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When working with radios, I've seen the term "dBm". What does this mean?

The wikipedia page gives a table showing the equivalent power in Watts. Is the dBm output just the power level of a transmitter?

How does this differ from signal strength measured in dB?

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A decibel is a unitless ratio and needs a reference point. Never use it without one. Especially you audio engineers! dBSPL!

Depending on the application, a signal strength may be measured in dBW or dBm. Both being units of power with a difference of 1000, conversion between dBw and dBm can be obtained by adding or subtracting 10log(1000) or 30.

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  • \$\begingroup\$ There are already multiple answers that say this. \$\endgroup\$
    – Kortuk
    Nov 16, 2010 at 19:41
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    \$\begingroup\$ Except none highlight the common misconception that it is a unit by itself. This is a problem many people have (including the OP) and it needs to be driven home. \$\endgroup\$
    – eruditass
    Nov 16, 2010 at 19:43
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    \$\begingroup\$ "I have a 10 dB attenuator" I used decibel without a reference point. \$\endgroup\$
    – Nick T
    Nov 16, 2010 at 20:49
  • \$\begingroup\$ @NickT, You referenced your input. @Eruditass, I did in the last section of my post. \$\endgroup\$
    – Kortuk
    Nov 17, 2010 at 1:35
  • \$\begingroup\$ @Kortuk - That's somewhat my point; there it's implicit, in many other contexts the reference is as well (yes, it can suck). \$\endgroup\$
    – Nick T
    Nov 17, 2010 at 18:31
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What is it?

dBm is a dB scale relative to 1mW.

0dBm = 1mW

30dBm = 1W = 0dBW

Why use dBm?

Transmit power of 20dBm is 100mW. You know something interesting. This is the max you can output with Bluetooth or Zigbee operating at 2.4GHz.

If a transmit power is 10dB you know nothing about your power. You must know both your amplification factor, as was noted in comments, and you need to know what is being amplified.

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    \$\begingroup\$ I have seen many places that leave off the m, I think it's a poor lazy practice though. \$\endgroup\$
    – Kellenjb
    Nov 16, 2010 at 17:40
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    \$\begingroup\$ Transmit power cannot be 10dB, 10dB is a multiplication factor not an absolute value. Attenuation or amplification can be 10dB. \$\endgroup\$
    – bjarkef
    Nov 16, 2010 at 18:07
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    \$\begingroup\$ Your 10 dBm = 10 mW example is correct, but maybe misleading for people who aren't familiar with logarithmic scales. For example, 3 dBm is roughly 2 mW. 6 dBm roughly 4 mW. Just trying to make the nonlinearity clear. \$\endgroup\$
    – pingswept
    Nov 16, 2010 at 18:53
  • \$\begingroup\$ @pingswept, I am so used to it I did not think of that, I have changed it to 20dBm which is a more interesting value anyway. @Bjarkef, that is my point, I have heard many people say dB and think it was of value. you could reference nanaoWatts. \$\endgroup\$
    – Kortuk
    Nov 16, 2010 at 19:29
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Dave Jones has an interesting video on decibels. He also discusses dBm in the video.

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  • \$\begingroup\$ EEVblog #49 – Decibels (dB’s) for Engineers – A Tutorial \$\endgroup\$
    – csadam
    Nov 16, 2010 at 20:23
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Signal strength that is measured as dB with no m usually is actually dBm. However you need to be careful because high power applications may use dBW which you have to add 30 to get from dBW to dBm (30dBm=0dBW, 60dBm=30dBW...)

Kortuk is correct that 0dBm = 1mW. But also 0dBW = 1W.

10*log(linear_ratio) is what is used to get dB.

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    \$\begingroup\$ assumptions make an ass of u and me. most transceivers have output in dBm. most amps have a gain in dB as a function of input dBm or a set gain with max output power. \$\endgroup\$
    – Kortuk
    Nov 16, 2010 at 17:43
  • \$\begingroup\$ I've seen a data sheet for a Sony high power amp that left off the m everywhere. \$\endgroup\$
    – Kellenjb
    Nov 16, 2010 at 17:45
  • \$\begingroup\$ Sony amp; for audio? Most of the core audio amp parameters I'd think of would be relative (SNR, max gain...) versus absolute (noise floor, max output?). \$\endgroup\$
    – Nick T
    Nov 16, 2010 at 18:38
  • \$\begingroup\$ Sony amp for RF. \$\endgroup\$
    – Kellenjb
    Nov 16, 2010 at 19:01
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What is not explicitly stated so far in the answers is that while a power level should be stated with a reference unit - dBm or dBw or similar, a difference in power levels must not have a unit "I tuned the coil and brought the output up 4 dB"

This is because when you subtract logarithms, what you are actually doing is dividing (the exponential values), and in division the units cancel leaving just a ratio.

So while seeing dB by itself may indicate someone is being sloppy, in well-written engineering documents you will often see a mix of referenced units such as dBm to state absolute powers, and unreferenced ratios in dB to state relative differences.

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