Ohm's Law Calculator: Solve Voltage, Current, Resistance & Power

Ohm’s Law Calculator

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#	Given	Voltage	Current	Resistance	Power

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How Ohm's Law works

The law in one line

V = I × R

Voltage (V), measured in volts, is the electrical "pressure" pushing charge around a circuit. Current (I), in amperes, is how much charge actually flows. Resistance (R), in ohms (Ω), is how strongly the material opposes that flow.

Ohm's Law says the three are locked together: the current through a conductor is directly proportional to the voltage across it, and inversely proportional to its resistance. Double the voltage and the current doubles; double the resistance and the current halves.

A water analogy makes it intuitive: voltage is the water pressure from a pump, current is the flow rate through the pipe, and resistance is how narrow the pipe is. More pressure → more flow. Narrower pipe → less flow.

The triangle trick — rearranging the formula

Write V on top of a triangle, with I and R side by side underneath. Cover the quantity you want, and the layout of the remaining two is the formula:

Cover V → I and R sit side by side → V = I × R
Cover I → V sits over R → I = V ÷ R
Cover R → V sits over I → R = V ÷ I

The same trick works for the power triangle (P on top, V and I below): P = V × I, V = P ÷ I, I = P ÷ V.

Where power fits in — all 12 formulas

Power (P), in watts, is the rate at which the circuit converts electrical energy into heat, light, or motion: P = V × I. Combining it with Ohm's Law gives three ways to reach each quantity — which is why knowing any two values is always enough:

To find	From V & I	From V & R	From I & R	From P
Voltage	—	—	V = I×R	V = P/I or V = √(P×R)
Current	—	I = V/R	—	I = P/V or I = √(P/R)
Resistance	R = V/I	—	—	R = V²/P or R = P/I²
Power	P = V×I	P = V²/R	P = I²×R	—

A worked example

A 12 V supply drives a lamp that draws 2 A. What are its resistance and power?

Known: V = 12 V and I = 2 A.
Resistance: R = V ÷ I = 12 ÷ 2 = 6 Ω.
Power: P = V × I = 12 × 2 = 24 W.
Sanity check with a third formula: P = I² × R = 4 × 6 = 24 W ✓

That cross-check habit — computing the same quantity two ways — is exactly what this calculator's step panel shows you for every problem.

One caveat: Ohm's Law holds for "ohmic" components like resistors and wires, where R stays constant. Devices such as LEDs, diodes, and filament bulbs change resistance with temperature or voltage, so treat their results as a snapshot at one operating point.

Ohm’s Law connects voltage, current, resistance, and power, know any two and the other two follow directly, this calculator solves for whichever values you’re missing and visualizes the relationship.

How to use this calculator

Fill in exactly two of the four fields: Voltage, Current, Resistance, or Power.
Leave the two you want calculated empty.
Tap Solve to see all four values, the step-by-step solution, an Ohm’s law wheel, and a V-I characteristic chart.

What this calculator does

Given any two of voltage (V), current (I), resistance (R), or power (P), this calculator solves for the other two using Ohm’s Law and the power equation, showing the exact formula and steps used for each derived value.

V = I × R · I = V / R · R = V / I
P = V × I · P = I² × R · P = V² / R

Why there are multiple formulas for the same quantity

Because voltage, current, resistance, and power are all interrelated, each one can be expressed in terms of different pairs of the others, this is why power alone has three equivalent formulas depending on which two other quantities you already know, this calculator automatically picks the right formula pair based on which two values you provided.

Reading the Ohm’s law wheel and V-I chart

The Ohm’s law wheel visually organizes all the formula combinations in one reference diagram, useful for seeing at a glance how any two known quantities lead to the other two. The V-I characteristic chart plots how current changes with voltage for your specific resistance value, a straight line whose slope is determined by resistance, visually reinforcing why Ohm’s Law describes a linear relationship for a fixed resistor.

Frequently asked questions

Why are there three different formulas for power?

Power can be expressed using any two of the three other quantities (voltage, current, resistance), P = VI, P = I²R, and P = V²/R are all equivalent, this calculator picks whichever formula matches the two values you actually provided.

Why is the V-I characteristic chart a straight line?

For a fixed resistance, Ohm’s Law (V = IR) describes a direct linear relationship between voltage and current, the slope of that line is determined by the resistance value, this is what makes an ideal resistor’s V-I behavior linear rather than curved.