How to Calculate or Convert Ohms to and from Amps, Volts, Watts.

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Alternate Titles

• How to Figure Out Ohms
• How Many Ohms in Volts, Amps, and/or Watts
• Ohms - How to Calculate and Convert Power aka Watts / Volts / Amps to Ohms aka Resistance

If you are looking for the resistor color code info, it is about halfway down the page on the right-hand side.

Ohms Equals... The Quick, Easy Answer.

Dividing volts by amps will give you resistance in ohms. For examples and other formulas that will give you the answer, or more information in general; continue reading.

Letters are used to designate the different electrical properties.

• "P" is used to represent watts
• "E" and "I" are used to represent voltage and amperage respectively.
• "R" represents resistance in ohms.

Converting Volts and Amps to Ohms

• R = E/I Ohm's Law for resistance.

Converting Volts and Watts to Ohms

• R = E2/P Ohm's Law derivative for resistance.

Converting Watts and Amps to Ohms

• R = P/I2 Ohm's Law derivative for resistance.

Source

Ohms Is the Unit of Measurement for Resistance

The amount of electrical resistance in a circuit, or internally in a device, can be determined by knowing any two of either power, electromotive-force, or current.

In other words, the number of ohms can be determined by knowing any two of either watts, volts, or amps.

#1 Calculating Ohms Using Volts and Amps

All one does is divide volts by amps to get ohms. The next four statements all say the same thing and have the same meaning. Pick whichever one that works for you.

• Resistance equals Electromotive-force divided Current
• Resistance equals Voltage divided by Amperage
• Resistance equals Volts divided by Amps
• Ohms equals Volts divided by Amps

The two formulas say the exact same thing and have the exact same meaning. Pick whichever one that works for you and make it yours.

• R=E/I
• R=V/I

Some everyday examples:

A 12-volt car battery divided by .5 amps equals 24 ohms (maybe a car instrument cluster).

A 240-volt house current divided by 3 amps equals 80 ohms (maybe a washer or dryer).

A 12-volt car battery divided by 50 amps equals .24 ohms (maybe a starter for a small engine).

Some circuit / formula examples (there are a 1000 millivolts in a volt, there are a 1000 milliamps in 1 amp, there are 1000 milliwatts in a watt):

3 volts divided by 100 milliamps (.1 amps) equals 30 ohms.

300 volts divided by .2 amps (200 milliamps) equals 1500 ohms of resistance.

Some math / algebra examples:

50 ohms = 200 volts / 4 amps

20 ohms = 120 volts / 6 amps

100 ohms = 120 volts / 1.2 amps

*** Resistor Value Color Code ***

This resistor-value-color-code section was included just in case someone happened to have a need or use for it. | Source

Color code for first three bands

• Black = 0
• Brown = 1
• Red = 2
• Orange = 3
• Yellow = 4
• Green = 5
• Blue = 6
• Violet = 7
• Gray = 8
• White = 9

The fourth band indicates tolerance or manufacturing variance. A 5% variance is better quality than a 10% variance.

• Gold = 5%
• Silver = 10%

Resistor Value Color Code Examples

From left to right, the resistor has red-green-orange bands. That would translate to the resistor having a resistance value of 25000 ohms.

Blue-yellow-red means the resistor has a value of 6400 ohms.

Gray-brown-brown equals 810 ohms.

More Examples

Band Colors
Ohms
black-red-black
2
gray-green-black
85
red-blue-brown
260
gray-orange-red
8300
red-gray-orange
28000
gray-red-yellow
820000
blue-gray-green
6.8 megohms
violet-red-blue
72 megohms
green-gray-violet
580 megohms
gray-red-gray
8200 megohms
red-green-white
25000 megohms

#2 Calculating Ohms Using Watts and Volts

[To square a number means to multiply a number times itself. Example: 32=9.]

To determine the resistance in ohms, one squares the voltage, and then divides by watts. The next four statements say the same thing with the same meaning.

• Resistance equals Electromotive-force squared divided by Power
• Resistance equals Voltage squared divided by Watts
• Resistance equals Volts squared divided by Power
• Ohms equals Volts squared divided by Watts

The next four formulas all say the same thing and have the same meaning. Pick whichever one that works for you and make it yours.

• R=E2/P
• R=V2/P
• R = (E x E) / P
• R = (V x V) / P

Some examples:

Example #1:

The 120-volt device uses 200 watts. What is its internal electrical resistance? The formula is R=E2/P. 120 volts squared equals 14,400. So you now have 14400/200 which would equal an answer of 72 ohms.

Example #2:

The device uses 2 watts of power and requires a 6-volt power source. If a test meter was hooked up to it, what should it read?

R=E2/P. So the ohms will equal 36 divided by the 2 watts. The meter should read an 18 ohm resistance.

Some circuit / formula examples: (there are 1000 millivolts in a volt, there are a 1000 milliamps in 1 amp, there are 1000 milliwatts in a watt):

There are 60 volts running through a circuit board which uses 90 watts of power. What is the circuit board's internal resistance? 60 volts squared is 3600. So we have 3600/90 watts. The answer is a resistance of 40 ohms.

There are 3 volts running through a circuit that uses 1.8 watts. The 3 squared is 9. The 9 divided by 1.8 equals 5. So the answer is 5 ohms.

Some math / algebra examples:

If 48 volts and 500 watts, then R=482/500=2304/500=4.608 ohms

If 1.5 volts and 10 watts, then R=1.52/10=2.25/10=.225 ohms

Just a Tesla Coil demonstration at Fermilab.

Can click to enlarge. | Source

#3 Calculating Ohms Using Watts and Amps

[To square a number means to multiply a number times itself. Example: 52=25.]

To calculate resistance (in ohms), one squares the current (amps), and then divides it into the number of watts. The next four statements each say the same thing and have the same meaning.

• Resistance equals Current-squared, divided into Power
• Ohms equals amps-squared, divided into watts
• Resistance equals Power divided by Current-squared
• Ohms equals watts divided by amps-squared

Here are two representations of the same formula:

• R=P/I2
• R = P / (I x I)

An example:

You have 10 amps flowing through a device or circuit that is using 2500 watts of power. What is the internal resistance?

R=P/I2

R = 2500 watts divided by 102

R = 2500 divided by (10 x 10)

R = 2500 divided by 100

R = 25 ohms

Summary

R=E/I (Ohms equals volts divided by amps. Of the three formulas, this is the one that is used 90% of the time.)

R=E2/P

R=P/I2

The how-to convert amps / volts / watts to ohms page.

R = E/I Ohm's Law for resistance. | Source

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