Ohm's Law Calculator

How to use this calculator

Type the two quantities you know into their boxes. As soon as two independent values are present, the remaining fields fill in and are marked calculated. You can start from any pair — voltage and current, current and power, resistance and power, and so on — because the calculator holds all four forms of the relationship at once.

Each field has its own unit menu. Readings from a multimeter are often in millivolts or milliamps, while resistor values come in kilo-ohms or mega-ohms; pick the unit you have and the arithmetic is converted for you. If you overwrite a calculated value, the calculator keeps your newest entries and recalculates the field you edited least recently, so you can explore "what if" questions quickly.

The formula

V = I × R

P = V × I = I² × R = V² / R

V: voltage
I: current
R: resistance
P: power

Ohm's law says that, for an ohmic conductor at constant temperature, the current through it is proportional to the voltage across it. The constant of proportionality is the resistance. Combining it with the definition of electrical power, P = VI, gives the two extra forms I²R and V²/R, which are handy when you know resistance but not voltage, or vice versa.

The I²R form explains why long cables waste energy: doubling the current through the same wire quadruples the heat produced. That is the reason power grids transmit at high voltage and low current.

Worked example

A 12 V car battery is connected across a 4 Ω heating element. How much current flows, and how much power does the element dissipate?

Current from Ohm's law: I = V / R = 12 V / 4 Ω = 3 A.
Power: P = V × I = 12 V × 3 A = 36 W.
Check with the resistance form: P = I²R = 3² × 4 = 36 W.

The element draws 3 amperes and turns 36 watts of electrical power into heat.

Assumptions and limits

The component is ohmic: its resistance does not change with voltage. Diodes, LEDs and filament bulbs at switch-on are not.
Values are DC, or AC RMS values in a purely resistive circuit. Reactive loads (motors, capacitors) need impedance and power factor.
Wire and contact resistance are ignored unless you include them in R.

Frequently asked questions

Does Ohm's law apply to AC circuits?

For purely resistive loads, yes — use RMS voltage and current. For inductive or capacitive loads, replace resistance with impedance and multiply apparent power by the power factor to get real power.

How do I find the resistor for an LED?

Subtract the LED forward voltage from the supply voltage, then divide by the desired current. For a 5 V supply, a 2 V red LED and 20 mA: R = (5 − 2) / 0.02 = 150 Ω. Enter 3 V and 20 mA here to get the same answer.

Why does my calculated power not match the appliance label?

Labels list rated power at nominal voltage. Mains voltage varies, and heating elements change resistance as they warm up, so measured values usually differ by a few percent.

What is the difference between watts and volt-amperes?

Watts measure real power. Volt-amperes measure apparent power (V × I). They are equal only for resistive loads; otherwise W = VA × power factor.