Difference Between dB and dBm (With Table)

dB is usually associated with the measurement of sound waves, however, its usage is extended to the field of engineering, where it is primarily used to calculate the ratio between the power levels at two distinct points in optic networks and cables.

dBm is the outright measure of the power intensity at only one point. It does not take into account the difference of signal or power in the overall network.

dB vs dBm

The difference between dB and dBm is that dB is used to connote a relative difference in power, voltage, current, or signal strength between two points. dBm is used to denote the absolute measurement of power or signal at a given point in a cable.

dB essentially represents a ratio between the value measurements at two given points. dBm is the measurement of power at a point in decibels referenced against one milliwatt (mW).

Comparison Table Between dB and dBm

Parameters of ComparisondB  dBm  
DefinitiondB is generally used to measure the difference in the power, voltage, or signal strength between two points in a cable or network.dBm is used to measure the strength of a signal at a given point in the cable or network.
Full-FormThe abbreviation dB stands for Decibels.The abbreviation dBm stands for Decibel Milliwatts.
Reference pointOne dB is measured against the power input of an optical cable.One dBm is measured against one milliwatt.
Nature of the Unit of MeasurementdB is a relative, dimensionless unit of measurement as it connotes a ratio between two 2 values.dBm is an absolute measure of the signal strength at a particular point.
Uses in a Fiber-Optic NetworkdB is used to connote the optical loss in a fiber-optic network.dBm is used to connote the optical power in a fiber-optic network.

What is dB?

dB is the shorthand used to refer to Decibels. It is a logarithmic ratio between the two values of power intensity measured at two different points in a wire, cable or network. As a dimensionless unit, dB is a relative value. This unit of measurement is generally associated with the measurement of sound. However, it is also used to calculate the intensity of power, signals, and current.

The utility value of this measuring system is stored in its ability to facilitate the convenient reduction of large values –by converting the values into ratio- into manageable integers. This helps make calculations easier for engineers and scientists.

dB is a useful unit in the measurement of the increase or decrease of signal strength between two points in an amplifier system. This makes it an accepted system of measurement in the discipline of acoustic electronics.

The formula for calculating 1 dB is as follows:

1 dB = 10 log (P1/P2)

Where P1 is the power intensity at the first point and P2 is the power intensity at the second point.

What is dBm?

dBm is the abbreviation used to connote Decibel Milliwatts. It is the absolute measure of power at a given point in the cable or network. Closely related to dB, dBm measures power at a given point in reference with one milliwatt. dBm is calculated as ten times the log of the signal strength in milliwatts.

It denotes the amount of power an antenna can produce at any given point. dBm is also used in the study of optic signals. It is used to define the signal strength in various electronic equipment like wires, cables, signal boosters, etc. A strong signal of an amplifier will range around 70 dBm or higher, while a weaker signal will around 100 dBm or lower.

It is also used in the study of fiber optic networks. dBm is usually used to measure optical power in absolute terms. Hence, dBm can be classed as another unit of power that expresses the outright power value intensity at a point in milliwatts.

The formula for calculating 1 dBm is as follows:

1 dBm =10*log (P/1mW)

Where P stands for power in watts.

Main Differences Between dB and dBm

  1. The main difference between dB and dBm is that the former is defined as the difference in the power or signal intensity between any two given points, while the latter is defined as the absolute value or intensity of power. The latter is used to determine signal strength at a particular point.
  2. The full-form of each is different. The abbreviation dB stands for Decibels, while dBm is the commonly used abbreviation for Decibel Milliwatts.
  3. The absolute power or signal level signified by dBm is referenced against 1 milliwatt. On the other hand, dB measurements do not correspond to 1 milliwatt. dB values for a given optical cable are referenced against the power of the total input in the network.
  4. Unlike dB, dBm is an absolute unit of measurement. The former represents a relative number connoting the increase or decrease in signal strength. However the latter connotes the absolute measure of the signal strength at a given point.
  5. The function of dB and dBm is different in a fiber-optics network. In a fiber optical network, optical power tends to be measured by the dBm unit of measurement, while optical loss is measured using the dB ratio measurement system.


dB and dBm are interrelated concepts. Both these concepts are central to equations and calculations in the field of science, engineering, and optical signaling. However, there are minuscule differences between the two.

A close assessment of the concepts reveals that dB is a ratio measure. It is a dimensionless measure of the difference in power intensity between two given points in a network, wire, or cable. Whereas, dBm stands for an absolute, outright value that denotes the power intensity at solely a singular point in the network or cable.

dBm measures power with a reference to milliwatts, while dB measures it as simply a ratio without associating the calculation with any other units of power. Moreover, the uses of each concept are also a little different, although they are both used in the same disciplines.


  1. https://ieeexplore.ieee.org/abstract/document/6222357/