Cell phones measure received signal strength in dBm (decibels referenced to 1mW, a logarithmic scale where 1mW is equal to 0dBm). The bars on your phone are a visual representation of this measurement. You can see the numeric value for yourself in Field Test Mode.¹ To activate Field Test Mode on an iPhone dial *3001#12345#*

iPhone’s have two slightly different versions of Field Test Mode depending on which model of iPhone you have.

iPhones powered by an Intel modem which includes models sold by AT&T and T-Mobile (A1784 (iPhone 7), A1778 (iPhone 7 Plus), A1905 (iPhone 8), A1897 (iPhone 8 Plus) and A1901 (iPhone X)) will look like this:

Signal strength details can be found in the LTE>Serving Cell Meas> section. RSRP (Reference Signal Received Power) is the primary measurement used to determine the number of “bars”. rsrp0 and rsrp1 refer the two antennas in an iPhone (one at the top of the phone and one at the bottom). The 0 or 1 do not refer to a particular antenna, but rsrp0 displays the stronger of the signals measured by the two antennas.

Extra credit: SINR stands for Signal-to-Interference-plus-Noise Ratio – if the environment is RF “noisy” SINR is actually used to determine the number of “bars”. RSRQ (Reference Signal Received Quality) also takes into account noise and interference.

In iOS 11, the “bars” RSRP ranges are:

Prior to iOS 11, four “bars” or “dots” had the range -85 to -94 dBm, and there was a fifth dot for > -85 dBm.

iPhone’s powered by a Qualcomm modem, which includes models sold by Verizon or Sprint (A1660 (iPhone 7), A1661 (iPhone 7 Plus), A1863 (iPhone 8), A1864 (iPhone 8 Plus) and A1865 (iPhone X)) have a different Field Test Mode:

Signal strength measurements will be in the Serving Cell Measurements> section. RSSI (Received Signal Strength Indicator) is an alternate indicator of the signal strength. (Unlike RSRP, RSSI does not exclude noise and interference, and is not independent of bandwidth or data transfer activity).²

The “bars” are only displaying the signal received by the phone, but your phone is also transmitting a signal. It is often the transmitted signal from the phone that is the limiting factor in making or keeping a connection when you are far from a cell tower. reach case improves both the received and transmitted signals.

Links:

¹http://www.redmondpie.com/how-strong-is-your-iphone-signal-find-out-now-no-jailbreak-required/

² http://s4gru.com/entry/308-rssi-vs-rsrp-a-brief-lte-signal-strength-primer/