Wifi signal strength (dBm)

dBm is decibels relative to one milliwatt, and it is logarithmic — which is why the numbers are negative and why they behave unintuitively. 0 dBm is one milliwatt; −30 dBm is one microwatt; −60 dBm is one nanowatt. Every 3 dB is a doubling or halving of power and every 10 dB is a factor of ten, so the difference between −50 and −60 is ten times the power, not twenty per cent.

The thresholds worth knowing: −50 or better is excellent and means you are close to the access point. −67 is the standard design target for voice and video, and most enterprise wifi surveys are done to that figure. −70 still carries web and email but streaming will stall. −80 and below is not usefully connected, and a device will often stay associated while passing almost no data — which is worse than disconnecting, because it will not roam to a better access point.

The percentage that Windows and phones show is not a standard measurement. Each vendor maps dBm to a percentage with its own curve, so 50% on one device and 50% on another are different signals. The dBm figure is the real one; the percentage here is a linear map from −100 to −30, which is roughly what most adapters do.

One practical point: signal strength is only half the story. A strong signal on a congested channel performs worse than a moderate signal on a clear one, because wifi is shared airtime. If everything reads −55 and it is still slow, the problem is interference or channel contention, not distance.