⚡ Ohms Law Calculator (Voltage, Current, Resistance, Power)

By Shihab Mia · Updated 2026-06-27

Fill in exactly two of the four values below and the calculator works out the other two for you.

Voltage V (volts)

Current I (amps)

Resistance R (ohms)

Power P (watts)

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Two values entered. The other two are calculated above.

Ohm's law wheel: the core relationships
To find	Formula	In words
Voltage	V = I × R	Current times resistance
Power	P = V × I	Voltage times current
Power	P = I² × R	Current squared times resistance
Power	P = V² ÷ R	Voltage squared over resistance

This Ohms law calculator lets you enter any two of the four core electrical quantities, voltage (V), current (I), resistance (R), and power (P), and works out the remaining two for you. Type two known values, leave the other two blank, and read the answers instantly. Every calculation runs in your browser using the standard relationships V = I times R and P = V times I, so the results are exact and nothing is sent anywhere.

What is the Ohms Law Calculator?

Ohms law is the single most useful relationship in basic electronics. It states that the voltage across a resistor equals the current flowing through it multiplied by its resistance, written V = I times R. From this one equation you can rearrange to find any quantity when you know the other two: I = V divided by R, or R = V divided by I. Voltage is measured in volts, current in amperes (amps), and resistance in ohms.

Power extends Ohms law by telling you how fast electrical energy is being used or dissipated. The base power formula is P = V times I, measured in watts. By substituting Ohms law into it you get two more handy forms: P = I squared times R, and P = V squared divided by R. Together these four power expressions and the three Ohms law forms make up the Ohms law wheel, a circle of formulas that lets you reach any unknown from any pair of known values.

That is exactly what this calculator does. It looks at which two boxes you filled in, picks the matching pair of formulas, and computes the other two quantities in one step. For example, if you enter a 12 volt supply and a 2 amp current, it returns a resistance of 6 ohms and a power of 24 watts. If instead you knew the resistance and the power, it would take square roots to recover the voltage and current.

The most common slip is filling in the wrong number of boxes. Ohms law needs exactly two knowns to pin down a circuit, so this tool gently reminds you if fewer or more than two values are present. It also guards against division by zero, since a zero current or zero voltage in the wrong place makes a quantity undefined. Keeping the units straight, volts with amps with ohms with watts, is the other half of getting clean answers.

When to use it

Sizing a current limiting resistor for an LED when you know the supply voltage and the desired current.
Working out how many watts a heater, motor, or appliance draws from its voltage and current rating.
Checking that a resistor is rated for enough power before it overheats, using P = I squared times R.
Solving physics or electronics homework where you are given any two of voltage, current, resistance, or power.

How to use the Ohms Law Calculator

Decide which two of the four quantities you already know: voltage, current, resistance, or power.
Type those two values into their boxes and leave the other two empty.
Read the two calculated values in the result cards at once.
Use Clear all or a sample button to start a fresh calculation with a different pair.

Formula & method

Ohms law: V = I × R, so I = V ÷ R and R = V ÷ I. Power: P = V × I, which also equals I² × R and V² ÷ R. From resistance and power: V = √(P × R) and I = √(P ÷ R).

Worked examples

A 12 volt battery drives a current of 2 amps through a resistor. Find the resistance and the power.

You know V = 12 volts and I = 2 amps.
Resistance: R = V divided by I = 12 divided by 2 = 6 ohms.
Power: P = V times I = 12 times 2 = 24 watts.

Result: R = 6 ohms and P = 24 watts.

A device is rated at 60 watts and contains a 240 ohm resistive element. Find the voltage and current.

You know P = 60 watts and R = 240 ohms.
Voltage: V = square root of (P times R) = square root of (60 times 240) = square root of 14400 = 120 volts.
Current: I = square root of (P divided by R) = square root of (60 divided by 240) = square root of 0.25 = 0.5 amps.

Result: V = 120 volts and I = 0.5 amps.

The Ohms law wheel: every form rearranged

To find	From V and I	From V and R	From I and R	From P and one of V, I, R
Voltage V	given	given	V = I x R	V = P / I or V = sqrt(P x R)
Current I	given	I = V / R	given	I = P / V or I = sqrt(P / R)
Resistance R	R = V / I	given	given	R = V^2 / P or R = P / I^2
Power P	P = V x I	P = V^2 / R	P = I^2 x R	given

Quick reference: common 12 volt examples

Voltage (V)	Current (I)	Resistance (R)	Power (P)
12 V	0.5 A	24 ohms	6 W
12 V	1 A	12 ohms	12 W
12 V	2 A	6 ohms	24 W
12 V	3 A	4 ohms	36 W
5 V	0.02 A	250 ohms	0.1 W
230 V	10 A	23 ohms	2300 W

Common mistakes to avoid

Filling in only one value, or all four. Ohms law needs exactly two known quantities to define a circuit. With only one, the answer is undefined; with all four, the numbers may not even agree. Enter precisely two values and leave the rest blank.
Mixing up the units. Volts, amps, ohms, and watts must be used in their base units. Entering milliamps as if they were amps, or kilo ohms as ohms, throws the result off by a factor of a thousand. Convert to base units first.
Confusing power with energy. Power in watts is the instantaneous rate of energy use, not the total energy consumed. To get energy in watt hours, multiply the power by the number of hours it runs. Ohms law alone gives you power, not a kilowatt hour bill.
Using Ohms law on non resistive parts. The simple V = I times R holds for ordinary resistors. Diodes, capacitors, inductors, and semiconductors do not follow a single fixed resistance, so plugging them into Ohms law gives only a rough snapshot at one operating point.

Glossary

Voltage (V): The electrical pressure or potential difference that pushes current through a circuit, measured in volts.
Current (I): The rate of flow of electric charge through a conductor, measured in amperes (amps).
Resistance (R): The opposition a component offers to the flow of current, measured in ohms. Higher resistance means less current for the same voltage.
Power (P): The rate at which electrical energy is converted to heat, light, or motion, measured in watts and equal to voltage times current.
Ohms law: The rule that voltage equals current multiplied by resistance, written V = I times R, valid for ordinary resistive components.
Ohms law wheel: A circular chart that arranges the twelve rearranged forms of Ohms law and the power formulas so you can find any quantity from any pair.

Frequently asked questions

What is Ohms law in simple terms?

Ohms law says the voltage across a resistor equals the current through it times its resistance: V = I times R. Rearranged, current is I = V divided by R and resistance is R = V divided by I. It lets you find any one of the three when you know the other two.

How do I calculate power from voltage and current?

Multiply them: power equals voltage times current, P = V times I, with the answer in watts. A 12 volt supply delivering 2 amps uses 12 times 2 = 24 watts. You can also use P = I squared times R or P = V squared divided by R.

Why do I have to enter exactly two values?

Two known quantities are the minimum needed to pin down a resistive circuit, and they uniquely determine the other two. With one value the system is underdetermined, and with three or four the entries could conflict, so the tool asks for exactly two.

How do I find resistance from power and current?

Use R = P divided by I squared. For example, 24 watts at 2 amps gives R = 24 divided by 4 = 6 ohms. This calculator applies that formula automatically when you fill in the power and current boxes.

Can I use this for AC circuits?

For purely resistive AC loads using RMS values, Ohms law and P = V times I work directly. For loads with significant capacitance or inductance you also need impedance and power factor, which this basic calculator does not model.

What units does the calculator expect?

Base SI units: volts for voltage, amperes for current, ohms for resistance, and watts for power. Convert milliamps, kilo ohms, or milliwatts to their base units before entering them so the results come out correct.