How to Calculate Ohms

An ohm is the unit used to measure an objects resistance to electrical current. Something with very low ohms will be a good conductor, while something with very high ohms will be a good insulator. When working with DC circuits, an objects current, voltage, and resistance operate in a manner that can be predicted using ohms law.

When calculating ohms law, it is best to use the measurements of voltage(V,) amperes(A,) and ohms(Ω.) Units should be converted to these units for calculation, but can be changed back afterwards.

To determine current, it is necessary to know voltage and resistance.

Using ohms law, it is possible to figure out an objects current, providing the resistance and voltage are known. It is also possible to determine the Resistance or Voltage of an item, providing the current is know, in addition to the voltage or resistance respectively.

It is important to note that in most text book equations, the abbreviations used to describe voltage and current are slightly different than the Units used to measure them. This can be a little confusing.

For the purpose of the following examples:

  • I = Current(A)
  • R= Resistance (ohms)
  • E= voltage of the battery (V)

OHMS Law States That:

  • E = IR
  • I = E/R
  • R = E/I

Below, you will find some examples showing how ohms law can be used.

Question: Given a 12 volt battery with a resistance of 4ohms, what is the current?


  • I = E/R
  • I = (12/4)
  • I = 3A

Question: Given a battery with a 50ohm resistance and a current of 1A, what is the voltage?


  • E = IR
  • E = (50 * 1)
  • E = 50 V

Question: Given a 12V battery that operates at 3A, what is the resistance?


  • R = E/I
  • R = 12/3
  • R = 4 ___ ohms

The Importance of Ohms Law

The power of ohms law becomes apparent, it is possible to determine Voltage, Current, or Resistance, providing you have two of the units to work with.

How is Power Measured

Power is the rate that energy is used and is usually measured in watts. The watt is represented by a 'W', but in formulas that figure an items wattage, an uppercase 'P' is used.

An objects Power can be measured providing you have the Voltage(E) and Amperes(I.)

To calculate an objects power, you can use the following formulas:

  • P = EI
  • P = E^2/R
  • P=I^2R

Some Examples of how to calculate power can be seen below:

Question: Given a circuit that is operating at 12V and 5A, how much power is being used?


  • P = EI
  • P = 12 * 5
  • P = 60 W

Question: Given a circuit operating at 6V and with a 5ohm resistance, what is the power being used?


  • P = E^2 / R
  • P = (6*6) / 5
  • P = 7.2 W

Question: Given a circuit with a resistance of 20ohms operating with a current of .5A, what is the power being used?


  • P = I^2R
  • P=(.5*.5*20)
  • P= 5W

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