A Beginners Guide to Calculating Resistance in Parallel Circuits

This is Part 1 in 3-part series about parallel and series circuits. Check out: Part 2: Calculating Resistance in Parallel Circuits, Part 3: A Beginners Guide to Batteries in Series and Parallel.

So you’re learning about electricity, maybe for school or a DIY project. You already understand how voltage, current, and resistance differentiate, and now you stepping into learning about series and parallel circuits.

In my case, I found I could understand series circuit behavior easily enough. But one concept I struggled to grasp was parallel circuits.

Sure, I could memorize the rules for a short time when I needed them – but I would always forget and be back at the same place.

When it comes to parallel circuits, and electricity for that matter, there’s a difference between getting it… and getting it.

So let’s try something today. I am going to sketch out a parallel circuit and try to explain how resistance behaves as simply as possible. I’ll also show you an interesting trick I learned that will help you solve any parallel circuit in the future.

How to Calculate the Total Resistance in a Parallel Circuit?

Sketch out the circut - it always helps to visulalise your circuit to calculate resistance. 
Identify where the parallel resistors are in your circuit. 
Add all of the parallel resistors up using the formula: 
1/RT = 1/R + 1/R..

Add the remaining resistirs up as would a series circuit: RT = R1 + R2 + R3

An Example Parallel Circuit

Here we have a basic 12V DC circuit. We start with one resistor wired in series valued at 100 ohms, then there are resistors wires in parallel valued at 20, 30, and 50 ohms. The final stage of the circuit is another resistor in series, values at 60 ohms.

Let’s solve this, one step at a time.

In a conventional circuit, the current will flow from the positive to the negative. The first resistor we need to take into account is the 100 Ohm resistor.

R (Total)  = 100 + ....

Next, our current is going to run into three parallel resistors.

We have to do something a little different when our current runs into multiple resistors at the same time.

To get the resistance for this stage of the circuit, we need to add all of the resistors together. This is the formula to add resistors in parallel:

1 / R = 1 / R1 + 1 / R2 + 1 / R3

Adding in our numbers, we get:

1/ R = 1/20 + 1/30 + 1/50
1/ R = 31 / 300
R = 9.68

R (Total)  = 100 + 9.68 + ...

See how we changed the parallel circuit to a series circuit by swapping out those three parallel resistors for a one total resistance value of 9.68 Ohms?

Try to think of parallel resistance as one big series resistor that you need to calculate.

From here, it’s easy to calculate the total resistance of the circuit.

R (Total)  = 100 + 9.68 + 60

R (Total) = 169.68 Ohms

Summing Up

Here are the steps you need to follow to calculate the total resistance the first time, every time:

Sketch out the circut – it helps to visulalise your circut to calculate resistance.
Identify where the parallel resistors are in your circut.
Calcule parallel resistors up to make up one value.
Add all the resitors up as you would for series circuit

You might now be asking, but what about current?

Well, that one deserves its own separate answer, you can read part two of this series on parallel circuits here.

I hope you have learned a thing or two today. If you like it, why not give it a share.

This is Part 1 in 4-part series. Check out: Part 2: Current, Part 3: Voltage, and Part 4: Batteries.