Ohm's Law Calculator: Voltage, Current & Resistance

An Ohm's law calculator finds voltage, current, or resistance in an electric circuit when the other two values are known. It applies Ohm's law, a rule that connects these three quantities: voltage equals current multiplied by resistance, written as V = I × R.

What voltage, current, and resistance mean

Voltage (V) is the electrical push that moves electrons through a circuit. It is measured in volts.

Current (I) is the rate at which electrons flow past a point in the circuit. It is measured in amperes, often shortened to amps (A).

Resistance (R) is how much a material opposes the flow of current. It is measured in ohms (Ω).

A common comparison uses water in a pipe. Voltage works like water pressure. Current works like the flow rate. Resistance works like how narrow the pipe is. More pressure or a wider pipe increases flow. More narrowing reduces it.

Ohm's law formula

The base formula is:

V = I × R

Two other forms come from rearranging it:

I = V ÷ R

R = V ÷ I

All three formulas describe the same relationship. Which one to use depends on which value is missing.

How to calculate voltage

Multiply current by resistance.

V = I × R

Example: a circuit has a current of 2 A and a resistance of 5 Ω. V = 2 × 5 = 10 V

How to calculate current

Divide voltage by resistance.

I = V ÷ R

Example: a circuit has a voltage of 12 V and a resistance of 4 Ω. I = 12 ÷ 4 = 3 A

How to calculate resistance

Divide voltage by current.

R = V ÷ I

Example: a circuit has a voltage of 9 V and a current of 3 A. R = 9 ÷ 3 = 3 Ω

Example: sizing a resistor for an LED

A 9 V battery powers an LED that needs 20 mA (0.02 A) of current and has a forward voltage drop of 2 V. The resistor in series with the LED must absorb the rest of the voltage.

Voltage across the resistor: 9 V − 2 V = 7 V

Resistance needed: R = 7 ÷ 0.02 = 350 Ω

Since 350 Ω is not a standard resistor size, a builder would pick the nearest common value, such as 330 Ω or 390 Ω.

How to use the calculator

The calculator has three fields: Voltage (V), Current (I), and Resistance (R). A user fills in exactly two of them and leaves the third blank. The calculator computes the missing value, rounds it to two decimal places, and shows the formula it used.

Filling in all three fields, or fewer than two, produces an error asking for exactly two values. Negative numbers are rejected, since voltage, current, and resistance are treated here as zero or greater. Anything that is not a plain number, such as text or a figure too large to represent, is rejected the same way. If the two entered values would force a division by zero, such as calculating current with a resistance of 0 Ω, the calculator shows an error instead of an undefined result.

Limits of Ohm's law

Ohm's law applies to ohmic materials: conductors, such as most metal wires and resistors, where current rises in direct proportion to voltage. It does not directly apply to components like diodes, LEDs, or transistors, because their current does not rise in a straight line with voltage.

The law also assumes a steady temperature, since resistance in many materials changes as they heat up. For alternating current (AC) circuits, engineers use impedance instead of plain resistance, since capacitors and inductors add effects that depend on frequency.

History

Georg Simon Ohm (1789–1854), a German physicist, published this relationship in 1827 after measuring how current changed across wires of different lengths and materials. Other scientists doubted his findings at first. The Royal Society later recognized his work with the Copley Medal in 1841. In 1881, an international congress of electrical engineers named the unit of resistance the ohm in his honor.

Frequently asked questions

What is Ohm's law in simple terms?

It is the rule that voltage equals current multiplied by resistance: V = I × R. Raising the voltage or lowering the resistance increases the current.

What are the three Ohm's law formulas?

V = I × R for voltage, I = V ÷ R for current, and R = V ÷ I for resistance. Each is a rearrangement of the same relationship.

What is a real example of Ohm's law?

Sizing a current-limiting resistor for an LED. With a 5 V supply, an LED with a 2 V forward voltage drop, and a target current of 20 mA, the needed resistance is (5 − 2) ÷ 0.02 = 150 Ω.

Does Ohm's law work for AC circuits?

Not directly. AC circuits use impedance instead of plain resistance, since capacitors and inductors respond differently depending on the frequency of the current.

Does Ohm's law apply to every component?

No. It applies to ohmic materials, such as plain resistors and most wires. It does not apply to diodes, LEDs, or transistors, since their current does not rise in a straight line with voltage.

Why do voltage, current, and resistance need different units?

Because they measure different physical quantities. Volts measure electrical pressure, amperes measure flow rate, and ohms measure opposition to that flow. Mixing up units in a calculation produces a wrong answer even when the formula is correct.