Typically, cellular signal strength ranges from -50 dBm to -120 dBm.
The closer you are to 0 the better. On average, though, a strong outside signal clocks in somewhere around -50 to -70 dBm. Good signal strength ranges between -80 to -90 dBm. Anything below -100 dBm is considered poor signal strength.
The use of a cell phone signal booster can improve your existing dBm exponentially. According to the logarithm, the signal strength doubles with every third decibel, meaning that a dBm signal strength of something like -70 is actually twice the power of -73 dBm. The numbers may not make it look like a great change has occurred, but in reality, the decibels have grown exponentially.