Ohm's Law Calculator

Ohm's Law

Ohm's Law describes how voltage, current and resistance relate in a circuit. The current I (in amps) through a conductor is proportional to the voltage V (in volts) across it, and inversely proportional to its resistance R (in ohms).

In practical terms: for a fixed resistance, raising the voltage raises the current by the same proportion, and lowering it lowers the current. It is the relationship you fall back on constantly when designing or fault-finding a circuit.

The law is written as:

V = IR

where:

• V is the voltage across the conductor (volts, V),
• I is the current flowing through it (amperes, A),
• R is its resistance (ohms, Ω).

Rearranging it

You can rearrange V = IR to find whichever value you're missing:

• To find the current I:
  
• To find the voltage V:
  V = IR
• To find the resistance R:
  

Example

Say you have a 200 Ω resistor with 2 A flowing through it. The voltage across it is:

V = IR = 2A × 200Ω = 400V

Where you'll use it

Ohm's Law is how you pick a resistor to set a particular current at a given voltage, and how you work back to an unknown value when you know the other two. It applies to any single component or any group of them, provided you keep the units consistent, volts, amps and ohms, and don't mix in millamps or kilohms without converting first.