Simple Ohm's Law Calculator

(The Calculator is at the bottom of the page if you want to skip ahead)

In working with wiring and related in your vehicle you will need to know a bit about Ohm's Law. This is a very simple set of formulas that help you calculate Power, Voltage, Current and Resistance. It's often seen in Wheel with the various formula embedded in it.

Why do I need to know about this stuff? Well if you are doing any sort of wiring you need to know the current capability of your circuit to, for example, determine the proper wire size. Also note that this is for Direct Current (DC) as that is primarily what you use in your vehicle. These formula apply only to DC, and will be different for Alternating Current (AC) that you might find in your home. As mentioned elsewhere check out this Toyota Training Document on elementary automotive electrical systems.

Real World Example

(well maybe not so real)

Let's say you want to connect up TWO 100W driving lights to your 4x4 and they didn't tell you how many amps (current) each light draws. The more likely cause is you lost all of the documentation and only remember the 100 Watts Rating on the lights. At this point you could just hook one up and measure the current That is, if you didn't also lend your multi-meter to a buddy, so now you must use math to solve the problem or hook it up and see if the wiring catches fire and burns down your truck.

What do you know about this problem?

Car Battery Voltage is approx. 12 Volts (nominal, lets stick with this for now)
Each Bulb is 100W and I have 2

So how much Current is being Drawn by all 2 bulbs at 12 Volts? That's Easy, each bulb draws 8.34 Amps at 12 Volts, and I have 2 bulbs each running at 12 Volts, so that's right at 16.7Amps. Wait, how is the magic accomplished. By this simple formula -

I = P / V

Where :

I = Current in Amps
P = Power in Watts
V = Force in Volts

So each Bulb's current is calculated as follows -

Current in Amps = 100 Watts / 12 Volts
8.33 = 100/12

Now that I have 2 bulbs that's about 16.7 Amps total and that will tell me that I am going to need at least plan for a 20 amp circuit and calculate wire size based on that. Shazam!

Ohm's Law Notation

Their are a few terms that you need to know about and then you are on your way to becoming an electrical engineer. Here they are the important symbols and what they mean.

I = Current in Amps
P = Power in Watts
V = Force in Volts, Sometimes Also called 'E' and used interchangeably
R = Resistance in Ohms

Ohm's Law and Common Formulas

You can find out any of the OTHER 2 values if you know the other 2. This makes it easy to solve for most any common electrical problem. Here are a few that are common and helpful with you Automotive work. The ENGLISH SPEAK is given to help understand what each formula is trying to find given what known values.

Note :
'/' is the Divide Operation
Sqrt(X) means take the Square Root of X (Your Calculator will have this button)
V² is V Squared or essentially V multiplied by V

Finding The Resistance

R = V / I Given The Voltage and Current What's The Resistance
R = V² / P Given the Voltage and Power What's the Resistance
R = P / I²Given the Power and the Current What's the Resistance

Finding The Voltage

V = I x R Given The Current and Resistance What's the Voltage
V = P / I Given The Power and the Current What's the Voltage
V = Sqrt(P x R) Given the Power and Resistance What's the Voltage

Finding The Current

I = V / R Given the Voltage and the Resistance What's the Current
I = P / V Given the Power and the Voltage What's the Current
I = Sqrt(P / R) Given the Power and the Resistance What's the Current

Finding The Power

P = I x V Given Current and Voltage What's the Power
P = V² / R Given Voltage and Resistance What's the Power
P = I² x R Given Current and Resistance What's the Power