Ohm's Law Calculator

What is an Ohm’s law calculator?

An Ohm’s law calculator solves the relationship V = I × R together with electrical power, returning any missing values when you supply two of voltage, current, resistance and power. It links the four basic quantities of a DC circuit. Electronics students, hobbyists, electricians, lab technicians and engineers use it to size resistors, check current draw and find power dissipation without doing the algebra by hand.

What is the V = I × R formula?

The core of Ohm’s law is V = I × R: voltage equals current multiplied by resistance. Power adds a second relationship, P = V × I = I²R = V²/R. Each symbol has a fixed SI unit, so the results come out directly with no conversion:

• V — voltage (potential difference), in volts (V).
• I — current, in amperes or amps (A).
• R — resistance, in ohms (Ω). It must be non-zero when it acts as a divisor.
• P — power, in watts (W), the rate at which energy is used.

You enter any two of these four values and the calculator computes the rest. Which formula it uses depends on the pair you give it. The table below shows every case:

You know	It finds	Using
V and I	R, P	R = V / I, P = V × I
V and R	I, P	I = V / R, P = V² / R
I and R	V, P	V = I × R, P = I² × R
P and V	I, R	I = P / V, R = V² / P
P and I	V, R	V = P / I, R = P / I²
P and R	V, I	V = √(P × R), I = √(P / R)

Every divisor must be non-zero, and each result is reported to six significant figures.

Worked examples

Each example uses only the formulas above, so you can reproduce every answer by entering the same two values into the calculator. They all describe the same circuit from different starting points.

Example 1 — known voltage and current

A circuit runs at 12 V and draws 2 A. Find the resistance and power.

R = V / I = 12 / 2 = 6 Ω P = V × I = 12 × 2 = 24 W

The resistance is 6 ohms and the power is 24 watts.

Example 2 — known voltage and resistance

The same 12 V supply feeds a 6 Ω resistor. Find the current and power.

I = V / R = 12 / 6 = 2 A P = V² / R = 12² / 6 = 144 / 6 = 24 W

The current is 2 amps and the power is 24 watts, matching Example 1.

Example 3 — known current and resistance

A 6 Ω resistor carries 2 A. Find the voltage and power.

V = I × R = 2 × 6 = 12 V P = I² × R = 2² × 6 = 4 × 6 = 24 W

The voltage is 12 volts and the power is 24 watts.

Example 4 — known power and resistance

A 6 Ω resistor dissipates 24 W. Find the voltage and current.

V = √(P × R) = √(24 × 6) = √144 = 12 V I = √(P / R) = √(24 / 6) = √4 = 2 A

The voltage is 12 volts and the current is 2 amps, closing the loop with the first three examples.

Ohm’s law quick reference

This chart fixes the resistance at a constant 6 Ω and steps the voltage up, computing current from I = V / R and power from P = V × I. It shows how current rises in step with voltage while power grows with the square of voltage.

Voltage (V)	Resistance (Ω)	Current I = V/R (A)	Power P = V·I (W)
3	6	0.5	1.5
6	6	1	6
9	6	1.5	13.5
12	6	2	24
24	6	4	96
48	6	8	384

Doubling the voltage from 12 V to 24 V doubles the current but quadruples the power, because power depends on V² at fixed resistance.

Common uses

Ohm’s law underpins almost every electrical and electronic calculation:

• Electronics homework and exams — solving for voltage, current, resistance or power in DC circuit problems.
• Choosing a resistor — picking the value and wattage rating that gives the right current for an LED or sensor.
• Electrical work — checking the current a load draws and whether a wire or fuse is rated for it.
• Power supply sizing — finding the wattage a device needs from its voltage and current.
• Bench and lab testing — predicting current draw before powering a prototype to avoid overloading parts.

Tips and common mistakes

• Use SI units. Volts, amps, ohms and watts work together with no conversion. Milliamps (mA) or kilohms (kΩ) must be converted to amps and ohms first, or the result will be off by a factor of 1000.
• Do not divide by zero. Finding R from V and I needs a non-zero current, and finding I from V and R needs a non-zero resistance. Zero in a divisor makes the result undefined.
• Match power to the right pair. Use P = V × I when you know volts and amps, P = I²R when you know amps and ohms, and P = V²/R when you know volts and ohms — they all give the same answer for a given circuit.
• Watch the squared terms. At fixed resistance, doubling the voltage quadruples the power, not doubles it, because P depends on V². The same applies to current in P = I²R.
• Keep amps and milliamps straight. A reading of 200 mA is 0.2 A; entering 200 instead overstates current and power by 1000 times.
• Six significant figures is display precision, not measurement certainty. The answer is only as accurate as the values you typed in.

Limitations and accuracy

This calculator applies the ideal form of Ohm’s law for a DC circuit with a single, constant (ohmic) resistance at a steady temperature. It assumes V, I and R stay proportional, which holds for resistors and most metal conductors near room temperature but not for non-ohmic devices such as diodes, transistors, filament bulbs or thermistors, whose resistance changes with voltage or temperature. It also ignores wire resistance, contact resistance and, in AC circuits, the effects of reactance and impedance — for AC you would substitute impedance Z for R and account for phase. Treat the tool as an education and estimation aid: the arithmetic is exact for your inputs, but confirm that your real circuit behaves ohmically before relying on a figure.

For related calculations, try the kinetic energy calculator for the energy of moving objects, the density calculator to find mass from volume, or the combined gas law calculator for pressure, volume and temperature — and browse more in the chemistry and physics category.