How to use this Ohm's law calculator
- Pick the two values you already know — for example voltage and resistance.
- Type them in and choose the units (mV, V, kV, µA, mA, A, Ω, kΩ, MΩ, mW, W).
- The other two values appear instantly, together with the formula that was used and a safe resistor wattage.
You do not need to press a button. Change any number and the results update.
The Ohm's law formulas
Ohm's law links three quantities: voltage (V) in volts, current (I) in amps and resistance (R) in ohms. Power (P) in watts follows from them.
| You know | Find current | Find voltage | Find resistance | Find power |
|---|---|---|---|---|
| V and R | I = V ÷ R | — | — | P = V² ÷ R |
| I and R | — | V = I × R | — | P = I² × R |
| V and I | — | — | R = V ÷ I | P = V × I |
| P and V | I = P ÷ V | — | R = V² ÷ P | — |
Choosing the resistor wattage
Every resistor has a power rating — commonly ⅛ W, ¼ W, ½ W, 1 W and 2 W. Running a resistor near its limit makes it hot, shifts its value and shortens its life. A simple rule used by engineers is to pick a rating at least twice the power it will dissipate. The calculator does this for you.
Common mistakes
- Mixing units. 20 mA is 0.02 A, not 20 A. The unit drop-downs avoid this.
- Using Ohm's law on an LED or motor directly. These are not simple resistors. Apply Ohm's law to the resistor in series instead — see the LED resistor calculator.
- Forgetting power. A value can be right in ohms and still burn out because its wattage is too small.
Where you'll use it
Ohm's law is the first calculation in almost every electronics project: setting LED current, sizing pull-up resistors for an Arduino, checking how much current a sensor draws, building a voltage divider or working out how hot a resistor will get. Learn the theory step by step in our guide, Ohm's law explained with examples.
