Wi-Fi coverage estimator.

Wi-Fi Coverage Estimator: how it works

The access point's transmit power plus antenna gain, minus the weakest signal you will accept, is the signal budget in dB. Subtract an allowance for clutter and the loss of every wall, and what is left is the path loss the open air may use. Free-space path loss in dB is 20 log10(distance in metres) + 20 log10(frequency in Hz) - 147.55, so the tool solves that for distance.

Worked example. 2.4 GHz (2437 MHz), 20 dBm transmit power, 3 dBi antenna gain, a weakest acceptable signal of -70 dBm, a 15 dB allowance and 3 walls at 6 dB each. Budget = 20 + 3 - (-70) = 93 dB. Subtract 15 dB and 18 dB of walls: 60 dB is left. That gives a radius of about 9.8 m (32 ft), an area of about 301 m2 (3,239 sq ft), and 1.58% of the power remains after the walls (10 to the power of -1.8). With no walls the same budget reaches about 78 m, which is why open-air numbers on a box are not what you get indoors.

Limits. The 6 dB per wall comes from Wikipedia's Link budget article, which gives about 6 dB for a wood stud wall with drywall on both sides at 2.4 GHz; other materials and bands lose a different amount, so enter your own figure. Real signal also bounces, shadows and drops with floors. Wikipedia's Wi-Fi article gives about 20 m (66 ft) as the range of an access point indoors, in the same order as this tool's wall-cluttered results. Treat the result as a starting point and verify with a phone signal reading on site.