Ohm's Law Calculator

Calculate voltage, current, resistance, or power using Ohm's Law (V = I × R). Enter any two values to find the others.

Inputs

Voltage(V)

Current(A)

Resistance(Ω)derived

R = V / I — calculated from the values you entered. Clear one of those to edit this instead.

Power(W)derived

P = V × I — calculated from the values you entered. Clear one of those to edit this instead.

Tab to move between fields

The circuit, live

12 volts pushes 2 amps through 6 ohms, dissipating 24 watts — the bulb is glowing brightly.

Results

Voltage

12 V

Current2 A

Resistance6 Ω

Power24 W

Formulas UsedR = V / I, P = V × I

StatusSolved

The V-I-R-P wheel

Using V and I — the highlighted formulas are the ones being applied.

VVolts

12V

V = I × R
V = P / I
V = √(P × R)

you entered

IAmps

I = V / R
I = P / V
I = √(P / R)

you entered

ROhms

6Ω

R = V / I←
R = V² / P
R = P / I²

calculated

PWatts

24W

P = V × I←
P = V² / R
P = I² × R

calculated

Ohm's law

1.Divide volts by amps
$R = V / I$
This is the definition of resistance rather than a consequence of it: an ohm IS one volt per amp. Dividing the two you measured gives the resistance that must be present.
2.Multiply volts by amps for power
$P = V * I$
A volt is a joule per coulomb and an amp is a coulomb per second, so their product is joules per second — watts. The charge unit cancels, which is why the multiplication is dimensionally allowed at all.

Where the units go

volts ÷ amps A→Ω
volts × amps C→W

Georg Ohm published the proportionality between current and voltage after measuring it on wires of varying length, using a thermocouple for a steady source. The work was poorly received at first — his own ministry considered it insufficiently rigorous.

Georg Simon Ohm, Die galvanische Kette, mathematisch bearbeitet, Berlin·1827

How to Use This Calculator

Enter any two of the four values — voltage (V), current (A), resistance (Ω), or power (W) — and leave the other two at 0.
The calculator solves for the two you left blank using Ohm's Law (V = IR) and Joule's Law (P = VI).
Review the calculated voltage, current, resistance, and power, plus the formulas used to derive them.
Check the status field to confirm the values were solved (it prompts for a second value if only one was entered).
Use for basic circuit analysis, resistor selection, and power dissipation calculations.