Calculate the signal-to-noise ratio from signal and noise power levels.
Signal-to-noise ratio (SNR) is the difference, in dB, between the received signal power and the background noise floor: SNR(dB) = signal(dBm) − noise(dBm). Because both values are already logarithmic (dBm), the SNR calculation is a simple subtraction rather than a division. SNR is a better predictor of connection quality than signal strength alone, since a strong signal in a noisy environment can perform worse than a moderate signal in a quiet one — the modulation and coding scheme a radio can sustain depends directly on how far the signal sits above the noise.
Signal-to-noise ratio
SNR(dB) = signal(dBm) − noise(dBm)
A strong signal (e.g. -50 dBm) next to a high noise floor (e.g. -55 dBm) gives only 5 dB of SNR — poor quality — while a weaker signal (-70 dBm) with a very quiet noise floor (-95 dBm) gives 25 dB — good quality. Signal strength alone doesn't reveal this.
Roughly 25 dB or higher is generally needed for a solid, high-throughput connection. Below about 15-20 dB, expect noticeably reduced data rates as the radio falls back to more robust but slower modulation schemes to maintain a usable link.
Other wireless devices on the same or overlapping channels, non-WiFi interference (microwave ovens, Bluetooth, cordless phones), electrical equipment, and even thermal noise in the receiver itself all contribute to raising the effective noise floor.
Yes, if the noise floor exceeds the signal power (e.g. signal at -85 dBm with noise at -80 dBm gives -5 dB SNR), meaning the receiver essentially cannot distinguish the intended signal from background noise, and the link will fail.