How to See Your iPhone’s Precise Signal Strength
Dial *3001#12345#* and tap the green call button to put your iPhone into a secret Field Test Mode
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Instead of dots, you should now see a negative number in the upper left, like -102. This is your exact signal strength, measured in decibels, called the Received Signal Strength Indication (or RSSI if you want to impress technical support). The higher the number, the better, but note that these are negative numbers, so -1 would be an outstanding signal, while -1000 would be beyond poor. In the real world, you’ll probably see signal strengths somewhere between -40 (a five-bar signal) and -120 (a one-bar signal).
