dBm or dBmW (decibel-milliwatts) is a unit of level used to indicate that a power level is expressed in decibels (dB) with reference to one milliwatt (mW). It is used in radio, microwave and fiber-optical communication networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. dBW is a similar unit, referenced to one watt (1,000 mW).
The decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio. The dBm is also dimensionless, but since it compares to a fixed reference value, the dBm rating is an absolute one.
The dBm is not a part of the International System of Units (SI) and therefore is discouraged from use in documents or systems that adhere to SI units (the corresponding SI unit is the watt). However, the unit decibel (dB), without the 'm' suffix, is permitted for relative quantities, but not accepted for use directly alongside SI units. Ten decibel-milliwatts may be written 10 dB (1 mW) in SI.: 7.4
In audio and telephony, dBm is typically referenced relative to a 600-ohm impedance, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.
A power level of 0 dBm corresponds to a power of 1 milliwatt. A 10 dB increase in level is equivalent to a ten-fold increase in power. Therefore, a 20 dB increase in level is equivalent to a 100-fold increase in power. A 3 dB increase in level is approximately equivalent to doubling the power, which means that a level of 3 dBm corresponds roughly to a power of 2 mW. Similarly, for each 3 dB decrease in level, the power is reduced by about one half, making −3 dBm correspond to a power of about 0.5 mW.
To express an arbitrary power P in mW as x in dBm, the following expression may be used:
Conversely, to express an arbitrary power level x in dBm, as P in mW:
Below is a table summarizing useful cases:
|526 dBm||3.6×1049 W||Black hole collision, the power radiated in gravitational waves following the collision GW150914, estimated at 50 times the power output of all the stars in the observable universe.|
|420 dBm||1×1039 W||Cygnus A, one of the most powerful radio sources in the sky|
|296 dBm||3.846×1026 W||Total power output of the Sun|
|120 dBm||1 GW||Experimental high-power microwave (HPM) generation system, 1GW at 2.32 GHz for 38 ns|
|105 dBm||32 MW||AN/FPS-85 Phased Array Space Surveillance Radar, claimed by the US Space Force as the most powerful radar in the world.|
|95.5 dBm||3,600 kW||High-frequency Active Auroral Research Program maximum power output, the most powerful shortwave station in 2012|
|80 dBm||100 kW||Typical transmission power of FM radio station with 50-kilometre (31 mi) range|
|62 dBm||1.588 kW = 1,588 W||1,500 W is the maximal legal power output of a US ham radio station.|
|60 dBm||1 kW = 1,000 W||Typical combined radiated RF power of microwave oven elements|
|55 dBm||~300 W||Typical single-channel RF output power of a Ku-band geostationary satellite|
|50 dBm||100 W||Typical total thermal radiation emitted by a human body, peak at 31.5 THz (9.5 μm)
Typical maximal output RF power from a ham radio HF transceiver
|40 dBm||10 W||Typical power-line communication (PLC) transmission power|
|37 dBm||5 W||Typical maximal output RF power from a handheld ham radio VHF/UHF transceiver|
|36 dBm||4 W||Typical maximal output power for a citizens band radio station (27 MHz) in many countries|
|33 dBm||2 W||Maximal output from a UMTS/3G mobile phone (power class 1 mobiles)
Maximal output from a GSM850/900 mobile phone
|30 dBm||1 W = 1000 mW||
DCS or GSM 1,800/1,900 MHz mobile phone. EIRP IEEE 802.11a (20 MHz-wide channels) in either 5 GHz subband 2 (5,470–5,725 MHz) provided that transmitters are also IEEE 802.11h-compliant, or U-NII-3 (5,725–5,825 MHz). The former is EU only, the latter is US only. Also, maximal power allowed by the FCC for American amateur radio licensees to fly radio-controlled aircraft or operate RC models of any other type on the amateur radio bands in the US.
|29 dBm||794 mW|
|28 dBm||631 mW|
|27 dBm||500 mW||Typical cellular phone transmission power
Maximal output from a UMTS/3G mobile phone (power class 2 mobiles)
|26 dBm||400 mW|
|25 dBm||316 mW|
|24 dBm||251 mW||Maximal output from a UMTS/3G mobile phone (power class 3 mobiles)
1,880–1,900 MHz DECT (250 mW per 1,728 kHz channel). EIRP for wireless LAN IEEE 802.11a (20 MHz-wide channels) in either the 5 GHz subband 1 (5,180–5,320 MHz) or U-NII-2 and -W ranges (5,250–5,350 MHz & 5,470–5,725 MHz, respectively). The former is EU only, the latter is US only.
|23 dBm||200 mW||EIRP for IEEE 802.11n wireless LAN 40 MHz-wide (5 mW/MHz) channels in 5 GHz subband 4 (5,735–5,835 MHz, US only) or 5 GHz subband 2 (5,470–5,725 MHz, EU only). Also applies to 20 MHz-wide (10 mW/MHz) IEEE 802.11a wireless LAN in 5 GHz subband 1 (5,180–5,320 MHz) if also IEEE 802.11h-compliant (otherwise only 3 mW/MHz → 60 mW when unable to dynamically adjust transmission power, and only 1.5 mW/MHz → 30 mW when a transmitter also cannot dynamically select frequency).|
|22 dBm||158 mW|
|21 dBm||125 mW||Maximal output from a UMTS/3G mobile phone (power class 4 mobiles)|
|20 dBm||100 mW||EIRP for IEEE 802.11b/g wireless LAN 20 MHz-wide channels in the 2.4 GHz Wi-Fi/ISM band (5 mW/MHz).
Bluetooth Class 1 radio. Maximal output power from unlicensed AM transmitter per US FCC rules 15.219
|19 dBm||79 mW|
|18 dBm||63 mW|
|17 dBm||50 mW|
|15 dBm||32 mW||Typical wireless LAN transmission power in laptops|
|10 dBm||10 mW|
|7 dBm||5.0 mW||Common power level required to test the automatic gain control circuitry in an AM receiver|
|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|
|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||Maximal received signal power of wireless network (802.11 variants)|
|−13 dBm||50.12 μW||Dial tone for the Precise Tone Plan found on public switched telephone networks in North America|
|−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 receives one nanowatt per square metre from a magnitude +3.5 star|
|−70 dBm||100 pW|
|−73 dBm||50.12 pW||"S9" signal strength, a strong signal, on the S meter of a typical ham or shortwave radio receiver|
|−80 dBm||10 pW|
|−100 dBm||0.1 pW||Minimal received signal power of wireless network (802.11 variants)|
|−111 dBm||0.008 pW = 8 fW||Thermal noise floor 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|
|−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 kelvins)|
|−∞ 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).
In United States Department of Defense practice, unweighted measurement is normally understood, applicable to a certain bandwidth, which must be stated or implied.
In European practice, psophometric weighting may be, as indicated by context, equivalent to dBm0p, which is preferred.
In audio, 0 dBm often corresponds to approximately 0.775 volts, since 0.775 V dissipates 1 mW in a 600 Ω load. The corresponding voltage level is 0 dBu, without the 600 Ω restriction. Conversely, for RF situations with a 50 Ω load, 0 dBm corresponds to approximately 0.224 volts, since 0.224 V dissipates 1 mW in a 50 Ω load. In general the relationship between the power level P in dBms and the RMS voltage V in volts across a load of resistance R (typically used to terminate a transmission line with impedance Z) is:
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 is available that includes a variety of examples not necessarily related to electrical or optical power.
The dBm was first proposed as an industry standard in the paper "A New Standard Volume Indicator and Reference Level".
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. (in support of MIL-STD-188).