Wi-Fi Signal Calculator

Wi-Fi tools report signal in dBm, a unit that looks simple and behaves counter-intuitively. The page converts a reading to actual power, estimates a quality percentage, and calculates signal-to-noise ratio from the noise floor, which is what decides the data rate a link can sustain.

What dBm means

dBm is a logarithmic measure of power relative to one milliwatt. 0 dBm is 1 mW; every 10 dB is a factor of ten and every 3 dB roughly a factor of two. Wi-Fi signals arrive at the receiver in the range of nanowatts or less, so their values are negative: -30 dBm is 0.001 mW, -50 dBm is 10 nW, -60 dBm is 1 nW and -90 dBm is one picowatt. The page shows the power in the most readable unit, so a -65 dBm reading comes out as 0.32 nW.

Quality percentage and its trap

Windows displays a percentage computed as 2 × (dBm + 100), clamped to the range 0 to 100. -50 dBm and anything stronger is 100 %, -70 dBm is 60 % and -90 dBm is 20 %. That is a convention and not a physical quantity. Other operating systems and vendor tools scale differently, and many show only bars, so a percentage from one device cannot be compared with another’s. The ratings (excellent, very good, good, fair, weak, poor) are also rules of thumb used in the industry, not figures from the 802.11 standards.

Signal-to-noise ratio

A strong signal is useless if the noise is equally strong. The noise floor of typical Wi-Fi environments lies between -90 and -100 dBm, and rises with interference from neighbouring networks, microwave ovens or poorly shielded electronics. The signal-to-noise ratio in dB is simply signal minus noise. The page notes that 25 dB or more supports the higher-order modulations that give the fastest rates. As a rough guide, 15 to 20 dB gives workable but slower links, and below about 10 dB the connection becomes unreliable.

A worked example

The first preset is a signal of -65 dBm with a noise floor of -95 dBm. The quality is 2 × (−65 + 100) = 70 %, rated Good, which the page says is the commonly cited minimum for voice and video. The power is 0.32 nW, and the signal-to-noise ratio is 30 dB, enough for high-order modulation and fast rates. A reference table lists -30 dBm as 100 % and Excellent, -60 dBm as 80 %, -70 dBm as 60 % and Fair, -80 dBm as 40 % and Weak, and -90 dBm as 20 % and Poor.

Limits

Signal level and SNR are only two inputs to real throughput. Channel width, interference from other networks, the number of spatial streams, the client’s antennas and the access point’s load all matter, and the 802.11 rate chosen is negotiated continuously. The page converts and rates the numbers you type; it cannot measure your Wi-Fi. For antenna and cable gains in a radio link, use the Link Budget Calculator.

Related tools

• Free-Space Path Loss Calculator estimates how much signal is lost over distance.
• Link Budget Calculator adds antenna gains and receiver sensitivity.
• Network Jitter Test measures the stability of the connection you are rating.
• Fibre Loss Budget Calculator does the equivalent sum for optical links.