DBm

dBm or dB_mW (decibel-milliwatts) is a unit of power level using a logarithmic decibel (dB) scale. It is measured compared to one milliwatt (mW). People who work with radio, microwave, and fiber-optical communication often use dBm to measure the power of system transmissions. This helps them express both very large and very small values easily. dBW is a similar unit, but it is measured compared to one watt (1000 mW).

The decibel (dB) is a dimensionless unit. It is used to show the ratio between two values, like the signal-to-noise ratio. The dBm is also dimensionless, but it compares to a fixed reference value, making it an absolute quantity.

The dBm is not part of the International System of Units (SI). Because of this, it is not recommended for use in documents or systems that follow SI units. However, the unit decibel (dB) for relative quantities, without any suffix, is a non-SI unit that is accepted for use alongside SI units.

In audio and telephony, dBm is usually compared to a 600-ohm impedance, which is common in telephone voice networks. In radio-frequency work, dBm is usually compared to a 50-ohm impedance.

Unit conversions

A power level of 0 dBm is the same as having 1 milliwatt of power. If the level goes up by 10 dB, the power becomes ten times bigger. So, a rise of 20 dB means the power is one hundred times bigger. A small rise of 3 dB is about the same as doubling the power, so 3 dBm is close to having 2 milliwatts of power. Each time the level drops by 3 dB, the power gets about half as big, so −3 dBm is close to having 0.5 milliwatts of power.

To change any power measured in milliwatts to dBm, you can use a special math formula. And to change a dBm level back to milliwatts, there is another math formula you can use.

Table of examples

Main article: Orders of magnitude (power)

Standards

The strength of a signal can be measured in different ways around the world. In the United States, measurements are usually made without special changes and need to mention the bandwidth being used. In Europe, a different method called psophometric weighting might be used instead.

In audio equipment, 0 dBm is about equal to 0.775 volts because this uses up 1 milliwatt of power in a special type of circuit. For radio frequency equipment with a different kind of circuit, 0 dBm is about equal to 0.224 volts.

The dBm measurement was first suggested as a standard way to measure power in 1940. It is mainly used for measuring light and electrical power, not for other kinds of power like heat.