What Is The Difference Between dB, dBm and dBi?

dB vs dBm vs dBi - The differences

dB, dBm and dBi are acronyms you may commonly see in relation to wifi routers.

They look similar and are used for similar measurements so it is important to be absolutely sure of what they mean before you choose a router, or are thinking of making modifications to a router.

In the rest of this article we will try to explain what each measurement means in as simple a way as possible and more importantly how they are used in relation to your routers.

What is dB?

dB stands for decibel, which is of course used to measure sound, but is also a relative measurement of two different power levels. As such the number before dB is a ratio, not an absolute measurement.

For example, in the two sections below we will talk about dBm, which is the power in relation to one milliwatt, and dBi, which is the power in relation to an isotropic reference antenna.

What is dBm?

dBm stands for decibels relative to a milliwatts and it refers to the power ratio in decibels when referenced to milliwatts. dBm is normally associated with power emitted from an amplifier. Power itself is usually measured in milliwatts - represented as mW.

What this means for a wifi router is that it is a measure of the power coming out of the antenna which can affect the range your router can work at. If your dBm is too low, you may find that your connection is weak and you are unable to use your network as you would like.

Likewise, if it is too high, it can amplify background 'noise' , which can affect the quality of your wireless connections. There are also guidelines and laws on how much power can be applied - country dependent.

What is dBi?

dBi stands for decibels relative to an isotropic reference antenna. When you see this in relation to routers it refers to the maximum effectiveness of the antenna on the wifi router. As such dBi is the standard term used when it comes to measuring an antenna’s gain.

A typical home wifi router antenna will come in at around 4-9 dBi, which should be enough for a small indoor area like an apartment or small house.

It is possible to buy antennas with a much higher dBi. In fact, some antennas built for outdoor use can come in at 15 or even 24 dBi. These tend to be directional antennas used in point-to-point or point-to-multipoint connections.

dB vs dBm vs dBi: The Differences?

Well firstly dB on its own doesn’t really mean anything. A dB just represents gain or loss.

Without referencing it to something else it is impossible to know what the number is representing. dB is however used to provide a dBm or dBi measurement...

The difference between dBm and dBi is quite simply that they have different points of reference:

  • dBm = power. The amount of relative power transmitted by the amplifier.
  • dBi = gain. The amount of 'gain' an antenna puts on a signal. Normal router antennas (omni-directional) will cover more horizontal area with higher 'gain', but not cover the vertical area as much.
Omni-directional Antennas - with Varying Gain
dB vs dBm vs dBi - Low Gain

As you can see, low gain (low dBi) antennas offer less horizontal coverage, but greater vertical coverage.

dB vs dBm vs dBi - High Gain

High gain antennas offer much greater horizontal coverage, but poorer vertical coverage.

The amount of coverage area will again be affected by the amount of dBm.

One thing to note is that due to these different points of reference, manufacturers will often market their product using the dBi rating, simply because it will use a higher number. Consumers will naturally assume this makes the product better, as it has a higher number. But of course, this isn't always the case.

Hopefully that has cleared up any questions you may have had about decibels! If not, please comment below.

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