dBm (sometimes dBmW or Decibelmilliwatts) is an abbreviation for the power ratio in decibels (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 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, 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 signaltonoise ratio.
In audio and telephony, dBm is typically referenced relative to a 600 ohm impedance,^{[1]} while in radio frequency work dBm is typically referenced relative to a 50 ohm impedance.^{[2]}
Unit conversions
A power level of 0 dBm corresponds to a power of 1 milliwatt. 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. 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, or vice versa, the following equivalent expressions may be used:

\begin{align} x &= 10 \log_{10} \frac{P}{ 1 \mathrm{mW}} \end{align}
idem with P in watts

\begin{align} x &= 30+10 \log_{10} \frac{P}{ 1 \mathrm{W}} \end{align}

\begin{align} P &= 1 \text{mW} \cdot 10^{\frac{x}{10}}\\ P &= 1 \text{W} \cdot 10^{ \frac{ x30 }{10} } \end{align}
where P is the power in W and x is the power level in dBm. Below is a table summarizing useful cases:
Power level

Power

Notes

80 dBm

100 kW

Typical transmission power of FM radio station with 50kilometre (31 mi) range

62 dBm

1.588 kW = 1,588 W

1500 W is the maximum legal power output of a U.S. ham radio station.^{[3]}

60 dBm

1 kW = 1,000 W

Typical combined radiated RF power of microwave oven elements

50 dBm

100 W

Typical thermal radiation emitted by a human body Typical maximum output RF power from a ham radio HF 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 station (27 MHz) in many countries

33 dBm

2 W

Maximum output from a UMTS/3G mobile phone (Power class 1 mobiles) Maximum output from a GSM850/900 mobile phone

30 dBm

1 W = 1,000 mW

Typical RF leakage from a microwave oven DCS or GSM 1,800/1,900 MHz mobile phone. EIRP IEEE 802.11a (20 MHzwide channels) in either 5 GHz Subband 2 (5,470–5,725 MHz) provided that transmitters are also IEEE 802.11hcompliant, or UNII3 (5,725–5,825 MHz). The former is EU only, the latter is US only.

29 dBm

794 mW


28 dBm

631 mW


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


25 dBm

316 mW


24 dBm

251 mW

Maximum 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 MHzwide channels) in either the 5 GHz Subband 1 (5,180–5,320 MHz) or UNII2 & 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 MHzwide (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 MHzwide (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

Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles)

20 dBm

100 mW

EIRP for IEEE 802.11b/g Wireless LAN 20 MHzwide channels in the 2.4 GHz ISM band (5 mW/MHz). Bluetooth Class 1 radio. Maximum output power from unlicensed AM transmitter per U.S. Federal Communications Commission (FCC) rules 15.219.^{[4]}

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

More precisely (to 8 decimal places) 1.9952623 mW

2 dBm

1.6 mW


1 dBm

1.3 mW


0 dBm

1.0 mW = 1,000 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

Maximum received signal power of wireless network (802.11 variants)

−20 dBm

10 µW


−30 dBm

1.0 µW = 1,000 nW


−40 dBm

100 nW


−50 dBm

10 nW


−60 dBm

1.0 nW = 1,000 pW

The Earth receives one nanowatt per square metre from a magnitude +3.5 star^{[5]}

−70 dBm

100 pW


−73 dBm

50.12 pW

"S9" signal strength, a strong signal, on the Smeter of a typical ham or shortwave radio receiver

−80 dBm

10 pW


−100 dBm

0.1 pW

Minimum 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 wellexpressed 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 Freespace 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 Volts dissipates 1 mW in a 600 Ω load.^{[6]} dBu measures against this reference voltage 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 Volts dissipates 1 mW in a 50 Ω load.
The dBm is not a part of the International System 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 (dB), being a unitless ratio of two numbers, is perfectly acceptable.^{[7]}
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^{[6]} in the paper "A New Standard Volume Indicator and Reference Level".^{[8]}
See also
References
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MILSTD188).

^ Bigelow, Stephen. Understanding Telephone Electronics. Newnes. p. 16.

^ Carr, Joseph (2002). RF Components and Circuits. Newnes. pp. 45–46.

^ "Part 97  Amateur Radio". ARRL. Retrieved 20120921.

^ FCC Web Documents citing 15.219

^ Radiant Flux of a Magnitude +3.5 Star

^ ^{a} ^{b} Davis, Gary (1988). The Sound Reinforcement Handbook. Yamaha. p. 22.

^ Thompson and Taylor 2008, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811

^ Chinn, H.A.; D.K. Gannett, R.M.Moris (January 1940). "A New Standard Volume Indicator and Reference Level". Proceedings of the Institute of Radio Engineers 28 (1): 1–17.
External links

The dBm calculator for impedance matching

Convert dBm to watts
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