dBm vs dBi – What’s the Difference?

dBi (decibel-isotropic) and dBm (decibel-milliwatts) are units used in different contexts to measure different properties in the field of radio frequency (RF) and telecommunications. This article includes a detailed explanation of each and their differences.

Table of Contents

dBi (Decibel-Isotropic)

  • Definition: dBi is a unit of measurement that describes the gain of an antenna relative to an isotropic radiator, which is a theoretical antenna that radiates power uniformly in all directions.
  • Context: It is used to quantify how much more power an antenna can direct in a specific direction compared to an isotropic antenna.
  • Usage: dBi is often used in specifications for antennas to indicate how effectively they can radiate power in a preferred direction.
  • Example: An antenna with a gain of 3 dBi can direct power three times more effectively in its favored direction compared to an isotropic antenna.
Antenna directive gain diagram

dBm (Decibel-Milliwatts)

  • Definition: dBm is a unit of power measurement relative to 1 milliwatt. It expresses the power level in decibels (dB) relative to 1 mW.
  • Context: It is used to measure the absolute power levels of RF signals, audio signals, and other types of power.
  • Usage: dBm is used in various applications, including telecommunications, audio engineering, and RF engineering, to indicate signal strength, power output of transmitters, and received signal strength.
  • Example: A power level of 10 dBm is equivalent to 10 milliwatts, while -10 dBm is 0.1 milliwatts. Use the dBm to Watt Calculator.

Key Differences

Type of Measurement

  • dBi: Measures the gain of an antenna relative to an isotropic radiator. It is a relative measurement.
  • dBm: Measures absolute power relative to 1 milliwatt. It is an absolute measurement.


  • dBi: Used in antenna design and specifications to describe how well an antenna directs energy in a specific direction.
  • dBm: Used to express power levels in circuits, systems, and signal strength in RF and audio applications.

Nature of Value

  • dBi: Represents a comparative gain value, indicating how much more (or less) effectively an antenna radiates compared to an isotropic source.
  • dBm: Represents an absolute power value, indicating the actual power level of a signal.

Practical Example

Scenario: You have a wireless router with an antenna gain of 5 dBi, and the router is transmitting at a power level of 20 dBm.

  • Antenna Gain (dBi): The 5 dBi gain means the antenna directs the power 5 dB more effectively in its primary direction compared to an isotropic antenna.
  • Power Level (dBm): The 20 dBm power level indicates that the router is transmitting at 100 milliwatts (since 20 dBm = 100 mW).


  • dBi is used to describe the directional gain of an antenna relative to an isotropic radiator.
  • dBm is used to describe absolute power levels relative to 1 milliwatt.

Understanding the difference between these units is crucial for designing and analyzing RF systems, as they provide insights into how effectively antennas transmit signals and what power levels are involved in the communication process.

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