Calculators

Resistance Calculator

Calculate electrical resistance using voltage and current, or determine the resistance of a conductor from its material resistivity, length and cross-sectional area.

What Is Electrical Resistance?

Electrical resistance is the opposition that a material or component presents to the flow of electric current.

Resistance is measured in ohms (Ω).

For a simple resistive circuit, resistance can be calculated from voltage and current using Ohm's law.

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Calculate Resistance from Voltage and Current

The resistance of a resistive load can be calculated using: 

R = V ÷ I

Where:

  • R = resistance in ohms (Ω)
  • V = voltage in volts (V)
  • I = current in amperes (A)
Enter voltage and current.

Example

A load has 24 V across it and draws 3 A. 

R = V ÷ I

R = 24 ÷ 3

R = 8 Ω

The resistance is therefore 8 Ω.

Resistance and Power

Resistance can also be related to electrical power. 

P = V² ÷ R

P = I² × R

These relationships are useful when determining the power dissipated by a resistor or resistive load.

Calculate Resistance from Voltage and Power

Enter voltage and power.

Calculate Resistance from Current and Power

Enter current and power.

Resistance of a Wire

The resistance of a uniform conductor depends on its material, length and cross-sectional area.

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The relationship is: 

R = ρL ÷ A

Where:

  • R = resistance in ohms (Ω)
  • ρ = resistivity of the material
  • L = conductor length
  • A = cross-sectional area

Wire Resistance Calculator

Enter the material resistivity, length and cross-sectional area.

Common Material Resistivity

The following are approximate room-temperature values. Actual resistivity varies with temperature, purity and material composition.

Material Approximate Resistivity (Ω·m)
Copper 1.68 × 10⁻⁸
Aluminium 2.82 × 10⁻⁸
Silver 1.59 × 10⁻⁸
Gold 2.44 × 10⁻⁸
Iron Approximately 1.0 × 10⁻⁷

Resistance of Copper Wire

Copper is widely used for electrical wiring because of its relatively low resistivity.

For example, increasing the length of a copper wire increases its resistance, while increasing its cross-sectional area decreases its resistance.

Effect of Wire Length

Resistance is directly proportional to conductor length. 

Longer wire → Higher resistance

Shorter wire → Lower resistance

If the length of a conductor is doubled while all other factors remain constant, its resistance approximately doubles.

Effect of Wire Thickness

Resistance is inversely proportional to cross-sectional area. 

Larger area → Lower resistance

Smaller area → Higher resistance

This is why thicker electrical conductors generally have lower resistance than thinner conductors made from the same material and having the same length.

Resistance and Temperature

The resistance of many conductors changes with temperature.

For many metallic conductors, resistance increases as temperature increases.

A commonly used approximation is: 

R = R₀ [1 + α(T - T₀)]

Where:

  • R₀ = resistance at reference temperature
  • α = temperature coefficient
  • T = final temperature
  • T₀ = reference temperature

Resistance in Series

When resistors are connected in series, their equivalent resistance is the sum of their individual resistances.

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Rtotal = R1 + R2 + R3 + ...
Enter at least two resistor values.

Resistance in Parallel

For resistors connected in parallel, the equivalent resistance is calculated from the reciprocal of the individual resistances.

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1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...
Enter at least two resistor values.

Resistance Units

Unit Symbol Value
Ohm Ω 1 Ω
Kilohm kΩ 1,000 Ω
Megohm MΩ 1,000,000 Ω

Example Resistance Conversions

1 kΩ = 1,000 Ω

4.7 kΩ = 4,700 Ω

10 kΩ = 10,000 Ω

1 MΩ = 1,000,000 Ω

2.2 MΩ = 2,200,000 Ω

Measuring Resistance

Resistance can be measured with a multimeter using its resistance function.

  1. Turn off the circuit.
  2. Disconnect the component where necessary.
  3. Select the appropriate resistance range.
  4. Place the probes across the component.
  5. Read the resistance.

Measuring a resistor while it is connected to other components can produce an incorrect reading because current can flow through other paths in the circuit.

Open Circuit

An open circuit has extremely high resistance because there is no continuous conductive path.

A multimeter may display:

  • OL
  • Infinity
  • Out of range

depending on the meter.

Short Circuit

A short circuit has very low resistance.

Ideally, a perfect short circuit has zero resistance, although real wires and connections always have some small resistance.

Resistance Troubleshooting

Measurement Possible Meaning
Expected resistance Component may be normal
OL / infinite Possible open circuit
Almost zero Ω Possible short circuit
Much higher than expected Possible damaged resistor or incorrect circuit measurement
Unstable reading Poor connection, damaged component or in-circuit effects

Important Notes

  • Resistance is measured in ohms.
  • Ohm's law can be used to calculate resistance from voltage and current.
  • Resistance depends on material, length and cross-sectional area.
  • Many conductors change resistance with temperature.
  • Series resistances add together.
  • Parallel resistance is lower than the smallest individual resistance.
  • In-circuit resistance measurements can be affected by other components.
  • Power dissipation must be considered when selecting resistors.

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