DBm
Encyclopedia
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
because of its capability to express both very large and very small values in a short form. Compare dBW, which is referenced to one watt (1000 mW).
Since it is referenced to the watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
, it is an absolute unit, used when measuring absolute power. By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
.
Unit conversions
Zero dBm equals one milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making −3 dBm equal to about 0.5 milliwatt.To express an arbitrary power P as x dBm, or vice versa, the following equations may be used:
or, 
and
or, 
where P is the power in W and x is the power ratio in dBm. Below is a table summarizing useful cases:
| dBm level | Power | Notes |
|---|---|---|
| 80 dBm | 100 kW | Typical transmission power Effective radiated power In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains... of FM radio station with adj=on 50 range |
| 60 dBm | 1 kW = 1000 W | Typical combined radiated RF power of microwave oven Microwave oven A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food... elements Maximum RF output power from a ham radio transceiver Transceiver A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s... allowed without special permission |
| 50 dBm | 100 W | Typical thermal radiation emitted by a human body Typical maximum output RF power from a ham radio HF High frequency High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency... transceiver |
| 40 dBm | 10 W | Typical PLC (Power Line Carrier) Transmit Power |
| 37 dBm | 5 W | Typical maximum output RF power from a handheld ham radio VHF/UHF transceiver |
| 36 dBm | 4 W | Typical maximum output power for a Citizens' band radio Citizens' band radio Citizens' Band radio is, in many countries, a system of short-distance radio communications between individuals on a selection of 40 channels within the 27-MHz band. Citizens' Band is distinct from the FRS, GMRS, MURS and amateur radio... station (27 MHz) in many countries |
| 33 dBm | 2 W | Maximum output from a UMTS/3G 3G 3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union... mobile phone (Power class 1 mobiles) Maximum output from a GSM850/900 mobile phone |
| 30 dBm | 1 W = 1000 mW | Typical RF leakage from a microwave oven - Maximum output power for DCS 1800 MHz mobile phone Maximum output from a GSM1800/1900 mobile phone |
| 27 dBm | 500 mW | Typical cellular phone transmission power Maximum output from a UMTS/3G mobile phone (Power class 2 mobiles) |
| 26 dBm | 400 mW | Access point for Wireless networking |
| 25 dBm | 316 mW | |
| 24 dBm | 250 mW | Maximum output from a UMTS/3G mobile phone (Power class 3 mobiles) |
| 23 dBm | 200 mW | Maximum output in interior environment from a WiFi 2.4Ghz antenna (802.11b/g/n). |
| 22 dBm | 160 mW | |
| 21 dBm | 125 mW | Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles) |
| 20 dBm | 100 mW | Bluetooth Bluetooth Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security... Class 1 radio, 100 m range Maximum output power from unlicensed AM transmitter per U.S. Federal Communications Commission Federal Communications Commission The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the... (FCC) rules 15.219. Typical wireless router Wireless router A Wireless router is a device that performs the functions of a router but also includes the functions of a wireless access point and a network switch. They are commonly used to allow access to the Internet or a computer network without the need for a cabled connection. It can function in a wired... transmission power. |
| 15 dBm | 32 mW | Typical WiFi WIFI WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting... transmission power in laptops. |
| 10 dBm | 10 mW | |
| 6 dBm | 4.0 mW | |
| 5 dBm | 3.2 mW | |
| 4 dBm | 2.5 mW | Bluetooth Class 2 radio, 10 m range |
| 3 dBm | 2.0 mW | More precisely (to 8 decimal places) 1.9952623 mW |
| 2 dBm | 1.6 mW | |
| 1 dBm | 1.3 mW | |
| 0 dBm | 1.0 mW = 1000 µW | Bluetooth standard (Class 3) radio, 1 m range |
| −1 dBm | 794 µW | |
| −3 dBm | 501 µW | |
| −5 dBm | 316 µW | |
| −10 dBm | 100 µW | Typical maximum received signal power (−10 to −30 dBm) of wireless network Wireless network Wireless network refers to any type of computer network that is not connected by cables of any kind. It is a method by which homes, telecommunications networks and enterprise installations avoid the costly process of introducing cables into a building, or as a connection between various equipment... |
| −20 dBm | 10 µW | |
| −30 dBm | 1.0 µW = 1000 nW | |
| −40 dBm | 100 nW | |
| −50 dBm | 10 nW | |
| −60 dBm | 1.0 nW = 1000 pW | The Earth Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets... receives one nanowatt per square metre from a magnitude Apparent magnitude The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere... +3.5 star Star A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth... |
| −70 dBm | 100 pW | Typical range (−60 to −80 dBm) of wireless received signal power over a network (802.11 variants) |
| −73 dBm | 50.12 pW | "S9" signal strength, a strong signal, on the S-meter S meter An S meter is an indicator often provided on communications receivers, such as amateur radio receivers or shortwave broadcast receivers. The scale markings are derived from a subjective system of reporting signal strength from S1 to S9 as part of the RST code... of a typical ham Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... or shortwave Shortwave Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used... radio receiver |
| −80 dBm | 10 pW | |
| −100 dBm | 0.1 pW | |
| −111 dBm | 0.008 pW = 8 fW | Thermal noise floor Johnson–Nyquist noise Johnson–Nyquist noise is the electronic noise generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage... for commercial GPS single channel signal bandwidth (2 MHz) |
| −127.5 dBm | 0.178 fW = 178 aW | Typical received signal power from a GPS satellite GPS satellite A GPS satellite is a satellite used by the NAVSTAR Global Positioning System . The first satellite in the system, Navstar 1, was launched February 22, 1978. The GPS satellite constellation is operated by the 50th Space Wing of the United States Air Force.... |
| −174 dBm | 0.004 aW = 4 zW | Thermal noise floor for 1 Hz bandwidth at room temperature (20 °C) |
| −192.5 dBm | 0.056 zW = 56 yW | Thermal noise floor for 1 Hz bandwidth in outer space (4 kelvin Kelvin The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all... s) |
| −∞ dBm | 0 W | Zero power is not well-expressed in dBm (value is negative infinity) |
The signal intensity (power per unit area) can be converted to received signal power by multiplying by the square of the wavelength and dividing by 4π (see Free-space path loss
Free-space path loss
In telecommunication, free-space path loss is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space , with no obstacles nearby to cause reflection or diffraction...
).
In United States Department of Defense
United States Department of Defense
The United States Department of Defense is the U.S...
practice, unweighted
Weighting filter
A weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.- Audio applications :...
measurement is normally understood, applicable to a certain bandwidth, which must be stated or implied.
In European practice, psophometric weighting
Psophometric weighting
Psophometric weighting refers to any weighting curve used in the measurement of noise. In the field of audio engineering it has a more specific meaning, referring to noise weightings used especially in measuring noise on telecommunications circuits...
may be, as indicated by context, equivalent to dBm0p, which is preferred.
The dBm is not a part of the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
and therefore is discouraged from use in documents or systems that adhere to SI units (the corresponding SI unit is the watt). However the straight decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
(dB), being a unitless ratio of two numbers, is perfectly acceptable.
Expression in dBm is typically used for optical and electrical power measurements, not for other types of power (such as thermal). A listing by power levels in watts
Orders of magnitude (power)
This page lists examples of the power in watts produced by various sources of energy. They are grouped by orders of magnitude, and each section covers three orders of magnitude, or a factor of one thousand.-zeptowatt :...
is available that includes a variety of examples not necessarily related to electrical or optical power.