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What is Ohm (Ω)? Unit of Electrical Resistance and Impedance

Ohm “Ω”: Definition, Formula, Measurement, Conversion and Calculation

What is Ohm Ω?

Ohm is a unit of electrical resistance, named after Georg Simon Ohm, a German physicist who discovered Ohm’s Law, which states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant.

What is Ohm - Ω - Unit of Resistance and Impedance

The concept of resistance is an essential component of electrical systems, and understanding it is crucial for anyone working with electricity or electronics, and are used extensively in circuit design, analysis, and troubleshooting.

Resistance is the property of a material that opposes the flow of electrical current, and is measured in ohms. The symbol for ohm is the Greek letter omega (Ω).

Resistance is influenced by various factors, including the type of material, its temperature, and its dimensions. The resistance of a material can be calculated using Ohm’s Law, and is defined as the ratio of the voltage across the material to the current flowing through it.

The unit of Ohm denoted by the symbol of Ω (the Greek letter omega) is used to measure the resistance between two points in a circuit or conductor. An Ohm is defined as:

One ohm is equal to the resistance of a conductor that allows a current of one ampere to flow through it when a voltage of one volt is applied across its terminals.


if a material has a resistance of 1 ohm, it will allow a current of 1 ampere to flow through it when a voltage of 1 volt is applied across it.


Ohm = 1 Volt ÷ 1 Ampere … (R = V ÷ I)


  • Ohm = Resistance in Ohms.
  • V = Voltage or Potential difference in Volts.
  • I = Electric current in Amperes.

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Good to Know:

  • The ohm is one of the seven base units in the International System of Units (SI) for measuring physical quantities.
  • The same unit of Ohm is also used to represent the measured value of Inductive reactance (XL), capacitive reactance (XC), Impedance (Z) and resistivity “ρ” (if area (a) and length (l) of the material is same).

Ohm’s Formulas & Equations with Related Quantities

Ohm from Amperes and Volts

  • R = V ÷ I
  • R = P ÷ I2
  • R = V2 ÷ P


Amps from Power Factor

R = Cos θ × Z


  • R is the resistance
  • Cos θ = Power factor
  • Z = Impedance of the circuit

Amps from Volts and Resistance

Resistance in AC resistive circuit

  • R = √(Z2 – X2)

Resistance in inductive circuit

  • R = √(Z2 + XL2)

Resistance in capacitive circuit

  • R = √(Z2 + XC2)


  • R = Resistance
  • Z = Impedance
  • XL = Inductive reactance
  • XC = Capacitive reactance

How to Measure Ampere?

The tool which is used to measure resistance in ohms is known as ohmmeter. In both analog and digital multimeters, there is an Ω mode for measuring resistance in AC and DC circuits.

To measure the electric resistance of an electrical element (such as resistorcapacitorinductordiode etc.) in Ω, simply put the two leads of the multimeter across the element and the display will show the exact value of resistance in Ω. You may follow the step by step guide posted in the previous article as “How to Measure Resistance in Ohms using Digital and Analog Multimeter?”.

How to Calculate Amps?

Based on the above given formula and equations for resistance in ohms (for different scenarios), we may calculate the value of electric resistance in ohms as follows.

Example 1:

If 3 amps are flowing in a conductor when 12V applied across it, find the value of resistance in Ohms.


According to Ohm’s law:

R = V ÷ I

Putting the values

R = 12V ÷ 3A

R = 4 Ohms

Example 2:

If the value of supply voltage across a 30W led bulb is 12V, Calculate the resistance of light bulb.


We know that

R = V2 ÷ P

Putting the values:

R = 122V ÷ 30W

R = 4.8 Ohms

Example 3:

Determine the value of resistance in ohms to glow a light bulb having 500W and 5Ω resistance .

Determine the value of resistance in ohms of a glowing bulb having the rating of 500W and 10A.


Using the derived equation of:

R = P ÷ I2

Putting the values:

R = 500W ÷ 102A

I = 5 Ohms.

Example 4:

What is the value of the electrical resistance in ohms for the circuit if the power factor is 0.9 and the impedance is 20 ohms?


using the power factor basic formula.

R = Cos θ × Z

R = 0.9 ÷ 20Ω

R = 18 Ohms

In this post, we learned about the definition of the Ohm (Ω), its unit of measurement, its discoverer, and the formula to calculate it in different scenarios. Additionally, we learned about how to measure and calculate the Ohm with solved examples.

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