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

Amps

Alternate titles

  • How to Figure Out Amps
  • How many amps in volts, watts and/or ohms.
  • Amps - How to Calculate and Convert Power aka Watts / Volts / Ohms to Amps aka Current

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This is one of four pages having to do with Ohm's Law and its derivatives.

  1. How to calculate Power aka Watts
  2. How to calculate Current aka Amps (You Are Here)
  3. How to calculate Electromotive Force aka Volts
  4. How to calculate Resistance aka Ohms

Amps Equals... The Quick, Easy Answer.

Current aka amps equals watts divided by volts. For examples and other formulas that will give you the answer, or more information in general; continue reading.

Converting Watts and Volts to Amps

  • I = P/E Ohm's Law derivative for current.

Converting Volts and Ohms to Amps

  • I = E/R Ohm's Law for current.

Converting Watts and Ohms to Amps

  • I = √(P/R) Ohm's Law derivative for current.

How to convert watts / volts / ohms to amps.

Source

Amps Is the Unit of Measurement for Current

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

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

Each set of two has its own formula. There are three formulas.

The algebraic term for "symbol" is "variable".

"I" is the industry standard to designate current by the unit of measurement, amps.

"P" is the industry standard to designate power by the unit of measurement, watts. Sometimes "W" is used.

"R" is the industry standard to designate resistance by the unit of measurement, ohms.

"E" and "V" are both used to designate electromotive-force by the unit of measurement, volts. The industry standard used to be "E", but now both "E" and "V" are being used interchangeably.

How to Convert Watts and Volts to Amps

#1 Calculating Amps Using Watts and Volts

All one does is divide watts by volts to get amps. The next four statements all say the same thing in different ways and have the same meaning. Whichever one that works best for you is fine.

  • Current equals Power divided by Electromotive-force
  • Current equals Wattage divided by Voltage
  • Current equals Watts divided by Volts
  • Amps equals Watts divided by Volts

The next two formulas say the same thing and have the same meaning. Whichever one that works best for you is fine.

  • I=P/E
  • I=P/V

Some everyday examples:

A 100-watt light bulb divided by the typical 120-volt house current means it's using .8333 amps. 100 watts divided by 120 volts equals .8333 amps.

A 6-watt car instrument cluster connected to the standard 12-volt battery uses half an amp. 6 watts divided by 12 volts equals .5 amps.

A 720 watt appliance feeding off a 240 volt line consumes 3 amps. 720 watts divided by 240 volts equals 3 amps.

A 600-watt starter for a small engine using the standard 12-volt battery uses 50 amps. 600 watts divided by 12 volts equals 50 amps.

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

A 300 milliwatt (.3 watts) circuit board connected to a 3-volt power supply uses 100 milliamps (.1 amps). 300 milliamps divided by 3 volts equals 100 milliamps.

A 60-watt circuit requiring a 300-volt input will use .2 amps (200 milliamps). 60 watts divided by 300 volts equals .2 amps.

Some math / algebra examples:

1.5 amps = 300 watts / 200 volts

.20833333 amps = 25 watts / 120 volts

.833333 amps = 100 watts / 120 volts

How to Convert Volts and Resistance ( ohms ) to Amps

The typical house circuit breaker is 20 amps.

Source
Source

#2 Calculating Amps Using Volts and Resistance ( ohms )

To calculate current (in amps), one divides the voltage by the resistance. The next four statements all say the exact same thing and have the same exact meaning. Which ever one you prefer is good.

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

The two formulas say the same thing and have the same results. It is only a matter of preference.

  • I=E/R
  • I=V/R

Some examples:

Example #1:

The sales clerk claims that your spiffy, new electronics device uses less than 2 amps. Is it true? Well, you know you're plugging it into a 120-volt outlet; so that's one variable already known. So, you hook up the VOM (see note below) to the device to ascertain it's resistance measurement. The measurement comes back as 72 ohms.

The formula is I=E/R. 120 volts divided by 72 ohms equals 1.6667 amps. The sales clerk was correct

Example #2:

Just how much amperage does that 4-battery, super-flashlight really use? Well, the four batteries give a 6-volt output. The meter says there is an 18 ohm resistance.

I=E/R. So you have 6/18, which equals .3333 amps.

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

There are 60 volts running through a circuit with a measured resistance of 40 ohms. So that would be 60/40, which gives you 1.5 amps.

There are 3 volts running through a circuit with a measured resistance of 5 ohms. So that would be 3/5, which gives you .6 amps (600 milliamps).

Some math / algebra examples:

I = 48/500 = .096

I = 1.5 / 10 = 150 milliamps

A general note about VOM's: A VOM is a multi-range, volt-ohm multi-tester meter. It is a test instrument to measure voltage, current, and resistance of an element or circuit. Do not buy one until you really know what you are doing. If you just want a meter or meters for measuring volts and/or amps only, you have more latitude. But when it comes to measuring resistance, cheaper meters are extremely inaccurate when it comes to measuring certain ranges. Really research the subject first.

How to Convert Power (watts) and Resistance (ohms) to Amps

Just a Tesla Coil demonstration at Fermilab.

Can click to enlarge.
Can click to enlarge. | Source

#3 Calculating Amps Using Power (watts) and Resistance (ohms)

This one is a little trickier. Square root is involved, so you will have to break out the calculator or spreadsheet. The next four statements say the same thing and have the same meaning.

  • To calculate Current, divide Power by Resistance, then derive the square root of the quotient
  • To calculate Current, take the square root of the answer to (Power being divided by Resistance)
  • Amps equals Watts being divided by ohms, then deriving the square root of the quotient
  • Amps equals taking the square root of the answer to (Watts being divided by Ohms

The formula is:

  • I=(P/R)

Some examples:

Example #1

How many amps are flowing through a 2500 watt circuit that has a resistance of 25 ohms?

I=√(P/R)

The 2500 watts is divided by the 25 ohms giving an answer (aka quotient) of 100. The square root of 100 is 10. The final answer is 10 amps.

Example #2

How many amps are flowing through a 1500 watt space heater that has a resistance of 9.6 ohms?

I=√(P/R)

The 1500 watts is divided by the 9.6 ohms giving an answer (aka quotient) of 156.25. The square root of 156.25 is 12.5. The final answer is 12.5 amps.

The Three Formulas for Amps

Summary

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

I=E/R (Amps equals volts divided by ohms.)

I=√(P/R) (Amps equals the square root of (watts divided by ohms).)

If you have any suggestions on how this reference article can be improved, please let us know. There is a Comments Section further down the page.

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

I = P/E Ohm's Law derivative for current.
I = P/E Ohm's Law derivative for current. | Source

By the way, if you didn't know algebra before; you do now. All algebra does is substitute variables for numbers. Algebra is easy to learn.

Base Numbering Systems

As one delves deeper into the science of electronics, different base numerical systems come into play.

For future reference...

Base 2, 4, 8, 16 Number System Lessons for Binary, Quaternary, Octal, and Hexadecimal

Coincidentally, these are the four primary base number systems used when it comes to all things computer programming.

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