Many people use the signal bars on their smartphone to judge connectivity, but they are not standard measurements of signal strength.
In fact, the way these bars reflect mobile signal varies greatly between devices, as each manufacturer uses a different algorithm. It can also vary between devices from the same manufacturer.
All you can see is your device displaying signal strength on a scale of several “bars”, where four bars indicates a stronger signal than three bars, for example.
Samsung explains how it works
Samsung explained to MyBroadband that the signal bars on its Galaxy devices measure signal strength using RSCP, which is given in dBm.
Signal strength and signal quality are both measured in decibels (dBm), which is expressed as a negative number.
Decibel values closer to 0 represent a stronger signal, and a signal strength beyond -98 dBm is considered poor.
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Source - MyBroadband.co.za