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Signal Strength dBm

You probably have asked yourself why when measuring signal strength dBm is the unit of choice? Or why is dBm signal strength always expressed as a negative number? Or what is dBm to begin with?

WiFi uses radio signals for transferring data wirelessly between computers and other devices. Very often you will find in various documents related to WiFi documents various positive or negative values expressed in dBm. So, here’s an in-depth explanation of what it is and what it has to do with the signal strength.


In short, dBm or decibel-milliwatt is a unit used to describe signal power level in decibels in relation to 1 milliwatt of energy. But what a signal strength dBm measurement then actually describes? Because the dB scale is logarithmic, it is used to easily describe both very large and very small values.

A decibel is a dimensionless unit, which means that it does not have a physical dimension associated with them. Instead, it describes a ratio between two values. With decibel-milliwatts signal, power is not directly expressed, but the ratio between the strength of 1 milliwatt signal and the measured one.


For signal strength dBm is a very useful unit of measurement because the power of radio signals changes logarithmically with the change of distance between transmitter and receiver. This is the most important thing to understand because for signal strength dBm is very useful to understand the signal strength loss or gain when using a device such as a WiFi booster like RangeXTD.

When diagnosing the quality of the WiFi signal strength dBm measurement shows how much the signal’s power decreases. The absolute measurement in watts or milliwatts would be meaningless and would require measuring signal power at two points.


The physical nature of radio wave signals is that their power decreases with the distance they travel from emitter to receiver. WiFi signal when the router’s antenna emits starts losing its power as it travels to your device. That’s why when measuring in dB signal strength you will see negative values, it is a measure of the factor of decrease compared to the reference value of 1 milliwatt.

In technical documentation of WiFi repeaters, you will find gain expressed as a positive value, because it is a measure of amplification of the signal strength.


If you have math knowledge about logarithms, the simplest explanation is that decibel is the base-10 logarithm. Otherwise, let me try to explain it with a bit more words. For signal strength dBm is used because it offers a handy level of granularity of measurement. Decibel is one-tenth of a bel, this is very handy to understand when trying to figure out what some number like -20dBm means.

Decibel-milliwatts describe the numerical factor by which signal power is higher or lower than 1 milliwatt signal. With each 10dBm increase signal strength increases 10 times, and with 10dBm decrease signal strength decreases 10 times. For example, a 10dBm signal is ten times stronger than 1 milliwatt, which is 10 milliwatts. 20dBm signal is a tenfold higher power than 10dBm, 30dBm is tenfold higher than 20dBm, and so on.

Thus converting decibels to bels is a useful quick hand way to calculate the gain or loss of power, by calculating 10 to the power of bels. For example, 20dBm is the increase of 20 decibels which are 2 bels, so it is a 102, or in other words a hundredfold increase.


When measuring cell signal strength dBm is also used, after all, it is a radio signal, and though of a different frequency from WiFi its strength can be measured in the same way, because the signal strength is independent of its frequency. The typical range in dBm phone signal strength is measured in the real world is between -30dBm and -110dBm.

If you convert that to milliwatts, it is between 10 femtowatts and 1 microwatt. In other words, between ten quadrillionth parts of watt and one-millionth of a watt. Usable signal strength is anything above -80dBm, or stronger than 10 picowatts. Anything below that will cause loss of signal and frequent interruptions, making it unusable.


The maximum speed at which data can be sent or received over a WiFi network is directly proportional to the signal strength. Higher signal strength will mean higher possible speed. This is why in areas of your home that suffer from poor WiFi signal coverage you will notice that when your download speeds decrease dramatically.

The speed also depends on the particular 802.11 variants your WiFi network is using, but the optimal dBm values are the same for all of them. The maximum signal strength of a typical WiFi network is -10dBm, while the minimum signal strength that a receiver can usefully receive is -100dBm. At this minimum, your device will only be able to connect to WiFi, but speed will be useless.

Some general rule is that signal strength in the range between -60 dBm and -75dBm is a good, -45dBm to -60dBm is very good, and -10dBm to -45dBm is excellent. But depending on your internet connection speed even signal strengths of -80dBm can be useful because the signal strength impacts the maximum speed at which your device can communicate with the wireless router.


Signal strength is increased by using some method of its amplification. For WiFi home networks the best way is by using WiFi repeaters because they always are equipped with boosters. These devices function by receiving the signal from your wireless router, boosting its strength, and then emitting it. They are also a perfect way to extend the range of your WiFi network equipment.

Due to the nature of radio signals, mainly because they very poorly transmit through solid objects such as walls and floors, your home will be covered in a patchwork of areas with signals of varying strength. WiFi repeaters solve this problem.S

April 10, 2021


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