Ohm's Law Calculator

Ohm’s law connects the three basic quantities of a circuit: voltage, current and resistance. Know any two and this calculator finds the third, along with the power the part uses, written with a sensible unit (2 mA, 4.7 kΩ).

How to Use It

1. Under Find, pick the value you don’t know: voltage, current or resistance.
2. Enter the other two in base units: volts (V), amperes (A) and ohms (Ω).
3. Press Calculate. The answer shows with its unit, plus the power and the full equation with your numbers.

Converting from prefixed units: 20 mA is 0.02 A, 4.7 kΩ is 4,700 Ω, and 1.5 kV is 1,500 V.

How It Works

Ohm’s law says the voltage across a resistor equals the current through it times its resistance. Rearranged, it gives each of the three:

The power, in watts, is how fast the part turns electrical energy into heat or light:

Worked Example

A 6 Ω resistor carries a current of 2 A. Its voltage is $\vA{V} = 2\,\text{A} \cdot 6\,\Omega = 12\,\text{V}$, and it uses $P = 12\,\text{V} \cdot 2\,\text{A} = 24\,\text{W}$ of power.

Going the other way: an LED circuit runs from a 5 V supply through a 250 Ω resistor. The current is $\vB{I} = 5\,\text{V} \div 250\,\Omega = 0.02\,\text{A}$, which the calculator shows as 20 mA.

Tips and Common Mistakes

• Convert prefixes first. Entering 20 for a 20 mA current is 1,000 times too much. Divide milli- values by 1,000; multiply kilo- values by 1,000.
• Check the power rating. A resistor that has to handle more watts than it’s rated for (often 0.25 W for small ones) will overheat.
• It’s for resistive parts. Ohm’s law holds for resistors and wires. LEDs, diodes and most real loads aren’t “ohmic”: their resistance changes with the current.
• Use the voltage across the part, not the supply voltage, when other parts share the circuit.
• Resistance can’t be zero when you’re solving for current, and current can’t be zero when you’re solving for resistance: the math divides by it.