Ohm's Law Calculator
Ohm's law ties together the three quantities that describe a simple circuit. Enter any two of voltage, current and resistance, and this calculator returns the third.
Formula
V = I × R
- V = voltage in volts
- I = current in amps
- R = resistance in ohms
2 A through 12 Ω: V = 2 × 12 = 24 V, and P = 24 × 2 = 48 W
The Formula
V = I × R
Rearranged for whichever value you need:
- Voltage: V = I × R
- Current: I = V ÷ R
- Resistance: R = V ÷ I
Voltage is measured in volts, current in amperes, and resistance in ohms (Ω).
Use Base Units
The formula only works in volts, amps and ohms, but components are labelled in milliamps and kilohms. This is the error that produces impossible answers.
- 1 mA = 0.001 A
- 1 kΩ = 1,000 Ω
- 1 MΩ = 1,000,000 Ω
12 V across a 2.2 kΩ resistor is 12 ÷ 2,200 = 5.45 mA. Enter 2.2 instead and you get 5.45 A, a thousand times too high.
Adding Power
Ohm's law pairs with the power formula to give three ways of finding watts:
- P = V × I
- P = I² × R
- P = V² ÷ R
This matters when choosing a resistor: the right resistance with too low a power rating will overheat and fail.
Quick Reference
| To find | From V and I | From V and R | From I and R |
|---|---|---|---|
| Voltage (V) | — | V = √(P × R) | V = I × R |
| Current (I) | I = P ÷ V | I = √(P ÷ R) | — |
| Resistance (R) | R = V² ÷ P | — | R = P ÷ I² |
| Power (P) | P = V × I | P = V² ÷ R | P = I² × R |
Frequently Asked Questions
- What is Ohm's law?
- It states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance, written as V = I × R.
- How do I calculate the resistor for an LED?
- Subtract the LED's forward voltage from the supply voltage, then divide by the desired current. A 2.1 V LED at 20 mA on a 9 V battery needs (9 − 2.1) ÷ 0.02 = 345 Ω, so use the next standard value up.
- Does Ohm's law apply to everything?
- No. It describes ohmic components whose resistance stays constant. Diodes, LEDs, filament bulbs and thermistors are not ohmic, and AC circuits with capacitors or inductors involve impedance rather than plain resistance.
- Why is current abbreviated as I?
- From the original French term intensité de courant, meaning intensity of current. It is the reason current is never abbreviated to C.