The amount of electric current in a circuit depends on two things: the amount of voltage, the force with which the current is pushed along, and the amount of resistance in the circuit. The relationship between current, voltage and resistance is known as Ohm's Law. This law states that resistance equals voltage divided by current, or R = V ÷ I. Ohm's Law is important in electronics. It allows us to work out the value of the components needed to build an electronic circuit.

Calculation triangle

Ohm's Law can actually be written in three different ways: resistance equals voltage divided by current (R = V ÷ I); current equals voltage divided by resistance (I = V ÷ R); voltage = current multiplied by resistance (V = I × R). Each of these three equations can be represented by a triangle.
​​​​​​​

Ohm's Law makes it easy to calculate voltage, current and resistance if you know the value of two of these three things. This is very useful in electronics. You can use it, for example, to find what value of resistor you will need for an electronic circuit. See this page to find out how.

Using Ohm's Law:
​​​​​​How much current?

Here is a very simple circuit with a battery and a resistor. The battery is a 12 V battery and the resistor has a resistance value of 600 Ω. How much current flows through the circuit?
​​​​​​​

From the triangle above, current equals voltage divided by resistance, I = V ÷ R
I = 12 ÷ 600

You can use your calculator to get the result:
I = 0.02

​​​​​​​I is measured in amps (A). So the current in the circuit is 0.02 A, or 20 milliamps (20 mA).