Ohm's law states that voltage equals current times resistance, written V = I × R. Power is V × I, so any two of these four quantities determine the other two.

Which units does this use?

Volts for voltage, amps for current, ohms for resistance and watts for power. Convert milliamps to amps by dividing by 1000 before entering.

How do you calculate power without knowing current?

Use the derived forms: P = V² ÷ R if you know voltage and resistance, or P = I² × R if you know current and resistance. This tool applies whichever form fits the pair you select.

How do you find resistance from power and current?

Rearrange to R = P ÷ I². For example, 50 W through 2 A needs R = 50 ÷ 4 = 12.5 ohms. Choose the current-and-power pair and the tool does this automatically.

What is the power dissipated by a resistor?

It is the electrical power converted to heat, P = I² × R watts. A 100-ohm resistor carrying 0.1 A dissipates 0.1² × 100 = 1 watt, so a quarter-watt resistor would be undersized.

Is my data sent anywhere?

No. Every calculation runs locally in your browser — nothing is uploaded or stored.

Ohm's Law Calculator

Ohm’s law links voltage (V), current (I) and resistance (R) with the relationship V = I × R, and electrical power (P) is P = V × I. Given any two of these four quantities, the other two are fully determined. This calculator is handy for sizing resistors, checking that a component stays within its power rating, and quick electronics bench work.

How it works

You choose which two of the four quantities you know, and the tool solves for the rest using Ohm’s law and the power equations. Depending on the pair, it applies the matching derived form:

You know	Voltage	Current	Resistance	Power
V and I	—	—	V ÷ I	V × I
V and R	—	V ÷ R	—	V² ÷ R
I and R	I × R	—	—	I² × R
V and P	—	P ÷ V	V² ÷ P	—
I and P	P ÷ I	—	P ÷ I²	—
R and P	√(P×R)	√(P ÷ R)	—	—

Example

Suppose you know the voltage is 12 V and the current is 2 A (the default pair):

Resistance R = V ÷ I = 12 ÷ 2 = 6 ohms
Power P = V × I = 12 × 2 = 24 watts

So a load drawing 2 A at 12 V behaves like a 6-ohm resistor dissipating 24 W. Pick the pair you know, type the values, and all four results update instantly — entirely in your browser, with nothing uploaded.