	R = V² ÷ P	R = P ÷ I²
Power (P)	P = V × I	P = I² × R	P = V² ÷ R

To plot how voltage changes as current changes, you can use our Desmos graphing calculator for plotting equations.

Important Points and Assumptions

This calculator assumes an ideal resistive DC circuit: a steady direct current supply, and a component that behaves like a simple resistor with a fixed value.

Real components heat up as current flows, and their resistance often changes as they get hotter. A bulb filament has much higher resistance when hot than when cold.

AC circuits are out of scope. In alternating current circuits, coils and capacitors add an effect called reactance, which also limits current. This simple form of Ohm’s law does not cover reactance, so do not use these results for AC design.

Safety note: Mains electricity is dangerous and can kill. These calculations are educational only. Do not work on live circuits unless you are qualified to do so.

Common Mistakes

• Mixing mA with A: 500 mA is 0.5 A, not 500 A. Divide milliamps by 1,000 to get amps before entering the value.
• Filling in too many boxes: The calculator needs exactly two values. With three or four, it cannot know which pair to trust.
• Using P = V × I with the wrong pair: The voltage and current must belong to the same component. Do not mix voltage across one part with current through another.
• Forgetting units: Enter resistance in ohms. 4 kΩ is 4,000 Ω, not 4 Ω.
• Entering zero: Zero resistance or current leads to division by zero, so the calculator asks for a value above zero.

How to Interpret Your Results

Check the size of each result. If current looks very large for a small device, you may have used the wrong unit. If power is far above a component’s rating, it could overheat.

At the same voltage, lower resistance means higher current and more power. At the same resistance, higher voltage also raises both.

Frequently Asked Questions

What is Ohm’s law in simple words?

It says the current through a resistor depends on the voltage across it and its resistance. More voltage pushes more current; more resistance allows less. The basic formula is V = I × R.

Can I find all four values from just two?

Yes. The four values are linked, so any two are enough to work out the other two. That is why the calculator asks for exactly two.

What units should I use?

Use volts for voltage, amps for current, ohms for resistance and watts for power. Convert smaller units first, such as milliamps to amps.

Why does the calculator reject zero or negative values?

Negative values do not make sense in this simple model, and zero resistance or current would require dividing by zero. Positive values keep the result possible and safe.

Does Ohm’s law work for AC circuits?

Only in a limited way. It works for the resistive part, but coils and capacitors add reactance, which this calculator does not include. AC work needs impedance, not just resistance.

Why does a real bulb not match the calculation exactly?

A bulb filament changes resistance as it heats up. Its cold resistance differs from its hot, lit resistance, so a simple calculation is a good guide, not an exact prediction.

References

See the National Institute of Standards and Technology (NIST) — NIST’s SI unit definitions for the volt, ampere and ohm. NIST maintains the standard definitions for these SI electrical units.