How does RSSI (dBm) relate to signal quality (percent) ?
Depending on your OS and application, WiFi signal strength is represented either as quality in percentage, or an RSSI value in dBm, i.e. -70db. RSSI is usually expressed in decibels relative to a milliwat (dBm) from zero to -120dBm and the closer it is to zero, the stronger the signal is. RSSI level less than -80dBm may not be usable, depending on noise.
While there is no simple precise solution that is used universally, we will try to explain the approximate correlation between signal (RSSI) and quality (percentage).
dBm >= -50 dBm = 100% quality
dBm <= -100 dBm = 0% quality
For RSSI signal between -50dBm and -100dBm,
quality ~= 2* (dBm + 100)
RSSI ~= (percentage / 2) - 100
High quality: 90% ~= -55dBm
Medium quality: 50% ~= -75dBm
Low quality: 30% ~= -85dBm
Unusable quality: 8% ~= -96dBm
See also: How to read RSSI/signal and SNR/noise ratings ?
Note: Most networks can work well with signal over -80dBm.