dBm

For other uses, see DBM (disambiguation).

A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)

dBm (sometimes dBmW) 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 signal-to-noise ratio.

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:

$x = 10 \log_{10}(1000P)\,$ or, $x = 10 \log_{10}P + 30\,$

and

$P = 10^{(x/10)}/1000 \,$ or, $P = 10^{(x-30)/10} \,$

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 of FM radio station with 50-kilometre (31 mi) range
60 dBm	1 kW = 1000 W	Typical combined radiated RF power of microwave oven elements Maximum RF output power from a ham radio transceiver 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 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 = 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 Class 1 radio, 100 m range Maximum output power from unlicensed AM transmitter per U.S. Federal Communications Commission (FCC) rules 15.219.[1] Typical wireless router transmission power.
15 dBm	32 mW	Typical WiFi 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
−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[2]
−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 of a typical ham or shortwave radio receiver
−80 dBm	10 pW
−100 dBm	0.1 pW
−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.

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.^[3]

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.

See also

• dBW
• Decibel
• Transmission level point
• Orders of magnitude (power)

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

1. ^ http://www.hallikainen.org/FCC/FccRules/CiteFind/015219.htm
2. ^ Radiant Flux of a Magnitude +3.5 Star
3. ^ Taylor 1995, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811

External links

• The dBm calculator for impedance matching